Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator β′–EtMe₃Sb[Pd(dmit)₂]₂, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, while the low-temperature thermal conductivity significantly depends on the cooling rate, the high-temperature resistivity is almost perfectly independent of the cooling rate. These results indicate that in the samples with the finite residual term, the mean free path of the quasiparticles that carry the heat at low temperatures is governed by disorders, whose characteristic length scale of the distribution is much longer than the electron mean free path that determines the high-temperature resistivity. This explains why recent X-ray diffraction and nuclear magnetic resonance measurements show no cooling rate dependence. Naturally, these measurements are unsuitable for detecting disorders of the length scale relevant for the thermal conductivity, just as they cannot determine the residual resistivity of metals. Present results indicate that very careful experiments are needed when discussing itinerant spin excitations in β′–EtMe₃Sb[Pd(dmit)₂]₂.

DOI： 10.1038/s41598-022-13155-8

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Other Link： https://www.nature.com/articles/s41598-022-13155-8

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Despite extensive investigations, a topological state that hosts Majorana edge modes in the magnetic fieldinduced quantum disordered state of the Kitaev candidate material alpha-RuCl3 has been hotly debated. To gain more insight into this issue, we measured the thermal Hall conductivity kappa xy of various samples grown by the Bridgman method. The results show that the half-integer quantum thermal Hall effect is intimately related to the magnitude of longitudinal thermal conductivity and the Neel temperature at zero field, both of which are sample dependent. Samples exhibiting the half-integer quantum thermal Hall effect have larger zero-field thermal conductivity values than a threshold value, implying that a long mean free path of heat carriers is an important prerequisite. In addition, we find that samples with a higher Neel temperature exhibit a higher magnetic field at which quantization starts to occur. These results indicate that the quantization phenomenon is significantly affected by the impurity scatterings and the non-Kitaev interactions.

DOI： 10.1103/PhysRevB.106.L060410

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DOI： 10.1038/s42005-022-00873-8

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.105.024415

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.024415/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PORTFOLIO  

Charge-neutral excitations have been proposed to explain metal-like thermal transport in Kondo insulators. Here, the authors demonstrate the coupling between charge-neutral excitations and spin degrees of freedom in a Kondo insulator YbIr3Si7, which puts restrictions on current theories.Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr3Si7 reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr3Si7 is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.

DOI： 10.1038/s41467-021-27541-9

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The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, characterized by Cooper pairs condensed at finite momentum, has been a long-sought state that remains unresolved in many classes of fermionic systems, including superconductors and ultracold atoms. A fascinating aspect of the FFLO state is the emergence of periodic nodal planes in real space, but its observation is still lacking. Here we investigate the superconducting order parameter at high magnetic fields H applied perpendicular to the ab plane in a high-purity single crystal of FeSe. The heat capacity and magnetic torque provide thermodynamic evidence for a distinct superconducting phase at the low-temperature/high-field corner of the phase diagram. Despite the bulk superconductivity, spectroscopic-imaging scanning tunneling microscopy performed on the same crystal demonstrates that the order parameter vanishes at the surface upon entering the high-field phase. These results provide the first demonstration of a pinned planar node perpendicular to H, which is consistent with a putative FFLO state.

DOI： 10.1103/PhysRevLett.127.257001

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

FeSe is a unique high-$$T_c$$ iron-based superconductor in which nematicity, superconductivity, and magnetism are entangled with each other in the P-T phase diagram. We performed $$^{77}$$Se-nuclear magnetic resonance measurements under pressures of up to 3.9 GPa on 12% S-substituted FeSe, in which the complex overlap between the nematicity and magnetism are resolved. A pressure-induced Lifshitz transition was observed at 1.0 GPa as an anomaly of the density of states and as double superconducting (SC) domes accompanied by different types of antiferromagnetic (AF) fluctuations. The low-$$T_{\mathrm{c } }$$ SC dome below 1 GPa is accompanied by strong AF fluctuations, whereas the high-$$T_{\mathrm{c } }$$ SC dome develops above 1 GPa, where AF fluctuations are fairly weak. These results suggest the importance of the $$d_{xy}$$ orbital and its intra-orbital coupling for the high-$$T_{\mathrm{c } }$$ superconductivity.

DOI： 10.1038/s41598-021-96277-9

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Other Link： https://www.nature.com/articles/s41598-021-96277-9

Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science (AAAS)  

DOI： 10.1126/science.abd6701

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1882-0786/ac1ca4

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac1ca4/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science ({AAAS})  

DOI： 10.1126/science.aay5551

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Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the National Academy of Sciences  

We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi mathvariant="normal">F</m:mi><m:mi mathvariant="normal">e</m:mi><m:mi mathvariant="normal">S</m:mi><m:mi mathvariant="normal">e</m:mi></m:mrow><m:mrow><m:mn>1</m:mn><m:mo>−</m:mo><m:mi>x</m:mi></m:mrow></m:msub><m:msub><m:mrow><m:mi mathvariant="normal">S</m:mi></m:mrow><m:mrow><m:mi>x</m:mi></m:mrow></m:msub></m:math></inline-formula> superconductor. We observe two types of long-wavelength <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mi>X</m:mi><m:mi>Y</m:mi></m:math></inline-formula> symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>, a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mi>X</m:mi><m:mi>Y</m:mi></m:math></inline-formula> symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>. The intensity of the QEP grows with increasing sulfur concentration x and maximizes near critical concentration <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>x</m:mi></m:mrow><m:mrow><m:mi>c</m:mi><m:mi>r</m:mi></m:mrow></m:msub><m:mo>≈</m:mo><m:mn>0.16</m:mn></m:math></inline-formula>, while the pseudogap size decreases with the suppression of <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mrow><m:mi>T</m:mi></m:mrow><m:mrow><m:mi>S</m:mi></m:mrow></m:msub><m:mrow><m:mo>(</m:mo><m:mrow><m:mi>x</m:mi></m:mrow><m:mo>)</m:mo></m:mrow></m:math></inline-formula>. We interpret the development of the pseudogap in the quadrupole scattering channel as a manifestation of transition from the non-Fermi liquid regime, dominated by strong Pomeranchuk-like fluctuations giving rise to intense electronic continuum of excitations in the fourfold symmetric high-temperature phase, to the Fermi liquid regime in the broken-symmetry nematic phase where the quadrupole fluctuations are suppressed.

DOI： 10.1073/pnas.2020585118

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Other Link： https://syndication.highwire.org/content/doi/10.1073/pnas.2020585118

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevresearch.3.023069

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DOI： 10.1103/physrevx.11.011021

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The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe − S from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system. 1 x x

DOI： 10.1126/SCIADV.ABB9052

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DOI： 10.1103/physrevresearch.2.033367

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DOI： 10.1103/physrevresearch.2.033140

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Publishing type：Research paper (scientific journal)   Publisher：World Scientific Pub Co Pte Lt  

We performed [Formula: see text]-nuclear magnetic resonance measurements on [Formula: see text] ([Formula: see text] = 0.12 and 0.23). We found from the relaxation time [Formula: see text] that nematicity and antiferromagnetic fluctuations show cooperative coupling.

DOI： 10.1142/s0217984920400485

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DOI： 10.1103/PhysRevB.101.140407

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DOI： 10.1103/PhysRevLett.124.107001

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DOI： 10.1103/physrevresearch.2.013099

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.75.1.0_1718

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DOI： 10.1126/science.aaw4278

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© 2019, The Author(s). Unconventional superconductivity arises at the border between the strong coupling regime with local magnetic moments and the weak coupling regime with itinerant electrons, and stems from the physics of criticality that dissects the two. Unveiling the nature of the quasiparticles close to quantum criticality is fundamental to understand the phase diagram of quantum materials. Here, using resonant inelastic x-ray scattering (RIXS) and Fe − K β emission spectroscopy (XES), we visualize the coexistence and evolution of local magnetic moments and collective spin excitations across the superconducting dome in isovalently-doped BaFe2(As1−xPx)2 (0.00 ≤ x ≤ 0.52 ). Collective magnetic excitations resolved by RIXS are gradually hardened, whereas XES reveals a strong suppression of the local magnetic moment upon doping. This relationship is captured by an intermediate coupling theory, explicitly accounting for the partially localized and itinerant nature of the electrons in Fe pnictides. Finally, our work identifies a local-itinerant spin fluctuations channel through which the local moments transfer spin excitations to the particle-hole (paramagnons) continuum across the superconducting dome.

DOI： 10.1038/s42005-019-0236-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Photoexcitation is a very powerful way to instantaneously drive a material into a novel quantum state without any fabrication, and variable ultrafast techniques have been developed to observe how electron, lattice, and spin degrees of freedom change. One of the most spectacular phenomena is photoinduced superconductivity, and it has been suggested in cuprates that the transition temperature Tc can be enhanced from the original Tc with significant lattice modulations. Here, we show a possibility for another photoinduced high-Tc superconducting state in the iron-based superconductor FeSe. The transient electronic state over the entire Brillouin zone is directly observed by time- and angle-resolved photoemission spectroscopy using extreme ultraviolet pulses obtained from high harmonic generation. Our results of dynamical behaviors from 50 fs to 800 ps consistently support the favourable superconducting state after photoexcitation well above Tc. This finding demonstrates that multiband iron-based superconductors emerge as an alternative candidate for photoinduced superconductors.

DOI： 10.1038/s42005-019-0219-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41567-019-0552-2

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DOI： 10.1103/PhysRevB.100.060503

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DOI： 10.1038/s41467-019-11200-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41467-019-09869-5

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The 12%-S doped FeSe system has a high T of 30 K at a pressure of 3.0 GPa. We have successfully investigated its microscopic properties for the first time via Se-NMR measurements under pressure. The antiferromagnetic (AFM) fluctuations at the optimal pressure (~3 GPa) exhibited unexpected suppression compared with the AFM fluctuations at ambient pressure, even though the optimal pressure is close to the phase boundary of the AFM phase induced at the high-pressure region. In addition, we revealed that the SC phase at an applied field of 6.02 T exhibited a remarkable double-dome structure in the pressure–temperature phase diagram, unlike the SC phase at zero field. c 77

DOI： 10.7566/JPSJ.88.033703

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DOI： 10.1038/s41535-018-0141-0

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Recently, FeSe S systems have received much attention because of the unique pressure-temperature phase diagram. We performed Se-NMR measurements on a single crystal of FeSe S to investigate its microscopic properties. The shift of Se spectra exhibits anomalous enhancement at 1.0 GPa, suggesting a topological change in the Fermi surfaces, the so-called Lifshitz transition, occurs at 1.0 GPa. The magnetic fluctuation simultaneously changes its properties, which implies a change in the dominant nesting vector. 1-x x 0.88 0.12 77 77

DOI： 10.4279/PIP.110003

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Other Link： http://orcid.org/0000-0002-3881-331X

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FeSe is argued as a superconductor in the Bardeen-Cooper-Schrieffer Bose-Einstein condensation crossover regime where the superconducting gap size and the superconducting transition temperature Tc are comparable to the Fermi energy. In this regime, vortex bound states should be well quantized and the preformed pairs above Tc may yield a pseudogap in the quasiparticle-excitation spectrum. We performed spectroscopic-imaging scanning tunneling microscopy to search for these features. We found Friedel-like oscillations near the vortex, which manifest the quantized levels, whereas the pseudogap was not detected. These apparently conflicting observations may be related to the multiband nature of FeSe.

DOI： 10.1103/PhysRevLett.122.077001

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DOI： 10.1038/s41586-019-0923-y

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Other Link： http://orcid.org/0000-0002-6007-9617

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.2.0_1782

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_1952

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Publishing Group  

The structure of the superconducting gap in unconventional superconductors holds a key to understand the momentum-dependent pairing interactions. In superconducting FeSe, there have been controversial results reporting nodal and nodeless gap structures, raising a fundamental issue of pairing mechanisms of iron-based superconductivity. Here, by utilizing polarization-dependent laser-excited angle-resolved photoemission spectroscopy, we report a detailed momentum dependence of the gap in single-and multi-domain regions of orthorhombic FeSe crystals. We confirm that the superconducting gap has a twofold in-plane anisotropy, associated with the nematicity due to orbital ordering. In twinned regions, we clearly find finite gap minima near the vertices of the major axis of the elliptical zone-centered Fermi surface, indicating a nodeless state. In contrast, the single-domain gap drops steeply to zero in a narrow angle range, evidencing for nascent nodes. Such unusual node lifting in multi-domain regions can be explained by the nematicity-induced time-reversal symmetry breaking near the twin boundaries.

DOI： 10.1038/s41467-017-02739-y

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© 2018 The Authors. In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator-ytterbium dodecaboride (YbB12). The resistivity of YbB12, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass.

DOI： 10.1126/science.aap9607

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FeSe S has attracted much attention among iron-based superconductors because the pure sample undergoes nematic and superconducting (SC) phase transitions without magnetism. A pressure-induced antiferromagnetic (AFM) phase emerges upon applying pressure. In the pressure (P)-temperature (T) phase diagram for the 12%-S doped sample, the AFM phase is separated from the nematic phase at around 3.0 GPa, and SC transition temperature (T ) takes a maximum (∼30 K). We measured T of Se for the 12%-S doped FeSe at 3.0 GPa. We found from 1/T T that low-energy AFM fluctuations are not so much enhanced under pressure compared with those at ambient pressure. The result suggests changes of topology and nesting of Fermi surfaces during pressurizing process. 1-x x c 1 1 77

DOI： 10.1063/1.5042570

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DOI： 10.1038/s41586-018-0274-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

Unconventional superconductivity often competes or coexists with other electronic orders. In iron-based superconductors, a central issue has been the relationship between superconductivity and electronic nematicity, spontaneous breaking of the lattice rotational symmetry. Using spectroscopic-imaging scanning tunneling microscopy, we simultaneously investigated the electronic structure and the superconducting gap in FeSe1-xSx, where the nematicity diminishes above the nematic end point (NEP) at x = 0.17. The nematic band structure appears as anisotropic quasiparticleinterference patterns that gradually become isotropic with increasing x without anomalies at the NEP. By contrast, the superconducting gap, which is intact in the nematic phase, discontinuously shrinks above the NEP. This implies that the presence or absence of nematicity results in two distinct pairing states, whereas the pairing interaction is insensitive to the strength of nematicity.

DOI： 10.1126/sciadv.aar6419

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL ACAD SCIENCES  

The emergence of the nematic electronic state that breaks rotational symmetry is one of the most fascinating properties of the iron-based superconductors, and has relevance to cuprates as well. FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering, providing a significant opportunity to investigate the role of nematicity in the superconducting pairing interaction. Here, to reveal how the superconducting gap evolves with nematicity, we measure the thermal conductivity and specific heat of FeSe1-xSx, in which the nematicity is suppressed by isoelectronic sulfur substitution and a nematic critical point (NCP) appears at x(c) approximate to 0.17. We find that, in the whole nematic regime (0 <= x <= 0.17), the field dependence of two quantities consistently shows two-gap behavior; one gap is small but highly anisotropic with deep minima or line nodes, and the other is larger and more isotropic. In stark contrast, in the tetragonal regime (x = 0.20), the larger gap becomes strongly anisotropic, demonstrating an abrupt change in the superconducting gap structure at the NCP. Near the NCP, charge fluctuations of d(xz) and d(yz) orbitals are enhanced equally in the tetragonal side, whereas they develop differently in the orthorhombic side. Our observation therefore directly implies that the orbital-dependent nature of the nematic fluctuations has a strong impact on the superconducting gap structure and hence on the pairing interaction.

DOI： 10.1073/pnas.1717331115

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DOI： 10.11316/jpsgaiyo.73.2.0_1390

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Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the in situ scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In1-xZnx)5 films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the c-f hybridization gap, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn5. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn5, in both of which d-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

DOI： 10.7566/JPSJ.87.034702

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A long-standing controversial issue in the quest to understand the superconductivity in cuprates is the nature of the enigmatic pseudogap region of the phase diagram(1). Especially important is whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature T*. Here we report torque-magnetometry measurements of anisotropic susceptibility within the ab planes in orthorhombic YBa2Cu3Oy with exceptionally high precision. The in-plane anisotropy displays a significant increase with a distinct kink at the pseudogap onset temperature T*, showing a remarkable scaling behaviour with respect to T/T* in a wide doping range. Our systematic analysis reveals that the rotational symmetry breaking sets in at T* in the limit where the effect of orthorhombicity is eliminated. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which differs from the recently reported charge-density-wave transition that accompanies translational symmetry breaking(2-10).

DOI： 10.1038/nphys4205

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A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T-c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe1-xSx. By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T-c shows a striking enhancement. The completed phase diagram uncovers that high-T-c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T-c remains low near the nematic critical point.

DOI： 10.1038/s41467-017-01277-x

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In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe2(As1-xPx) 2 changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature T-c is depressed at high concentrations (x greater than or similar to 0.28), it shows an initial increase at lower x. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-T-c family.

DOI： 10.7566/JPSJ.86.083706

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The superconducting transition of FeSe1-xSx with three distinct sulfur concentrations x was studied under hydrostatic pressures up to similar to 70 kbar via bulk ac susceptibility. The pressure dependence of the superconducting transition temperature (T-c) features a small dome-shaped variation at low pressures for x = 0.04 and x = 0.12, followed by a more substantial T-c enhancement to a value of around 30 K at moderate pressures. In x = 0.21, a similar overall pressure dependence of T-c is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum T-c of 30 K is reached near the verge of the pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-T-c superconducting states at high pressure in this system.

DOI： 10.1103/PhysRevB.96.020502

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

There is growing evidence that the superconducting semimetal FeSe (T-c similar to 8K) is in the crossover regime between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits. We report on longitudinal and transverse thermal conductivities,kappa(xx) and kappa(xy), respectively, in magnetic fields up to 20 T. The field dependences of kappa(xx) and kappa(xy) imply that a highly anisotropic small superconducting gap forms at the electron Fermisurface pocket whereas a more isotropic and larger gap forms at the hole pocket. Below similar to 1.0 K, both kappa(xx) and kappa(xy) exhibit distinct anomalies (kinks) at the upper critical field H-c2 and at a field H* slightly below H-c2. The analysis of the thermal Hall angle (kappa(xy)/kappa(xx)) indicates a change of the quasiparticle scattering rate at H*. These results provide strong support to the previous suggestion that above H* a distinct field-induced superconducting phase emerges with an unprecedented large spin imbalance.

DOI： 10.7566/JPSJ.86.014707

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The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein condensate (BEC) limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature T-c = 8.5 K has been found to be deep inside the BCS-BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large non-linear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T-star similar to 20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance and transport data give evidence of pseudogap formation at similar to T-star. The multiband superconductivity along with electron-hole compensation in FeSe may highlight a novel aspect of the BCS-BEC crossover physics.

DOI： 10.1038/ncomms12843

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We report magnetic force microscopy (MFM) measurements on underdoped BaFe2(As1-xPx)(2) (x = 0.26) that showenhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel superconducting vortices and act as barriers for them to cross. The width of the stripes is hundreds of nanometers, on the scale of the penetration depth, well within the inherent spatial resolution of MFM and implying that the width is set by the interaction of the superconductor with the MFM's magnetic tip. Unlike similar stripes observed previously by scanning SQUID in the electron doped Ba(Fe1-xCox)(2)As-2, the stripes in the isovalently doped BaFe2(As1-xPx)(2) disappear gradually when we warm the sample towards the superconducting transition temperature. Moreover, we find that the stripes move well below the reported structural transition temperature in BaFe2(As1-xPx)(2) and that they can be much denser than in the Ba(Fe1-xCox)(2)As-2 study. When we cool in finite magnetic field we find that some vortices appear in the middle of stripes, suggesting that the stripes may have an inner structure, which we cannot resolve. Finally, we use both vortex decoration at higher magnetic field and deliberate vortex dragging by the MFM magnetic tip to obtain bounds on the strength of the interaction between the stripes and vortices. We find that this interaction is strong enough to play a significant role in determining the critical current in underdoped BaFe2(As1-xPx)(2).

DOI： 10.1103/PhysRevB.94.064510

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The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is one of the most intriguing properties of the iron-based superconductors (Fe SC), and has relevance for the cuprates as well. Establishing the critical electronic modes behind nematicity remains a challenge, however, because their associated susceptibilities are not easily accessible by conventional probes. Here, using FeSe as a model system, and symmetry-resolved electronic Raman scattering as a probe, we unravel the presence of critical charge nematic fluctuations near the structural/ nematic transition temperature, T-S similar to 90 K. The diverging behavior of the associated nematic susceptibility foretells the presence of a Pomeranchuk instability of the Fermi surface with d-wave symmetry. The excellent scaling between the observed nematic susceptibility and elastic modulus data demonstrates that the structural distortion is driven by this d-wave Pomeranchuk transition. Our results make a strong case for charge-induced nematicity in FeSe.

DOI： 10.1073/pnas.1606562113

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In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near x approximate to 0.17, the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

DOI： 10.1073/pnas.1605806113

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The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (T-c) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to similar to 15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above similar to 6GPa the sudden enhancement of superconductivity (T-c &lt;= 38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-T-c phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-T-c cuprates.

DOI： 10.1038/ncomms12146

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© 2016 American Physical Society.The resistivity ρ and Hall resistivity ρH are measured on FeSe at pressures up to P=28.3 kbar in magnetic fields up to B=14.5 T. The ρ(B) and ρH(B) curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of P and temperature (T≤110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order.

DOI： 10.1103/PhysRevB.93.180503

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The resistivity rho and Hall resistivity rho(H) are measured on FeSe at pressures up to P = 28.3 kbar in magnetic fields up to B = 14.5 T. The rho(B) and rho(H) (B) curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of P and temperature (T &lt;= 110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order.

DOI： 10.1103/PhysRevB.93.180503

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© 2016 American Physical Society.We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P=16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P∼8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dTc/dP observed in a range between P∼8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: kBTc/EF∼0.1 even at P=16.1 kbar.

DOI： 10.1103/PhysRevB.93.094505

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We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to P = 16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at P similar to 8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative dT(c)/dP observed in a range between P similar to 8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: k(B)T(c)/E-F similar to 0.1 even at P = 16.1 kbar.

DOI： 10.1103/PhysRevB.93.094505

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DOI： 10.1103/PhysRevB.93.060504

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We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high<br />
pressure up to $P$ = 16.1 kbar. We find a sudden change in SdH oscillations at<br />
the onset of the pressure-induced antiferromagnetism at $P$ $\sim$ 8 kbar. We<br />
argue that this change can be attributed to a reconstruction of the Fermi<br />
surface by the antiferromagnetic order. The negative d$T_c$/d$P$ observed in a<br />
range between $P$ $\sim$ 8 and 12 kbar may be explained by the reduction in the<br />
density of states due to the reconstruction. The ratio of the transition<br />
temperature to the effective Fermi energy remains high under high pressure:<br />
$k_BT_c/E_F$ $\sim$ 0.1 even at $P$ = 16.1 kbar.

DOI： 10.1103/PhysRevB.93.094505

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Mechanism of unconventional superconductivity in FeSe has been intensely<br />
scrutinized recently because of a variety of exotic properties unprecedented<br />
for other iron-based superconductors. A central unanswered question concerns<br />
the origin of the interaction that causes the nematic transition at $T_s=90\,K$<br />
without accompanying magnetic order. Elucidating the nature of spin excitations<br />
in the normal state is a key to addressing this issue. Here we report, from<br />
inelastic neutron-scattering measurements in FeSe single crystals, that<br />
high-energy spin excitation spectra of FeSe exhibit characteristic energy<br />
dependence with missing intensity at around 70-80$\,$meV, which are very<br />
different from other iron-based superconductors. Despite of the strongest<br />
electron correlations among the iron-based superconductor family, the spectra<br />
are qualitatively at variance with the local moment model and can be<br />
essentially described by the itinerant electron picture. Moreover, the<br />
dynamical spin susceptibility above $T_s$ is only weakly temperature dependent,<br />
which is in stark contrast to the Curie-Weiss behavior of the electronic<br />
nematic susceptibility, suggesting that the nematic transition is not driven by<br />
spin but by orbital degrees of freedom.

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DOI： 10.1103/PhysRevLett.115.219902

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We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the Gamma and M points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in d(xz)/d(yz) orbitals. The d(xz) band shifts upward at the Gamma point, while it moves downward at the M point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the Gamma and M points are rotated by 90 degrees with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.

DOI： 10.1103/PhysRevB.92.205117

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We report a comprehensive study of the effect of H+ irradiation on the critical current density J(c) and vortex pinning in an FeSe single crystal. The value of J(c) for FeSe is enhanced by more than a factor of 2 after 3-MeV H+ irradiation, which is explained by the introduction of point pinning centers. Vortex creep rates are found to be strongly suppressed after irradiation. Detailed analyses of the pinning energy based on collectivecreep- theory and an extended Maley's method show that the H+ irradiation enhances the value of J(c) before the flux creep and also reduces the size of the flux bundle, which suppresses the field dependence of J(c) owing to vortex motion. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.113102

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To elucidate the origin of nematic order in Fe-based superconductors, we<br />
report a Raman scattering study of lattice dynamics, which quantify the extent<br />
of $C_4$-symmetry breaking, in BaFe$_2$As$_2$ and FeSe. FeSe possesses a<br />
nematic ordering temperature $T_\mathrm{s}$ and orbital-related band-energy<br />
split below $T_\mathrm{s}$ that are similar to those in BaFe$_2$As$_2$, but<br />
unlike BaFe$_2$As$_2$ it has no long-range magnetic order. We find that the<br />
$E_g$ phonon-energy split in FeSe sets in only well below $T_\mathrm{s}$, and<br />
its saturated value is substantially smaller than that in BaFe$_2$As$_2$.<br />
Together with reported results for the Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ family,<br />
the data suggest that magnetism exerts a major influence on the lattice.

DOI： 10.1103/PhysRevB.93.060504

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We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal which is free from doping-introduced inhomogeneities and charged quasiparticle scattering because of its innate superconductivity. The critical current density J(c) is found to be almost isotropic and reaches a value of similar to 3 x 10(4) A/cm(2) at 2 K (self-field) for both H parallel to c and ab. The normalized magnetic relaxation rate S (=vertical bar d lnM/d lnt vertical bar) shows a temperature-insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate (S similar to 0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fishtail effect is absent for both H parallel to c and ab. Based on this observation, the origin of the fishtail effect is also discussed. Combining the results of J(c) and S, the vortex motion in the FeSe single crystal is found to be dominated by sparse, strong pointlike pinning from nanometer-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed.

DOI： 10.1103/PhysRevB.92.144509

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Magnetoresistivity rho(xx) and Hall resistivity rho(xy) in ultrahigh magnetic fields up to 88 Tare measured down to 0.15 K to clarify the multiband electronic structure in high-quality single crystals of superconducting FeSe. At low temperatures and high fields we observe quantum oscillations in both resistivity and the Hall effect, confirming the multiband Fermi surface with small volumes. We propose a novel approach to identify from magnetotransport measurements the sign of the charge carriers corresponding to a particular cyclotron orbit in a compensated metal. The observed significant differences in the relative amplitudes of the quantum oscillations between the rho(xx) and rho(xy) components, together with the positive sign of the high-field rho(xy), reveal that the largest pocket should correspond to the hole band. The low-field magnetotransport data in the normal state suggest that, in addition to one hole and one almost compensated electron band, the orthorhombic phase of FeSe exhibits an additional tiny electron pocket with a high mobility.

DOI： 10.1103/PhysRevLett.115.027006

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We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above similar to 18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at similar to 12 kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in mu SR measurements [M. Bendele et al., Phys. Rev. Lett. 104, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to similar to 25 kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists.

DOI： 10.7566/JPSJ.84.063701

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We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.

DOI： 10.1103/PhysRevB.91.174509

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We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.

DOI： 10.1103/PhysRevB.91.174509

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We report on As-75-NMR measurements in single crystalline BaFe2(As0.96P0.04)(2) for magnetic fields parallel to the orthorhombic [110](o) and [100](o) directions above the structural transition temperature T-S similar or equal to 121 K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-4p(x) and 4p(y) orbitals, which reaches vertical bar n(x) - n(y)vertical bar/vertical bar n(x) + n(y)vertical bar similar to 15% at T -&gt; T-S and is a natural consequence of the orbital ordering of Fe-3d(xz) and d(yz) electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below similar to 160 K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state.

DOI： 10.7566/JPSJ.84.043705

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We report the observation of the de Haas-van Alphen effect in IrTe2 measured using torque magnetometry at lowtemperatures down to 0.4K and in high magnetic fields up to 33 T. IrTe2 undergoes a major structural transition around similar to 283(1) K due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two-dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than 1m(e)), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas-van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling IrTe2.

DOI： 10.1103/PhysRevB.91.121105

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We use magnetic force microscopy (MFM) to characterize superconductivity across the superconducting dome in BaFe2(As1-xPx)(2), a pnictide with a peak in the penetration depth (lambda(ab)) at optimal doping (x(opt)), as shown in sample-wide measurements. Our local measurements show a peak at x(opt) and a T-C vs lambda(-2)(ab) dependence similar on both sides of x(opt). Near the underdoped edge of the dome lambda(ab) increases sharply, suggesting that superconductivity competes with another phase. Indeed, MFM vortex imaging shows correlated defects parallel to twin boundaries only in underdoped samples and not for x &gt;= x(opt).

DOI： 10.1103/PhysRevB.91.060504

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We report a combination of Fe K beta x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2(As1-xPx)(2). The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1-xPx)(2). We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2(As1-xPx)(2) (x = 0.055) and Ca0.78La0.22Fe2As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

DOI： 10.1103/PhysRevLett.114.047001

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DOI： 10.11316/jpsgaiyo.70.1.0_1938

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Junctions and interfaces consisting of unconventional superconductors provide<br />
an excellent experimental playground to study exotic phenomena related to the<br />
phase of the order parameter. Not only the complex structure of unconventional<br />
order parameters have an impact on the Josephson effects, but also may<br />
profoundly alter the quasi-particle excitation spectrum near a junction. Here,<br />
by using spectroscopic-imaging scanning tunneling microscopy, we visualize the<br />
spatial evolution of the local density of states (LDOS) near twin boundaries<br />
(TBs) of the nodal superconductor FeSe. The $\pi/2$ rotation of the<br />
crystallographic orientation across the TB twists the structure of the<br />
unconventional order parameter, which may, in principle, bring about a<br />
zero-energy LDOS peak at the TB. The LDOS at the TB observed in our study, in<br />
contrast, does not exhibit any signature of a zero-energy peak and an apparent<br />
gap amplitude remains finite all the way across the TB. The low-energy<br />
quasiparticle excitations associated with the gap nodes are affected by the TB<br />
over a distance more than an order of magnitude larger than the coherence<br />
length $\xi_{ab}$. The modification of the low-energy states is even more<br />
prominent in the region between two neighboring TBs separated by a distance<br />
$\approx7\xi_{ab}$. In this region the spectral weight near the Fermi level<br />
($\approx\pm$0.2~meV) due to the nodal quasiparticle spectrum is almost<br />
completely removed. These behaviors suggest that the TB induces a fully-gapped<br />
state, invoking a possible twist of the order parameter structure which breaks<br />
time-reversal symmetry.

DOI： 10.1103/PhysRevX.5.031022

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Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe2(As1-xPx)(2) is perhaps the clearest example to date of a high-temperature quantum critical superconductor, and so it is a particularly suitable system to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields do not follow the behaviour, predicted by conventional theory, resulting from the observed mass enhancement near the QCP. Our results imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realized in quantum critical superconductors.

DOI： 10.1038/ncomms6679

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We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As1-xPx)(2) (x = 0.30, T-c = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s(+/-) gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.

DOI： 10.1038/srep07292

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In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, such as in d-wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of ironpnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe2(As1 - P-x(x))(2) crystals via electron irradiation, we find from the low-temperature penetration depth measurements that the nodal state changes to a nodeless state with fully gapped excitations. Moreover, under further irradiation the gapped state evolves into another gapless state, providing bulk evidence of unconventional sign-changing s-wave superconductivity. This demonstrates that the topology of the superconducting gap can be controlled by disorder, which is a strikingly unique feature of iron pnictides.

DOI： 10.1038/ncomms6657

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We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/Fe, an order of magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering.

DOI： 10.1103/PhysRevB.90.144517

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We present a study of the superconducting gap structure in the iron-pnictide<br />
series BaFe2(As1-xPx)2. By measuring the variation of the specific heat as a<br />
function of temperature and magnetic field we are able to determine the number<br />
and Fermi surface location of the nodes in the superconducting gap. In<br />
particular, from measurements of the variation of the specific heat as the<br />
magnetic field is rotated in the ab plane of the sample we conclude that the<br />
nodes are in the [110] directions. Then from a quantitative analysis of the<br />
temperature and field dependence of the specific heat we further conclude that<br />
nodes exists on all Fermi surface sheets.

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We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors, YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm(-1) followed by a featureless continuum between 500 and 1500 cm(-1) in the conductivity data and a significant suppression in the scattering rate below 700-900 cm(-1). The latter result allows us to identify the energy scale associated with the pseudogap Delta PG. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100-200 cm(-1), which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model, which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below Delta PG is solely due to the pseudogap formation, whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.

DOI： 10.1103/PhysRevB.90.014503

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Since the 1985 discovery of the phase transition at T-HO = 17.5 K in the heavy-fermion metal URu2Si2, neither symmetry change in the crystal structure nor large magnetic moment that can account for the entropy change has been observed, which makes this hidden order enigmatic. Recent high-field experiments have suggested electronic nematicity that breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state in the absence of magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at T-HO with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure samples, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.

DOI： 10.1038/ncomms5188

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity κ and penetration depth λ in the heavily hole-doped regime of Ba1-xKxFe2As2. The residual thermal conductivity (κ/T)T→0K and T dependence of λ consistently indicate the fully gapped superconductivity at x=0.76 and the (line) nodal superconductivity at higher hole concentrations. The magnitude of κTTc|T→0K and the slope of λ(T) at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s-wave states with and without sign reversal between Γ-centered hole pockets. © 2014 American Physical Society.

DOI： 10.1103/PhysRevB.89.115112

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To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity. and penetration depth. in the heavily hole- doped regime of Ba1- x K x Fe2As2. The residual thermal conductivity (./ T) T. 0 K and T dependence of. consistently indicate the fully gapped superconductivity at x = 0.76 and the ( line) nodal superconductivity at higher hole concentrations. The magnitude of. T Tc| T. 0 K and the slope of.( T) at low temperatures, both of which are determined by the properties of the low- energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s- wave states with and without sign reversal between - centered hole pockets.

DOI： 10.1103/PhysRevB.89.115112

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The nature of the pseudogap (PG) in high transition temperature (high T-c) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high T-c superconductivity can be universally associated with the PG formation. Here we provide direct evidence of the existence of the PG phase via angle-resolved photoemission spectroscopy in another family of high T-c superconductor, iron pnictides. Our results reveal a composition-dependent PG formation in the multiband electronic structure of BaFe (2)(As1-xPx)(2). The PG develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x, and is destroyed for x similar to 0.6, thus showing a notable similarity with cuprates. In addition, the PG formation is accompanied by inequivalent energy shifts in the zx/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the fourfold rotational symmetry.

DOI： 10.1103/PhysRevB.89.045101

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arXiv

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Fermi systems in the crossover regime between weakly coupled<br />
Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate<br />
(BEC) limits are among the most fascinating objects to study the behavior of an<br />
assembly of strongly interacting particles. The physics of this crossover has<br />
been of considerable interest both in the fields of condensed matter and<br />
ultracold atoms. One of the most challenging issue in this regime is the effect<br />
of large spin imbalance on a Fermi system under magnetic fields. Although<br />
several exotic physical properties have been predicted theoretically, the<br />
experimental realization of such an unusual superconducting state has not been<br />
achieved so far. Here we show that pure single crystals of superconducting FeSe<br />
offer the possibility to enter the previously unexplored realm where the three<br />
energies, Fermi energy $\varepsilon_{\rm F}$, superconducting gap $\Delta$ and<br />
Zeeman energy, become comparable. Through the superfluid response, transport,<br />
thermoelectric response, and spectroscopic-imaging scanning tunneling<br />
microscopy, we demonstrate that $\varepsilon_{\rm F}$ of FeSe is extremely<br />
small, with the ratio $\Delta/\varepsilon_{\rm F}\sim1 (\sim0.3)$ in the<br />
electron (hole) band. Moreover, thermal-conductivity measurements give evidence<br />
of a distinct phase line below the upper critical field, where the Zeeman<br />
energy becomes comparable to $\varepsilon_{\rm F}$ and $\Delta$. The<br />
observation of this field-induced phase provides insights into previously<br />
poorly understood aspects of the highly spin-polarized Fermi liquid in the<br />
BCS-BEC crossover regime.

DOI： 10.1073/pnas.1413477111

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report a combined study of the specific heat and de Haas-van Alphen effect in the iron-pnictide superconductor BaFe2(As1-xPx)(2). Our data when combined with results for the magnetic penetration depth give compelling evidence for the existence of a quantum critical point close to x = 0.30 which affects the majority of the Fermi surface by enhancing the quasiparticle mass. The results show that the sharp peak in the inverse superfluid density seen in this system results from a strong increase in the quasiparticle mass at the quantum critical point.

DOI： 10.1103/PhysRevLett.110.257002

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The NMR results in iron pnictides BaFe2(As1-xPx)(2) and Ba(Fe1-xCox)(2)As-2 are analyzed based on the self-consistent renormalization (SCR) spin-fluctuation theory. The temperature dependence of the NMR relaxation rate T-1(-1) as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi-liquid to Fermi-liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)(2) and Ba(Fe1-xCox)(2)As-2 possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom.

DOI： 10.1103/PhysRevB.87.174507

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We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe2(As 1-xPx)2, a material that qualifies as an archetypical clean superconductor, containing only sparse strong pointlike pins. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe1-xCox) 2As2 single crystals. Analysis reveals that the pinning force and energy distributions depend on the P content x. However, they are always much narrower than in Ba(Fe1-xCox) 2As2, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching Tc in BaFe2(As1-xPx)2. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers L̄∼90 nm, increasing with increasing P content x. This evolution can be understood as being the consequence of the P dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chainlike vortex arrangements, observed at all levels of P substitution. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.87.094506

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Temperature-dependent interplane resistivity ρc(T) was measured for the iron-based superconductor BaFe2(As 1-xPx)2 over a broad isoelectron phosphorus substitution range from x=0 to x=0.60, from nonsuperconducting parent compound to heavily overdoped superconducting composition with Tc≈10K. The features due to structural and magnetic transitions are clearly resolved in ρc(T) of the underdoped crystals. A characteristic maximum in ρc(T), found in the parent BaFe2As2 at around 200 K, moves rapidly with phosphorus substitution to high temperatures. At the optimal doping, the interplane resistivity shows T-linear temperature dependence without any crossover anomalies, similar to the previously reported in-plane resistivity. This observation is in stark contrast with dissimilar temperature dependencies found at optimal doping in electron-doped Ba(Fe 1-xCox)2As2. Our finding suggests that despite similar values of the resistivity and its anisotropy, the temperature-dependent transport in the normal state is very different in electron and isoelectron-doped compounds. Similar temperature dependence of both in-plane and interplane resistivities, in which the dominant contributions are coming from different parts of the Fermi surface, suggests that scattering is the same on the whole Fermi surface. Since magnetic fluctuations are expected to be much stronger on the quasinested sheets, this observation may point to the importance of the interorbital scattering between different sheets. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.87.104506

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Temperature-dependent interplane resistivity rho(c)(T) was measured for the iron-based superconductor BaFe2(As1-xPx)(2) over a broad isoelectron phosphorus substitution range from x = 0 to x = 0.60, from nonsuperconducting parent compound to heavily overdoped superconducting composition with T-c approximate to 10 K. The features due to structural and magnetic transitions are clearly resolved in rho(c)(T) of the underdoped crystals. A characteristic maximum in rho(c)(T), found in the parent BaFe2As2 at around 200 K, moves rapidly with phosphorus substitution to high temperatures. At the optimal doping, the interplane resistivity shows T-linear temperature dependence without any crossover anomalies, similar to the previously reported in-plane resistivity. This observation is in stark contrast with dissimilar temperature dependencies found at optimal doping in electron-doped Ba(Fe1-xCox)(2)As-2. Our finding suggests that despite similar values of the resistivity and its anisotropy, the temperature-dependent transport in the normal state is very different in electron and isoelectron-doped compounds. Similar temperature dependence of both in-plane and interplane resistivities, in which the dominant contributions are coming from different parts of the Fermi surface, suggests that scattering is the same on the whole Fermi surface. Since magnetic fluctuations are expected to be much stronger on the quasinested sheets, this observation may point to the importance of the interorbital scattering between different sheets. DOI: 10.1103/PhysRevB.87.104506

DOI： 10.1103/PhysRevB.87.104506

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We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe2(As1-xPx)(2), a material that qualifies as an archetypical clean superconductor, containing only sparse strong pointlike pins [in the sense of C. J. van der Beek et al., Phys. Rev. B 66, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe1-xCox)(2)As-2 single crystals. Analysis reveals that the pinning force and energy distributions depend on the P content x. However, they are always much narrower than in Ba(Fe1-xCox)(2)As-2, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching T-c in BaFe2(As1-xPx)(2). Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers (L) over bar similar to 90 nm, increasing with increasing P content x. This evolution can be understood as being the consequence of the P dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chainlike vortex arrangements, observed at all levels of P substitution. DOI: 10.1103/PhysRevB.87.094506

DOI： 10.1103/PhysRevB.87.094506

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In the iron pnictide superconductors, two distinct unconventional mechanisms<br />
of superconductivity have been put forth: One is mediated by spin fluctuations<br />
leading to the s+- state with sign change of superconducting gap between the<br />
hole and electron bands, and the other is orbital fluctuations which favor the<br />
s++ state without sign reversal. Here we report direct observation of peculiar<br />
momentum-dependent anisotropy in the superconducting gap from angle-resolved<br />
photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large<br />
anisotropy found only in the electron Fermi surface (FS) and the nearly<br />
isotropic gap on the entire hole FSs are together consistent with modified s+-<br />
gap with nodal loops, which can be theoretically reproduced by considering both<br />
spin and orbital fluctuations whose competition generates the gap modulation.<br />
This indicates that these two fluctuations are nearly equally important to the<br />
high-Tc superconductivity in this system.

arXiv

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

High energy electron irradiation is used to controllably introduce atomic-scale point defects into single crystalline Ba(Fe1-xCox)2As2, Ba(Fe1-xNix)2As2, and BaFe2(As1-xPx)2. The appearance of the collective pinning contribution to the critical current density in BaFe2(As1-xPx)2, and the magnitude of its enhancement in Ba(Fe1-xCox)2As2, conform with the hypothesis of quasi-particle scattering by Fe vacancies created by the irradiation. Whereas the insignificant modification of the temperature dependence of the superfluid density in Ba(Fe1-xCox)2As2 and Ba(Fe1-xNix)2As2 points to important native disorder present before the irradiation, the critical temperatures of these materials undergo a suppression equivalent to that observed in the much cleaner BaFe2(As1-xPx)2. This lends credence to the hypothesis of line nodes of the order parameter (at finite kz) in the former two materials. © Published under licence by IOP Publishing Ltd.

DOI： 10.1088/1742-6596/449/1/012023

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arXiv

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

High-resolution thermal-expansion and specific-heat data of isovalently substituted single-crystalline BaFe2(As1-xPx)(2) (0 &lt;= x &lt;= 0.33, x = 1) are presented. We show that crystals can be detwinned in situ in the capacitance dilatometer, allowing a study of all three independent crystallographic directions. From the thermal-expansion data, we determine the phase diagram via a thermodynamic probe, study the coupling of the spin-density wave (SDW) and superconducting order parameters, and determine various pressure dependencies of the normal and superconducting states. Our results show that in the underdoped region, superconductivity and SDW order coexist and compete with each other. The resulting phase diagram, however, exhibits a smaller coexistence region of SDW and superconductivity with a steeper rise of T-c on the underdoped side than in, e. g., Ba(Fe1-xCox)(2)As-2. On the overdoped side, where there is no sign of SDW order, the lattice parameters respond to superconductivity in much the same way as to the SDW on the underdoped side, which demonstrates the intimate connection between both kinds of order. Using thermodynamic relations, the uniaxial pressure derivatives of the superconducting critical temperature and the electronic Sommerfeld coefficient are determined from our thermal-expansion data together with the specific-heat data. We find that uniaxial pressure is proportional to P substitution and that the electronic density of states has a maximum at optimal doping. Overall, the coupling of the SDW and superconducting order to the lattice parameters of BaFe2(As1-xPx)(2) is found to be qualitatively very similar to that of the well-studied, supposedly electron-doped Ba(Fe1-xCox)(2)As-2 system.

DOI： 10.1103/PhysRevB.86.094521

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We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

DOI： 10.1103/PhysRevLett.109.027006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice(1,2), has been observed in the iron pnictide(3-7) and copper oxide(8-11) high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature T-s. Although recent experiments(3-7) have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase(3,5) or in the tetragonal phase under uniaxial strain(4,6,7), both of which break the 90 degrees rotational C-4 symmetry. Therefore, the question remains open whether the nematicity can exist above T-s without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe2(As1-xPx)(2), showing that the nematicity develops well above T-s and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides(8,12). By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures-one at T*, signifying a true nematic transition, and the other at T-s (&lt;T*), which we show not to be a true phase transition, but rather what we refer to as a 'meta-nematic transition', in analogy to the well-known meta-magnetic transition in the theory of magnetism.

DOI： 10.1038/nature11178

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, lambda(L). In two-dimensional systems, this ratio n/m* (similar to 1/lambda(2)(L)) determines the effective Fermi temperature, T-F. We report a sharp peak in the x-dependence of lambda(L) at zero temperature in clean samples of BaFe2(As1-xPx)(2) at the optimum composition x = 0.30, where the superconducting transition temperature T-c reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T-c/T-F at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

DOI： 10.1126/science.1219821

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Static and dynamic magnetic properties of lightly P-substituted BaFe2(As1-xPx)(2) were systematically investigated by P-31 NMR. The averaged internal magnetic field at the P site in the zero-temperature limit evaluated from the broadening of NMR spectra in the antiferromagnetic (AFM) phase is gradually suppressed toward x similar to 0.35 with increasing x, which provides definitive evidence for the existence of an AFM quantum critical point (QCP) at x similar to 0.35. The location of the AFM QCP is consistent with the previous estimation from temperature dependence of spin dynamics in the normal state, and the superconducting transition temperature T-c takes the maximum around the QCP. Our experiments, revealing a signature of a QCP extending up to room temperature, establish BaFe2(As1-xPx)(2) as one of the most accessible systems for unraveling the nature of quantum criticality and the relationship between AFM quantum criticality and unconventional superconductivity.

DOI： 10.1103/PhysRevB.85.184505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Epitaxial thin films of titanium perovskite oxyhydride ATiO(3-x)H(x) (A = Ba, Sr, Ca) were prepared by CaH2 reduction of epitaxial ATiO(3) thin films deposited on a (LaAlO3)(0.3)(SrAl0.5Ta0.5O3)(0.7) substrate. Secondary ion mass spectroscopy detected a substantial amount and uniform distribution of hydride within the film. SrTiO3/LSAT thin film hydridized at 530 degrees C for 1 day had hydride concentration of 4.0 X 10(21) atoms/cm(3) (i.e., SrTiO2.75H0.25). The electric resistivity of all the ATiO(3-x)H(x) films exhibited metallic (positive) temperature dependence, as opposed to negative as in BaTiO3-xHx powder, revealing that ATiO(3-x)H(x) are intrinsically metallic, with high conductivity of 10(2)-10(4) S/cm. Treatment with D-2 gas results in hydride/deuteride exchange of the films; these films should be valuable in further studies on hydride diffusion kinetics. Combined with the materials' inherent high electronic conductivity, new mixed electron/hydride ion conductors may also be possible.

DOI： 10.1021/ja302465c

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We investigate the k(z) dispersion in the hole Fermi surfaces (FSs) and the superconducting (SC) gaps of BaFe2(As0.65P0.35)(2) by employing bulk-sensitive angle-resolved photoemission spectroscopy (ARPES) with low energy photons. We find that this material exhibits a comparable magnitude of SC gaps for three-hole FSs at k(z) similar to pi. The comparable magnitude of the SC gap is observed also at k(z) similar to 0. Since each FS sheet has a distinct orbital character, these observations are an indication of the orbital-independent SC gaps in the hole FSs. (C) 2011 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2011.12.032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The coexistence of magnetism and superconductivity in the isovalent-P-substituted BaFe2(As1-xPx)(2) has been investigated microscopically by P-31-NMR measurements. We found that superconducting (SC) transition occurs in a magnetic region with static ordered moments and that the moments are reduced below the SC transition temperature T-c in the samples near the phase boundary of magnetism and superconductivity. Our results indicate that magnetism and superconductivity coexist spatially but compete with each other on the same Fermi surfaces. The coexistence state is qualitatively different from that observed in other unconventional superconductors and gives a strict constraint on the theoretical model for superconductivity in BaFe2(As1-xPx)(2).

DOI： 10.1143/JPSJ.81.033701

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By using high-quality single crystals, we quantitatively compare the transport properties between LiFeP and LiFeAs superconductors with compensated electron and hole carriers. The low-temperature resistivity follows the Fermi-liquid AT2 dependence with a factor of ∼3 difference in the coefficient A. This highlights weaker electron correlations in LiFeP, which is consistent with its ∼70 times lower upper critical field than that of LiFeAs. Our analysis of the magnetotransport data indicates that in LiFeP, the electron carriers with lighter masses exhibit stronger temperature dependence of inelastic scattering rate than the holes, which is the opposite to the LiFeAs case. This stark difference in the band-dependent inelastic scattering may be relevant to the recently reported contrasting superconducting gap structures in these two superconductors. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.060503

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

By using high-quality single crystals, we quantitatively compare the transport properties between LiFeP and LiFeAs superconductors with compensated electron and hole carriers. The low-temperature resistivity follows the Fermi-liquid AT(2) dependence with a factor of similar to 3 difference in the coefficient A. This highlights weaker electron correlations in LiFeP, which is consistent with its similar to 70 times lower upper critical field than that of LiFeAs. Our analysis of the magnetotransport data indicates that in LiFeP, the electron carriers with lighter masses exhibit stronger temperature dependence of inelastic scattering rate than the holes, which is the opposite to the LiFeAs case. This stark difference in the band-dependent inelastic scattering may be relevant to the recently reported contrasting superconducting gap structures in these two superconductors.

DOI： 10.1103/PhysRevB.85.060503

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We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T-c approximate to 18 Kand LiFeP with T-c approximate to 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally showsignificant enhancement, up to similar to 3 for LiFeP and similar to 5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.

DOI： 10.1103/PhysRevLett.108.047002

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High-precision measurements of magnetic penetration depth lambda in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting behaviors. In LiFeAs the low-temperature lambda(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density alpha lambda(-2), indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of similar to 1.33 angstrom.

DOI： 10.1103/PhysRevLett.108.047003

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We present measurements of the specific heat coefficient gamma (equivalent to C/T) in the low-temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe2(As0.7P0.3)(2). We find both a linear regime at higher fields and a limiting square root H behavior at very low fields. The crossover from a Volovik-like root H to a linear field dependence can be understood from a multiband calculation in the quasiclassical approximation assuming gaps with different momentum dependence on the hole-and electronlike Fermi surface sheets.

DOI： 10.1103/PhysRevB.84.184524

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The structure of the superconducting order parameter in the iron-pnictide superconductor BaFe2(As0.67P0.33)(2) (T-c = 31 K) with line nodes is studied by the angle-resolved thermal conductivity measurements in a magnetic field rotated within the basal plane. We find that the thermal conductivity displays distinct fourfold oscillations with minima when the field is directed at +/- 45 degrees with respect to the tetragonal a axis. We discuss possible gap structures that can account for the data, and conclude that the observed results are most consistent with the closed nodal loops located at the flat parts of the electron Fermi surface with high Fermi velocity.

DOI： 10.1103/PhysRevB.84.060507

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We report the specific heat discontinuity, Delta C/T-c, at T-c = 28.2 K of a collage of single crystals of BaFe2(As0.7P0.3)(2) and compare the measured value of 38.5 mJ mol(-1) K-2 with other iron pnictide and iron chalcogenide (FePn/Ch) superconductors. This value agrees well with the trend established by Bud'ko, Ni and Canfield, who found that Delta C/T-c proportional to aT(c)(2) c for 14 examples of doped Ba1-xKxFe2As2 and BaFe2-xTMxAs2, where the transition metal TM = Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for which Delta C/T-c is currently known and find Delta C/T-c proportional to aT(c)(1.9) c and a = 0.083 mJ mol(-1) K-4. A comparison with the elemental superconductors with T-c &gt; 1 K and with A-15 superconductors shows that, contrary to the FePn/Ch superconductors, electron-phonon-coupled conventional superconductors exhibit a significantly different dependence of Delta C on T-c, namely Delta C/Tc proportional to T-c(0.9). However Delta C/gamma T-c appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, for example, Delta C/gamma T-c = 2.4 for BaFe2(As0.7P0.3)(2). A discussion of the possible implications of these phenomenological comparisons for the unconventional superconductivity believed to exist in the FePn/Ch is given.

DOI： 10.1088/0953-8984/23/22/222201

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We report detailed measurements of the de Haas-van Alphen ( dHvA) effect in BaFe2P2, which is the end member of the superconducting series, BaFe2(As1-xPx)(2). Using high-purity samples, we are able to observe dHvA oscillations from all the sheets of the Fermi surface and hence build up a detailed picture of its structure. The results show the existence of a highly warped section of hole surface, which may be the origin of the nodal superconducting gap structure in this series. Importantly, we find that even for this nonsuperconducting end member, one of the hole surfaces almost exactly nests with the inner electron surface. This suggests that improved geometric nesting does not drive the increase in pairing strength with decreasing x in this system.

DOI： 10.1103/PhysRevB.83.220504

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The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle-resolved photoemission spectroscopy on BaFe2(As0.65P0.35)(2) and Ba0.6K0.4Fe2As2 to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

DOI： 10.1126/science.1202150

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We report the preparation and physical properties of reduced anatase TiO2-delta thin films obtained via a low-temperature (low-T) reduction using CaH2. The oxygen amounts were controlled in a wider range than ever reported. Some highly reduced anatase films showed resistivities as low as 10(-3) Omega cm at room temperature, both in metallic and semiconducting states. The most conducting metallic sample has very high carrier concentration of 1.6 x 10(21) cm(-3), comparable with those of metal-doped anatase films. Moreover, the magnetoresistance of the films changed its sign twice as a function of delta. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.035801

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We have studied the three-dimensional shapes of the Fermi surfaces (FSs) of BaFe2(As1-xPx)(2) (x = 0.38), where superconductivity is induced by isovalent P substitution and by angle-resolved photoemission spectroscopy. Moderately strong electron mass enhancement has been identified for both the electron and hole FSs. Among two observed hole FSs, the nearly two-dimensional one shows good nesting with the outer two-dimensional electron FS, but its orbital character is different from the outer electron FS. The three-dimensional hole FS shows poor nesting with the electron FSs. The present results suggest that the three dimensionality and the difference in the orbital character weaken FS nesting while partial nesting among the outer electron FSs of d(xy) character and/or that within the three-dimensional hole FS becomes dominant, which may lead to the nodal superconductivity.

DOI： 10.1103/PhysRevLett.106.117001

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Single crystals of CaFe2(As1-xPx)(2) are found to exhibit the tetragonal (T) to collapsed-tetragonal (cT) transition at T-cT less than or similar to 100 K for x &gt; 0.05. The c-axis shrinks by similar to 9% below T-cT, which substantially diminishes the interband nesting between the hole and electron bands. In sharp contrast to the superconducting T phase of AFe(2)(As1-xPx)(2) (A = Ba, Sr), where the anomalous non-Fermi liquid transport properties are observed, the resistivity, Hall coefficient, and magnetoresistance data in the Ca-based system all indicate that the standard Fermi liquid behaviors are recovered abruptly below T-cT, and the superconductivity disappears completely. The simultaneous disappearance of the superconductivity and the non-Fermi liquid transport enlightens the essential role of interband-associated fluctuations to the superconductivity in Fe pnictides.

DOI： 10.1103/PhysRevB.83.060505

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Non-Fermi liquid like charge transport properties and quantum critical behaviors observed in high-quality single crystals of BaFe[2](As[1-x]Px)[2] are reviewed. Near the SDW end point, charge transport coefficients, including resistivity, Hall coefficient and magnetoresistance, exhibit striking deviations from the conventional Fermi liquid properties. De Haas-van Alphen effect measurements in a wide range of superconducting dome reveal that thevolume of the Fermi surfaces shrink and the effective masses are enhanced as x approaches the critical concentration with highest Tc. It is also shown that antiferromagnetic fluctuations deduced from [31]P-NMR measurements are notably enhanced near this optimal composition, where magnetically ordered temperature θ deduced from relaxation rate is found to vanish. It is likely that these trends originate from the many-body interaction which gives rise to high-temperature superconductivity in iron-pnictides.

DOI： 10.14989/153265

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Measurements of the superconducting transition temperature, T-c, under hydrostatic pressure via bulk AC susceptibility were carried out on several concentrations of phosphorous substitution in BaFe2-(As1-xRx)(2). The pressure dependence of unsubstituted BaFe2As2, phosphorous concentration dependence of BaFe2(As1-xPx)(2), as well as the pressure dependence of BaFe2(As1-xPx)(2) all point towards an identical maximum T-c of 31 K. This demonstrates that phosphorous substitution and physical pressure result in similar superconducting phase diagrams, and that phosphorous substitution does not induce substantial impurity scattering.

DOI： 10.1143/JPSJ.79.123706

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Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe(2)(As(1-x)P(x))(2) is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.

DOI： 10.1103/PhysRevLett.105.267002

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Single crystals of BaFe2(As1-xPx)(2) are successfully grown. Resistivity measurements show a systematic reduction of structural and antiferromagnetic spin-density-wave (SOW) transition temperatures and a subsequent emergence of superconductivity upon P-doping. A dome shaped superconducting phase is shown in a wide range of x, with the highest transition temperature T-c approximate to 31 K locating near the end of SDW phase. A pronounced T-linear resistivity in the normal state is found in the vicinity of the optimally doping, showing a clear distinction from the conventional Fermi-liquid picture. The evolution of the resistivity and the phase diagram resemble those found in the parent BaFe2As2 under pressures, indicating that the substitution of P for As serve as chemical pressures. (c) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.137

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Superconductivity at 8 K was reported in beta-FeSe with a tetragonal PbO structure but not in delta-FeSe with a hexagonal NiAs structure. The superconducting transition temperature was then increased up to 37 K (on-set) by high pressure on polycrystalline samples. We report a study of the characterization on single crystals of FeSe superconductors. FeSe single crystals were grown using two methods, simple cooling from stoichiometric melt and flux using NaCl/KCl. While a ball-shaped aggregation composed of thick FeSe crystals was grown from the stoichiometric melt, thin hexagonal-shaped crystals with a size up to 1 mm were grown using the NaCl/KCl flux. The two different crystals indicated superconductivity around at 8 K in spite of their hexagonal shape, suggesting a non-superconducting compound with the NiAs-type hexagonal structure. According to the analyses using an X-ray diffractometer and a scanning electron microscope, the crystals contain microcracks that could be caused by stress. We assume that the hexagonal-shaped crystals grown at high temperature may turn partly to tetragonal beta-FeSe at the beta-delta structure transition temperature. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.10.041

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P-31 NMR was performed on the normal state in the newly discovered iron pnictide superconductor BaFe2(As0.67P0.33)(2) with maximum T-c of 30 K. The Knight shift is temperature-independent while the nuclear spin-lattice relaxation rate divided by temperature (T-1 T)(-1) exhibits a more than twofold increase on cooling. This contrasting behavior clearly indicates the development of low-lying antiferrornagnetic fluctuations in BaFe2(As0.67P0.33)(2) as observed in (Ba1-xKx)Fe2As2. The observed low-lying antiferromagnetic fluctuations may be universal in optimally doped "122" superconductors, in contrast to "1111" superconductors. (c) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.01.047

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Spin dynamics evolution of BaFe2(As1-xPx)(2) was probed as a function of P concentration via P-31 NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, the magnetically ordered temperature theta deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point, suggesting that the AF fluctuations associated with the quantum-critical point play a central role in the high-T-c superconductivity.

DOI： 10.1103/PhysRevLett.105.107003

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Magnetic measurements on optimally doped single crystals of BaFe2(As(1-x)Px)(2) (x approximate to 0.35) with magnetic fields applied along different crystallographic axes were performed under pressure, enabling the pressure evolution of coherence lengths and the anisotropy factor to be followed. Despite a decrease in the superconducting critical temperature, our studies reveal that the superconducting properties become more anisotropic under pressure. With appropriate scaling, we directly compare these properties with the values obtained for BaFe2(As1-xPx)(2) as a function of phosphorus content.

DOI： 10.1103/PhysRevB.82.094502

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We report specific-heat measurements on well characterized single crystals of the Fe-based superconductor BaFe2(As0.7P0.3)(2), from the same batch on which previous penetration depth, NMR, and thermal-conductivity measurements have observed a high density of low-energy excitations, which have been interpreted in terms of order-parameter nodes. Within the resolution of our measurements, the low-temperature limiting C/T is found to be linear in field, i.e., we find no evidence for a Volovik effect associated with nodal quasiparticles in either the clean or dirty limit. We discuss possible reasons for this apparent contradiction, which may be an intrinsic feature of the multiple Fermi-surface sheets present in the iron pnictide superconductors.

DOI： 10.1103/PhysRevB.81.214507

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We report magnetic penetration depth and thermal-conductivity data for high-quality single crystals of BaFe2(As1-xPx)(2)(T-c=30 K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba, K)Fe2As2 which has similar T-c and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-T-c superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable T-c.

DOI： 10.1103/PhysRevB.81.220501

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The normal-state charge transport is studied systematically in high-quality single crystals of BaFe2(As1-xPx)(2) (0 &lt;= x &lt;= 0.71). By substituting isovalent P for As, the spin-density-wave (SDW) state is suppressed and the dome-shaped superconducting phase (T-c less than or similar to 31 K) appears. Near the SDW end point (x approximate to 0.3), we observe striking linear temperature (T) dependence of resistivity in a wide T range, and remarkable low-T enhancement of Hall-coefficient magnitude from the carrier number estimates. We also find that the magnetoresistance apparently violates the Kohler&apos;s rule and is well scaled by the Hall angle Theta(H) as Delta rho(xx)/rho(xx) alpha tan(2) Theta(H). These non-Fermi-liquid transport anomalies cannot be attributed to the simple multiband effects. These results capture universal features of correlated electron systems in the presence of strong antiferro-magnetic fluctuations.

DOI： 10.1103/PhysRevB.81.184519

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Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx)(2) as a function of isoelectric substitution (As/P) for 0.41 &lt; x &lt; 1 (T-c up to 25 K). We find that the volumes of electron and hole Fermi surfaces shrink linearly with decreasing x. This shrinking is accompanied by a strong increase in the quasiparticle effective mass as x is tuned toward the maximum Tc. These results are not explained by simple band structure calculations, and it is likely that these trends originate from the same many-body interactions which give rise to superconductivity.

DOI： 10.1103/PhysRevLett.104.057008

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DOI： 10.1103/PhysRevB.81.020503

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P-31 and As-75 NMR measurements were performed in superconducting BaFe2(As0.67P0.33)(2) with T-c = 30 K. The nuclear-spin-lattice relaxation rate T-1(-1) and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T-1(-1) in the superconducting (SC) state decreases without a coherence peak just below T-c, as observed in (Ba1-xKx)Fe2As2. In contrast to other iron arsenide superconductors, the T-1(-1) proportional to T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in BaFe2(As1-xPx)(2) despite a comparable T-c value, an analogous phase diagram, and similar Fermi surfaces to (Ba1-xKx)Fe2As2.

DOI： 10.1103/PhysRevB.81.020503

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FeAs-based single crystals of Ba1-xKxFe2As2 and Sr1-xKxFe2As2 were grown in a Mo capsule at the temperatures from 1200 degrees C down to 850 degrees C by a FeAs-self-flux method Most of their sizes were approximately 0.2-0 5 mm. A larger single crystal of Sr Fe2As2 about 7mm in diameter was obtained by the same procedure, as appeared in a round-shape Most of the time, potassium-doped ;crystals tends to be smaller than undoped ones. An example of T-c for a Ba1-xKxFexAs2 single crystal measured with a SQUID magnetometer, which indicates T-c at 38 K. T-c s are sometimes lower dependent on the potassium composition The substitution reaction between Srfe(2)As(2) and potassium was examined. An undoped crystal absorbed potassium at around 600 degrees C and showed superconductivity by the reaction.

DOI： 10.4028/www.scientific.net/MSF.638-642.1412

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DOI： 10.1039/c004454g

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The normal-state charge transport is studied systematically in high-quality<br />
single crystals of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ ($0 \leq x \leq 0.71$). By<br />
substituting isovalent P for As, the spin-density-wave (SDW) state is<br />
suppressed and the dome-shaped superconducting phase ($T_c \lesssim 31$ K)<br />
appears. Near the SDW end point ($x\approx0.3$), we observe striking linear<br />
temperature ($T$) dependence of resistivity in a wide $T$-range, and remarkable<br />
low-$T$ enhancement of Hall coefficient magnitude from the carrier number<br />
estimates. We also find that the magnetoresistance apparently violates the<br />
Kohler&#039;s rule and is well scaled by the Hall angle $\Theta_H$ as<br />
$\Delta\rho_{xx}/\rho_{xx} \propto \tan^2\Theta_H$. These non-Fermi liquid<br />
transport anomalies cannot be attributed to the simple multiband effects. These<br />
results capture universal features of correlated electron systems in the<br />
presence of strong antiferromagnetic fluctuations.

DOI： 10.1016/j.physc.2012.07.005

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We report high-sensitivity microwave measurements of the in-plane penetration depth lambda(ab) and quasiparticle scattering rate 1/tau in several single crystals of the hole-doped Fe-based superconductor Ba1-xKxFe2As2 (x approximate to 0.55). While a power-law temperature dependence of lambda(ab) with a power similar to 2 is found in crystals with large 1/tau, we observe an exponential temperature dependence of the superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different levels of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe arsenides.

DOI： 10.1103/PhysRevLett.102.207001

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A new superconductivity in ternary intermetallics of La3Ni4X4, where X = Si and Ge, has been found. The superconducting critical temperatures, TcS, of La3Ni4Si4 and La3Ni4Ge4 defined as the onset of diamagnetism are 1.0 and 0.70 K, respectively. These superconductors have U3Ni4Si4-type crystal structure, with a combination of BaAl4-(ThCr2Si2-) and AlB2-type alternating layers. In this work, we present studies on low temperature specific heat in these nickel compounds to shed light on the superconducting natures. The Sommerfeld coefficient, gamma(n) for both compounds showed relatively high values: gamma(n) = 37.4 mJ/mol K-2 and 36.9 mJ/mol K-2 are derived for La3Ni4Si4 and La3Ni4Ge4, respectively. The Debye temperature Theta(D) in La3Ni4Si4 is estimated as 321 K, while that in La3Ni4Ge4 is given as 284 K. As well as the electron-phonon coupling constant, the electronic density of states at the Fermi level is estimated. It turned out that these materials have high density of states, which is in common to the analogous of La3Pd4Ge4. A new superconductivity in the compounds with U3Ni4Si4-type structure is discussed based on the thermodynamic properties. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.267

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A systematic investigation on the thermodynamic properties of La-based ternary intermetallic superconductors crystallizing in a U(3)Ni(4)Si(4)-type structure is presented. The U(3)Ni(4)Si(4)-type structure consists of a characteristic intergrowth of periodic BaAl(4) (ThCr(2)Si(2))- and AlB(2)-type segments. Pristine low temperature specific heat data for recently discovered members La(3)Ni(4)Si(4) and La(3)Ni(4)Ge(4) with T(c)s of 1.0 and 0.7 K, respectively, are presented as well as La(3)Pd(4)Ge(4) with the highest T(c) of 2.5 K in the U(3)Ni(4)Si(4)-type group. Owing to the higher Tcs of U(3)Ni(4)Si(4)-type superconductors than the related ThCr(2)Si(2)-type compounds, comparisons are drawn in our investigations of the ternary intermetallics of LaPd(2)Ge(2), LaNi(2)Si(2), and LaNi(2)Ge(2) having a ThCr(2)Si(2)-type structure. Our investigations of the thermodynamic properties show that La(3)Ni(4)Si(4) and La(3)Ni(4)Ge(4) have higher values gamma(n), N(E(F)), and Theta(D) than La(3)Pd(4)Ge(4). The same trend was found in ThCr(2)Si(2)-type compounds of LaPd(2)Ge(2), LaNi(2)Si(2), and LaNi(2)Ge(2). It turns out that the difference in Tc between La(3)Pd(4)Ge(4), La(3)Ni(4)Si(4), and La(3)Ni(4)Ge(4), as well as the relatively higher Tc of the U(3)Ni(4)Si(4)-type superconductors than of the related ThCr(2)Si(2)-type compounds, are largely due to the strength of electron-phonon coupling.

DOI： 10.1088/0953-8984/20/38/385204

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The specific heat of Li-2(Pd1-xPtx)(3)B [x = 0, 0.5, 1] was measured. The heat jumps due to the superconducting transition were observed at 7.0, 3.9, and 2.5 K (T-c), respectively. These compounds have the same cubic structure (P4(3) 32) without mirror or inversion symmetry. Being noncentrosymmetric, it is interesting then to study the symmetry of superconducting gap function and Cooper pairs. The behavior of the electronic-specific heat C-es of Li-2(Pd1-xPtx)(3)B at low temperatures &lt;T-c/3 showed different features with varying x. The temperature dependence of C,, for one end member, Li2Pd3B, follows an exponential behavior as in conventional superconductors, while that of Li2Pd1.5Pt1.5B and Li2Pt3B tends to follow a quadratic term rather than an exponential one. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.249

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Low temperature specific heat measurements are performed in a novel ternary superconductor La3Pd4Ge4, which forms U3Ni4Si4-type crystal structure consisting of the combination of alternating units of AlB2- and BaAl4 (ThCr2Si2)-type layers. The Sommerfeld coefficient gamma(n) = 27.0mJ/molK(2) and the Debye temperature Theta(D) = 256K are derived from the normal state specific heat. The electron-phonon coupling constant and the electronic density of states at the Fermi level are also derived. Temperature dependence of the superconducting electronic specific heat is well described by the BCS exponential behavior with gap Delta(0) = 0.30meV. The results suggest La3Pd4Ge4 has fully opened isotropic gap in the superconducting state. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2007.10.347

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Superconductivity in the ternary intermetallics of La(3)Ni(4)X(4) (X = Si and Ge) with the combination of AlB(2) and BaAl(4) layers is reported. Both compounds are type II superconductors with critical temperatures (T(c)) of 1.0 and 0.70 K for X = Si and Ge, respectively. The lower and the upper critical fields (H(c1)(0) and H(c2)(0)) for X = Si are 107 Oe and 16.5 kOe, whereas those for X = Ge are 68 Oe and 2.6 kOe, respectively. The gradient -dH(c2)/dT of La(3)Ni(4)Si(4) is extraordinarily higher than those of the other three analogues of La(3)Ni(4)Ge(4) and La(3)Pd(4)X(4). The magnetic penetration depth (lambda(0)) of 202 nm and Ginzburg-Landau coherence length (xi(0)) of 14 nm are derived for X = Si, while they are 206 and 36 nm for X = Ge.

DOI： 10.1088/0953-8984/20/7/075202

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Double superconducting transition in PrOs4Sb12 has been studied carefully by performing the local magnetization measurements using micro-Hall probe and the magneto-optical imaging. Local magnetization curves for each element of the Hall probe reveal that there exists a significant difference in the transition temperature at each point; two phases which give different T-c's are stabilized in the crystal. Magneto-optical imaging at T &lt; T-c2 and T-c2 &lt; T &lt; T-c1 also reveals a non-uniform superconducting nature. The characteristics of superconducting double transition in PrOs4Sb12 are discussed based on the local field inductions at multi-points.

DOI： 10.1143/JPSJS.77SA.327

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Superconductivity in Li2Pd3B and Li2Pt3B was confirmed at 7.0 K and 2.6 K, respectively. It is clear that their superconductivity is induced by Li addition. Their specific heat was measured under magnetic field by a heat-pulse relaxation method. Those materials have the same cubic structure (P4(3)32) composed of distorted octahedrons, whose corners are occupied by Pd/Pt. This structure does not have mirror or inversion symmetry along any directions. Those compounds are very interesting in relation to the symmetry of superconductivity. Sommerfeld (gamma T) and Debye (beta T-3) terms of Li2Pd3B were given as gamma = 8.3 mJ mol(-1) K-2 and beta = 1.4 mJ mol(-1) K-1. The Debye temperature is theta(D) = 202 K. The value of Delta C/gamma T at T-c was calculated to be 2.1. In the same manner, those parameters were described for Li2Pt3B as gamma = 9.6 mJ mol(-1) K-2, beta = 0.85 mJ mol(-1) K-1, theta(D) = 240 K, and Delta C/gamma T-c = 0.75. The superconducting state at zero magnetic field and the normal state achieved after quenching the superconductivity over H-c2 were compared and analyzed to understand the feature of those materials. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.05.015

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Local magnetization measurements were performed by micro Hall-probe technique in the superconducting state of PrOs4Sb12, in which the time-reversal symmetry is broken. We have observed positive, i.e. ferromagnetic like, local magnetic induction just below the superconducting transition temperature, which changes into a usual Meissner signal as temperature goes down. The signal is dependent on the measuring points of the sample surface, and may be interpreted as a sign of the dilute magnetic moment arising from spontaneous current in the superconducting state. The characteristics of the superconductivity in PrOs4Sb12 including the presence of possible spontaneous magnetization is discussed based on the local magnetization measurements. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.05.004

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Double superconducting transition in PrOs4Sb12 has been studied carefully by local magnetization measurements using Hall probe array. Local magnetization curves for each element of the Hall probe reveal that there exists a significant difference in the transition temperature at each point; two phases which give different T-c are stabilized in the crystal. The characteristics of superconducting double transition in PrOs4Sb12 are discussed based on the local magnetization measurements at multi-points. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.134

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The temperature- and field-dependent specific heat C(T,H) measurement on the noncentrosymmetric superconductors Li2Pd3B, Li2Pt3B, and Li-2(Pd0.5Pt0.5)(3)B were carried out over a wide range of T and H. C(T,H) of the limit compounds is found to be distinctly different: within the superconducting phase, the electronic specific heat of the Pd-based compound follows C-e(S)(T &lt; T-c=6.95 K,0 T)=3 exp[-16.4/T] J/mol K while for the Pt-based compound C-e(S)(T &lt; T-c=2.56 K,0 T)=alpha T-2 mJ/mol K, where alpha=7.3 mJ/mol K-3. An increase in H tends to reduce weakly the exponential argument and the coefficient of the quadratic term. The above-mentioned findings are in agreement with the description that, due to the difference in the strength of the spin-orbit coupling, the pairing symmetry in the Pd- (Pt-) based compound is predominately a spin-singlet (spin-triplet) state. Within the very low-temperature range, the field-induced normal-state of the Pd-based limit manifests C-N(T,7T&gt;H-c2)=9.3T+1.1T(3) mJ/mol K while that of the Pt-based isomorph shows C-N(T,7T&gt;H-c2)=9.3T+0.92T(3) mJ/mol K. For the intermediate alloy Li-2(Pt0.5Pd0.5)(3)B, C(T,H) mimics that of the Pt-based compound; specifically, C-e(S)(T &lt; T-c=3.9 K,0 T)=alpha T-2 mJ/mol K where alpha=4.6(2) mJ/mol K-3. The ratio of the coefficients of the quadratic terms in Li-2(Pt0.5Pd0.5)(3)B and Li2Pt3B, alpha Pd-Pt0.5(0.5)/alpha(Pt), is equal to Z(Pt0.5)Pd(0.5)(2)/Z(Pt)(2), confirming that the line nodes are driven by the spin-orbit interaction. The finding that these nodes can survive even in a 50% Pd-diluted material gives extra support to the reported claim that the x-dependent evolution of T-c, H-c1, H-c2, and (partial derivative H-c2/partial derivative T)(Tc) is almost linear.

DOI： 10.1103/PhysRevB.76.104506

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Li2Pd3B and Li2Pt3B show superconducting transition at the temperatures, 7.4 K and 2.6 K, respectively. These materials have the same cubic structure (P4(3)32) composed of distorted octahedrons, whose corners are occupied by Pd/Pt. This structure does not have mirror or inversion symmetry along any directions. It is a very interesting feature of those materials in relation to the symmetry of superconductivity. The samples were prepared by a two-stage arc-melting method. Their superconductivity is induced by Li addition. Controlling the Li stoichiometry is a key to synthesize high quality samples in the preparation process of arc-melting because Li is volatile at high temperatures. The specific heat measurements were carried out by heat-pulse relaxation and semi-adiabatic methods. Pressure effect was measured with a SQUID magnetometer using a low temperature hydrostatic micropressure cell at pressures up to around 1 GPa. The superconducting transition temperatures of both Li2Pd3B and Li2Pt3B decreased linearly with pressure. The pressure dependence, dT(c)/dP, were 0.091 and 0.090 K/GPa, respectively, (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.162

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DOI： 10.1016/j.molimm.2006.10.026

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

Superconductivity in two Li-containing compounds of Li2Pd3B and Li2Pt3B was recently discovered. These materials, showing the superconducting transition at 7.2 K and 2.6 K, respectively, have the same cubic structure (P4(3)32) composed of distorted octahedrons without mirror or inversion symmetry along any directions. This is a very interesting feature of those materials in relation to the symmetry of superconductivity. Resistivity measurements in magnetic fields gave their upper critical fields, H-c2(0) = 45 kOe and 19 kOe, respectively. Their specific heat was measured using a heat-pulse relaxation method. The Sommerfeld coefficient (gamma) and Debye temperature (theta(D)) terms of Li2Pd3B were given as gamma=9.5 mJmol(-1)K(-2) and theta(D)=228 K. The value of Delta C/gamma T at T-c was calculated to be 1.7. In the same manner, those parameters were described for Li2Pt3B as gamma=9.6 mJmol(-1)K(-1), theta(D)=240 K, and Delta C/gamma T-c=0.75, respectively. Since Delta C/gamma T-c in the weak-coupling limit by the BCS theory is 1.43, the value of 1.7 for Li2Pd3B is slightly higher. The electronic specific heat of Li2Pd3B at a zero magnetic field follows the typical exponetial behavior discribed in the BCS theory, while that of Li2Pt3B shows quadratic behavior. This result suggests the line nodes exist in the superconducting gap of Li2Pt3B driven by the spin-orbit interaction.

DOI： 10.4028/www.scientific.net/MSF.561-565.2079

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Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

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We performed local magnetization measurements using micro-Hall probe in filled skutterudite PrOs4Sb12, which shows a superconducting state with broken time-reversal symmetry. It turns out that there appears a remarkable dip in the local magnetization hysteresis near zero field, which is characteristic of chiral superconductors, while temperature dependence of local magnetization shows no distinctive anomaly related to the spontaneous field. Possible situations of the spontaneous field is discussed from the results of local magnetization.

DOI： 10.1063/1.2354878

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We performed local magnetization measurements in the superconducting state of PrOS4Sb12 Using micro-Hall probe. For a magnetic field applied along one of the principal axes, a remarkable dip structure in the magnetization hysteresis loop was observed close to zero local magnetic induction. The anomaly is most pronounced when the magnetic field is applied only in a certain axis of the crystal, and is not observed along other two axes. We believe that the anomaly close to zero induction is universal to the spin-triplet superconductors with broken time-reversal symmetry. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.02.050

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In order to get some insight into the origin of the anomalous angular dependence of H-c2 in a layered intermetallic compound CaAlSi, electronic, superconducting, and structural properties are compared between CaAlSi and CaGaSi. The angular dependence of H-c2 in CaGaSi is well described by the anisotropic GL model. Parallel to this finding, the pronounced lattice modulation accompanying the superstructure along the e-axis in CaAlSi is absent in CaGaSi. A relatively large specific heat jump at the superconducting transition in CaAlSi compared with CaGaSi indicates the presence of strong electron-phonon coupling in CaAlSi, which may cause the superstructure and the anomalous angular dependence of H-c2. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.01.020

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Nature of vortices in a highly anisotropic superconductor Bi2Sr2CaCu2O8+y is investigated in the presence of largely tilted magnetic fields. Direct visualizations of vortex distributions were performed under large in-plane fields up to 4000Oe by magneto-optical imaging technique. Our visualization clearly revealed inhomogeneous evolutions of the vortex penetration forming one-dimensional stripes. Comparative studies on the local magnetization revealed the existence of novel multiple peak structures in hysteresis loops. These peaks appear only in the increasing branch of the hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. The origin of the novel anomalies is discussed on the basis of inhomogeneous vortex distribution. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.71.224505

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Nature of vortices in a highly anisotropic superconductor Bi2Sr2CaCu2O8+y is investigated in the presence of largely tilted magnetic fields. Direct visualizations of vortex distributions were performed under large in-plane fields up to 4000 Oe by magneto-optical imaging technique. Our visualization clearly revealed inhomogeneous evolutions of the vortex penetration forming one-dimensional stripes. Comparative studies on the local magnetization revealed the existence of novel multiple peak structures in hysteresis loops. These peaks appear only in the increasing branch of the hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. The origin of the novel anomalies is discussed on the basis of inhomogeneous vortex distribution.

DOI： 10.1103/PhysRevB.71.224505

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Local magnetization hysteresis is studied in a highly anisotropic superconductor Bi2Sr2CaCu2O8+y under tilted magnetic fields using a micro Hall probe. At moderately high temperatures, we found novel multiple peak structures in the local magnetization hysteresis curves. These peaks appear only in the increasing branch of hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. Possible origins of the novel peak structures are discussed on the basis of the evolution of the peaks with temperature. (C) 2004 Published by Elsevier B.V.

DOI： 10.1016/j.physc.2004.01.067

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Vortices in organic superconductor kappa-(BEDT-TTF)(2)X {X = Cu(NCS)(2), Cu[N(CN)(2)]Br} are interesting for their anisotropic character. To investigate their dynamical behavior, ac complex susceptibility and dc electrical resistance measurements were done in the presence of vortices introduced by dc magnetic fields. In the course of experiments, we found that the dc field dependence of complex susceptibility changes drastically for moderate ac magnetic fields, i.e. the shielding effect of the system (-chi') has a peak structure and the dissipation term (chi") shows a dip. In addition to the anomalies in the susceptibility, it turned out that the electrical resistance has a dip structure under the same de magnetic fields. These anomalies imply that the vortices are less mobile there. (C) 2003 Published by Elsevier Science B.V.

DOI： 10.1016/S0921-4534(02)02763-6

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We investigated dynamical properties of vortices in the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) through resistance measurements. In the course of experiments, we found time dependent resistance oscillations with periods of 10-100sec. It indicates that the vortex system undergoes the alternating change in the state between a highly mobile state and less mobile state. This oscillation was found to appear in a wide region of the magnetic field and in a narrow region of the electric current that drives vortices.

DOI： 10.1016/S0379-6779(02)01141-4

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Vortices penetrating κ-(BEDT-TTF)2Cu(NCS)2 are interesting for its strongly two dimensional character. We investigated the behavior of vortices introduced by a dc-magnetic field through the shielding effect of the ac-magnetic field. For moderately strong ac-fields, we found an anomalous peak in the dc-field dependence of the shielding effect. The temperature dependence of the peak position is expressed as Hp = H0(Tc/T - 1). It turned out that the peak appears at the dc-field where the effective pinning potential on vortices has its maximum. We have confirmed this by estimating the pinning potential from the relaxation of the shielding, effect, which is caused by thermal creep of vortices and depends logarithmically on time.

DOI： 10.1016/S0379-6779(00)01155-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Sequoia SA  

Dynamical Properties of vortices introduced into a crystal of κ-(BEDT-TTF)2Cu(SCN)2 by a dc magnetic field were investigated through the shielding effect against an rf (1.5MHz) magnetic field applied parallel to the 2D plane. We found that the behavior of vortices changes with the change of the rf-frequency. When the dc field is applied in a direction slightly tilted from the 2D plane, rf shielding becomes much stronger than in zero field, indicating that the rf field is blocked by the presence of the vortices.

DOI： 10.1016/S0379-6779(98)00471-8

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DOI： 10.1016/0921-4526(93)90524-A

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DOI： 10.11316/jpsgaiyo.73.2.0_1755

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DOI： 10.11316/jpsgaiyo.73.1.0_2340

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DOI： 10.11316/jpsgaiyo.73.2.0_1492

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DOI： 10.11316/jpsgaiyo.73.1.0_1672

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DOI： 10.11316/jpsgaiyo.73.1.0_1672

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DOI： 10.11316/jpsgaiyo.73.1.0_2282

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DOI： 10.11316/jpsgaiyo.72.2.0_1337

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