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

We report the synthesis of a new layered iridium oxyfluoride, Sr2Ir(O, F)(6-delta), using a topochemical fluorination method. The c axis is elongated compared to that in the mother compound Sr2IrO4, because the fluorine layer is inserted into the rock-salt layer while tetragonal symmetry is preserved. Resistivity measurements for the compound show nonmetallic behavior similar to that of Sr2IrO4. In magnetization measurements, suppression of antiferromagnetic order and paramagnetic behavior was observed after topochemical fluorination. Meanwhile, muon spin relaxation (mu SR) experiments suggest the development of antiferromagnetic spin correlation between Ir spins. Furthermore, fluorine content was estimated at 1.5 the chemical composition of the fluorinated product being Sr(2)IrO(x)F(1.5 )with x = 3.25, assuming Ir4+. Suppression of magnetic order in Sr2Ir(O, F)(6-delta) is attributed to antiferromagnetic instability between IrO2 layers by elongation of the c axis due to fluorine intercalation.

DOI： 10.1103/PhysRevMaterials.4.013403

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Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, T-c, as high as 6.6 K at ambient pressure, in order to complete the T-c-pressure (p) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T-c value decreased with an increase in pressure, but the onset superconducting transition temperature, T-c(onset), increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R - T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p-wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

DOI： 10.1088/1361-648X/ab4605

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DOI： 10.1088/1367-2630/ab4159

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_2307

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DOI： 10.11316/jpsgaiyo.74.1.0_1664

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We performed X-ray diffraction (XRD) and DC magnetic susceptibility measurements to elucidate the crystal structure and superconducting properties of KSr2Nb3O10. From the diffraction pattern indexing, it was found that KSr2Nb3O10 crystallizes with monoclinic symmetry, space group P2(1)/m(11). We succeeded in preparing high temperature (HT) and low temperature (LT) phases of KSr2Nb3O10 powder samples synthesized by a conventional solid state reaction and an ion-exchange reaction, respectively. Superconductivity was observed at 4 K by Li intercalation and it was found that the superconducting volume fraction of the LT phase (similar to 1.4%) is clearly larger than that of the HT phase (0.07%).

DOI： 10.1016/j.physb.2017.09.060

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

© 2018 American Physical Society. We report the crystal structure and physical properties of Sr2-xLaxIrO4 synthesized by mechanical alloying. The magnetic transition temperature TN and electrical resistivity decreased with increasing La doping, consistent with previous studies involving single-crystalline samples. We also identified the relationship between TN and tetragonal distortion (c/a) in this system. This result suggests that the magnetism of the Sr214 system is strongly correlated with its crystal structure. Zero-field muon spin rotation/relaxation studies revealed that short-range antiferromagnetic ordering is realized in Sr1.9La0.1IrO4; also, the spin-glass state is stabilized in the low-temperature region. The Ir moment estimated from the longitudinal field μSR results is 0.045μB, ten times smaller than that of Sr2IrO4 (∼0.4μB), indicating that electrons are introduced into the Ir atoms.

DOI： 10.1103/PhysRevB.97.064425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L-3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Neel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

DOI： 10.1039/c8cp03756f

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

The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr2IrO4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds.

DOI： 10.1103/PhysRevB.96.041106

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

© The Author(s) 2016. Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0; Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors.

DOI： 10.1038/srep33303

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

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2 Delta(s)), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2 Delta(s) in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2 Delta(s) to higher energies, peaking at values of 3 Delta(s) for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.

DOI： 10.1038/srep23424

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

Single crystals of Ba1-xKxFe2As2 with 0.15 <= x <= 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38K at x = 0.4 to 3.4K at x = 1.0, in good agreement with the results for polycrystalline samples.

DOI： 10.7566/JPSJ.85.034718

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2256

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2315

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_2161

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/jpscp.3.015039

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DOI： 10.7566/jpscp.3.017020

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The antiferromagnetic insulator La1.5Ca0.5CoO4 has been investigated by Co L-2,L-3-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements and Co L-2,L-3-edge resonant soft X-ray magnetic scattering (RXMS) measurement to determine the Co electronic structures associated with magnetic ordering. Co L-2,L-3-edge linear-dichroic XAS shows that Co2+ takes a high-spin (HS) state and Co3+ takes a low-spin (LS) state. Using Co L-2,L-3-edge RXMS, we directly determined that an antiferromagnetic order is formed with a HS state of Co2+ ions. Moreover, the spin and orbital angular momenta of the Co2+ HS state are quantitatively estimated to be 1.1 +/- 0.1 and 1.0 +/- 0.1, respectively, and to align parallel in the ab plane by utilizing the cluster model calculation. The large orbital angular momentum of the Co2+ HS state originates from the small D-4h-symmetry crystal field splitting of t(2g) levels, which is comparable with the spin-orbit coupling constant of the Co 3d orbital.

DOI： 10.7566/JPSJ.83.044705

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

SrFeO2 is an insulating antiferromagnet with a remarkably high transition temperature in spite of its quasi-two-dimensional crystal structure. The magnetic exchange coupling is, however, very sensitive to a local mode involving transverse displacements of O and Fe, resulting in zigzag patterns of distortion. The buckling driven by rising temperatures is enhanced just as the Fe magnetic moment is reduced, implying a strong spin-lattice coupling. It is suggested that the undulations lead to orbital disorder by distorting the three possible paths to exchange interactions.

DOI： 10.1103/PhysRevLett.112.097001

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In resonant soft X-ray scattering measurement, we need the detector which can tune its solid angle continuously in vacuum. We have constructed a 4-blade-slit detector system for high vacuum and set it in the quasi-3-axis diffractometer. With this system, we have studied two magnetic structures in antiferromagnetic insulator La1.5Ca0.5CoO4 by Co L-2,L-3-edge resonant soft X-ray scattering. We revealed that the underlying Co electronic structures for these magnetic structures are the same, since the observed energy spectra take the same lineshape.

DOI： 10.1088/1742-6596/425/20/202003

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Other Link： http://stacks.iop.org/1742-6596/425/i=20/a=202003?key=crossref.42680327c4609995bb92ff6de0e11841

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Lattice vibrations from 20 K to room temperature in lithium alanates LiAlH4 and Li3AlH6, which decompose to release hydrogen over 423 K, were investigated by neutron spectroscopy. For both alanates, the overall spectra already start to broaden at 100-150 K with increasing temperature. The spectral lines assigned to the librational (rotational) modes of the polyhedral Al hydrido complexes, [AlH4](-) and [AlH6](3-), exhibit not only broadening but also softening at around 250-300 K. These results suggest that the decomposition stems from low-temperature bulk lattice instability/anharmonicity, in particular, due to the weakening of binding between the complexes and their surroundings. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4712053]

DOI： 10.1063/1.4712053

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We report the crystal structure parameters of clathrate-type copper-oxide Cu6O8YCl, which belongs to he Cu6O8MX (M = cation, X = anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu6O8YCl was identified to be a cubic structure with the space group Fm (3) over barm. The Cl atom is located at the 4a, which is the center position of the Cu6O8 cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (U-iso) at room temperature which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu6O8 cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds.

DOI： 10.1088/1742-6596/391/1/012129

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Neutron scattering experiments were performed on single crystals of magnetic impurity doped cobalt oxides La1.5Ca0.5CoO 4 to characterize the charge and spin orders. We newly found contrasting impurity effects. Two types of magnetic peaks are observed at q=(0.5,0,L) with L=half-integer and integer in La1.5Ca 0.5CoO4, while magnetic peak at L=half-integer (integer) was only observed in Mn (Fe)-substituted sample. Although Mn and Fe impurities degrade charge and magnetic order, Cr impurity stabilizes the ordering at x=0.5. Based on the crystal structural analysis of Cr doped sample, we found that the excess oxygen and change of octahedron around Co3+ were realized in Cr doped sample.

DOI： 10.1088/1742-6596/340/1/012061

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：PHYSICAL SOC JAPAN  

A renewed Ge-crystal monochromator was installed in a neutron spectrometer at JRR-3. The beam size at the sample position is vertically about 1.5 times as large as before, while the neutron luminosity is unchanged. Wellcollimated, monochromatic beams with quite different incident energy (Ei) are available through a slight rotation in the monochromator-crystal angle by making use of discrete Bragg reflections of Ge. The ability of inelastic-neutronscattering measurements is examined through observation of magnetic Brillouin scattering with extremely high resolution and with a supplementary use of two Eis. © 2011 The Physical Society of Japan.

DOI： 10.1143/JPSJS.80SB.SB012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Using Co L-2,L-3 resonant soft X-ray scattering measurement, we reveal that the magnetic orderings with q = (1/2, 0, 1/2) and (1/2, 0, 1) originate from different magnetic domains. Temperature dependence of the magnetic ordering peaks clearly shows that there is a finite difference between the transition temperatures of these magnetic orderings. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2010.12.037

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We performed magnetic susceptibility and inelastic neutron scattering measurements on the frustrated spinel Mg1-xCr2O4-x (0.02 <= x <= 0.11). Susceptibility shows that Curie-Weiss temperature is almost independent of x, meaning that exchange interactions sustain. Inelastic neutron scattering reveals that the spin-hexamer excitations also persist with almost the same energy around 4 meV in a lowest temperature phase, whereas the resonance width increases with increasing x. These results suggest that nearest-neighboring Cr center dot center dot center dot Cr direct exchange predominantly forms the hexamers regardless of lattice defect and superexchange via Mg and O sites.

DOI： 10.1088/1742-6596/320/1/012040

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

In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 K. A precise evaluation of T-c from zero-resistivity temperatures revealed that T-c shows a slightly distorted dome-shaped curve with maximum T-c(offset) (30 K) at 6 GPa. With the application of pressure, the temperature dependence of resistivity above T-c changes dramatically to a linear dependence, that the high-T-c phase appears above 3 GPa. We found a striking correlation between T-c and the Se height: the lower the Se height, the more enhanced is T-c. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that high-temperature superconductivity can appear only around the optimum anion height (similar to 1.38 angstrom). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of T-c of iron-based superconductors containing various anions.

DOI： 10.1103/PhysRevB.81.205119

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The isovalent substitution of Te for Se in the superconducting alpha-FeSe raises T-C where the average chalcogen-Fe bond angle decreases and the chalcogen-Fe distance increases. Locally, however, the Se and Te ions do not share the same site and have two distinct z coordinates, in contrast to what is presumed in the P4/nmm symmetry. The local bond angle between the chalcogens and Fe increases with the substitution, consistent with the rise in T-C, the Fe-Te bonds become shorter than in the binary FeTe, while the Fe-Se bonds stay the same as in the binary. Ab initio calculations based on spin density functional theory yielded an optimized structure with distinct z coordinates for Se and Te, in addition to a stronger hybridization of Te with Fe.

DOI： 10.1103/PhysRevB.81.134524

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The lattice dynamics of Ba1-xKxFe2As2 (x = 0.00,0.27) have been studied by inelastic X-ray scattering measurement at room temperature. K doping induces the softening and broadening of phonon modes in the energy range E = 10-15 meV. Analysis with a Born-von Karman force-constant model indicates that the softening results from reduced interatomic force constants around (Ba,K) sites following the displacement of divalent Ba by monovalent K. The phonon broadening may be explained by the local distortions induced by the K substitution. Extra phonon modes are observed around the wave vector q = (0.5, 0, 0) at E = 16.5 meV for the x = 0.27 sample. These modes may arise either from the local disorder induced by K doping or from electron-phonon coupling.

DOI： 10.1143/JPSJ.79.014714

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A transition to a superconducting state was recently observed in the binary alloy of FeSe1-xTex system where T-C rises with increasing x. The substitution of the larger Te for Se ion results in no additional charges but increases the internal chemical pressure. Earlier studies suggested that the crystal structure maintains the tetragonal P4/nmm symmetry with the substitution of Te where the average bond angle, alpha, decreases considerably from similar to 104 degrees in FeSe to 100.5 degrees in the mixed phase of FeSe0.5Te0.5. With the use of pulsed neutron power diffraction and the Rietveld analysis, the crystal structure refinement for FeSe0.5Te0.5 yielded very large thermal factors in the superconducting phase indicative of the presence of structural distortions that may be significant in understanding the electronic and magnetic properties of this system.

DOI： 10.1088/1742-6596/251/1/012009

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We studied magnetic phase diagram and x-dependence of spiral spin correlation in magnetically ordered phase of (Co1-xZnx)Cr2O4 by magnetization and neutron scattering experiments. The spiral correlation survives in a wide x range from 0 to 0.9. As x increases, the correlation length decreases from xi similar to 23 angstrom at x = 0, and saturates at xi similar to 2.5 angstrom at around x = 0.75, which is close to the nearest neighboring Cr3+-Cr3+ distance. The pitch is almost constant from x = 0 to 0.75, then abruptly shortens above around x = 0.75. The correlation is isotropic at x = 0.25 but anisotropic at x = 0.75. We discuss that the spiral correlation is of short-range owing to unquenched geometric frustration, and begins to collapse with the minimization of correlation length above around x = 0.75, possibly as the precursor of dynamic spin hexamer caused by high geometric frustration.

DOI： 10.1143/JPSJ.78.084704

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Using elastic neutron scattering on single crystals of La(1-x)A(x)CoO(3) (A=Ca2+, Sr2+, and Ba2+), we found the development of magnetic superstructures below the global magnetic transition to be strongly dependent on the size of the A-site dopant, << r(A)>>, in an unusual way. Upon reducing the << r(A)>> (i.e., as with Ca doping), only a commensurate ferromagnetic cluster phase is evident. On expanding the << r(A)>>, the tendency toward coexistence of competing ferromagnetic and antiferromagnetic orders increases giving rise to an inhomogeneous ground state. The antiferromagnetic ordered state, initially incommensurate, continuously strengthens and becomes commensurate with long-range order and a characteristic cubic wave vector of Q(c)=(0.25,0.25,0.25) with x. The two competing order parameters become comparable in magnitude indicative of the phase-separated nature of the cobalt perovskite system.

DOI： 10.1103/PhysRevB.80.052402

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We determined the crystal structural parameters of FeSe1-xTex (x = 0.5, 0.625, 0.75, 0.875) and FeSe by using the neutron powder diffraction technique. It was found that the superconducting transition temperature T-c of FeSe1-xTex increased with the (Se,Te)-Fe-(Se,Te) bond angle alpha in the Te-doped samples. However, T-c of FeSe did not exhibit a similar relation. In order to explain this contradiction, we studied the temperature dependence of the z position of chalcogens, which is strongly coupled to the spin fluctuation by theoretical calculations. The z position in FeSe showed a remarkable shift, indicating that strong spin fluctuation can affect the superconductivity in FeSe.

DOI： 10.1143/JPSJ.78.074718

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High-pressure resistivity experiments were performed on FeSe0.5Te0.5. The onset temperature of T, (T-c(onset)) increases rapidly from 13.5 to 26.2K upon applying pressure up to 2GPa. Above 2GPa, T-c decreases linearly and the metallic phase was observed at P = 14GPa. The same relationship between normalized T-c and pressure in both FeSe0.5Te0.5 and FeSe suggests a universal pressure dependence in this system. This report presents the first observation of a phase transition from the Superconducting to the metallic phase on the P-T-c phase diagram.

DOI： 10.1143/JPSJ.78.063705

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Neutron scattering experiments were performed on the single crystals of layered-perovskite cobaltate La2-xSrxCoO4. For a commensurate value of x=0.5, magnetic reflections due to high-spin (HS) state of Co2+ are observed in (H, O, L) plane. Further, in (H, H, L) plane, we found super-lattice reflections with wave vector q(m)=(1/4, 1/4, 1). The latter magnetic peaks cannot be understood by the Co2+ spin configurations, indicating that these peaks are originated from Co3+ spin ordering. From the hole-doping dependence, short ranged charge and spin orderings were observed below x=0.5, while those of peaks were suppressed at x=0.6. Our results suggest that double exchange interaction between the HS state of Co2+ and the intermediate spin (IS) state of Co3+ mediate the 15 state in this system. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.nima.2008.11.141

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DOI： 10.11316/jpsgaiyo.64.2.3.0_386_2

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We have measured the longitudinal and transverse sound velocities of La2-xCaxCoO4 (0.4≤x≤0.7) propagating along [100], [110], and [001] from 200 K down to 3 K with a frequency of 10 MHz. It was found that c66 shows a remarkable softening below Co 2+ spin-ordering temperature TN in the whole doping range, while c11-c12/2 exhibits a clear hardening below Co 3+ spin-ordering temperature TN2 only when x=0.5. To explain the observed anomaly, the Landau free energy for the La 2-xCaxCoO4 system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter (Q1, Q2), having a spatial modulation along [110] or [1̄ 10]. We argued that this represents orbital order, and it is this order that leads to the softening of c66. © 2008 The American Physical Society.

DOI： 10.1103/PhysRevB.78.144108

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We have measured the longitudinal and transverse sound velocities of La2-xCaxCoO4 (0.4 &lt;= x &lt;= 0.7) propagating along [100], [110], and [001] from 200 K down to 3 K with a frequency of 10 MHz. It was found that c(66) shows a remarkable softening below Co2+ spin-ordering temperature T-N in the whole doping range, while c(11)-c(12)/2 exhibits a clear hardening below Co3+ spin-ordering temperature T-N2 only when x=0.5. To explain the observed anomaly, the Landau free energy for the La2-xCaxCoO4 system has been constructed, including the coupling energy between elastic strains and a two-dimensional order parameter (Q(1),Q(2)), having a spatial modulation along [110] or [&lt;overline&gt;1&lt;/overline&gt;10]. We argued that this represents orbital order, and it is this order that leads to the softening of c(66).

DOI： 10.1103/PhysRevB.78.144108

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Using neutron and resonant X-ray scattering, we studied crystal structure and charge-ordering pattern in La1.5Ca0.5CoO4. Taking A2mm with a-b type twin structure into account, Co2+/Co3+ checkerboard charge ordering pattern was suggested in La1.5Ca0.5CoO4 by neutron scattering structure analysis. In order to clarify the charge ordering pattern and the valences of the two Co sites, we performed a resonant X-ray scattering (RXS) measurement near the Co K-edge. We found a resonant signal at (3, 0, 0) reflection which corresponds to the charge ordering with a Co2+/Co3+ checkerboard pattern, and the Co valences of charge ordering were fully charge disproportionated. From the X-ray fluorescence spectra, we found the pre-edge peak structures at E = 7.7 10 and 7.713 keV, corresponding to 1S --> t(2g) and 1s ---> e(g) transitions, respectively. The polarization dependence of the latter one suggests that the anisotropy e(g) orbital is realized in La1.5Ca0.5CoO4.

DOI： 10.1143/JPSJ.77.044601

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

Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La2-xCax-CoO4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3 <= x <= 0.8. For a commensurate value of x = 0.5 in (H, 0, L) plane, two types of superlattice reflections concomitantly appear at low temperature; one corresponds to a checkerboard charge ordered pattern of Co2+/ Co3+ ions and the other is magnetic in origin. Further, the latter magnefic-superlattice peaks show two types of symmetry in the reflections, suggesting anti ferromagnetic-stacking (AF-S) and ferromagnetic-stacking (F-S) patterns of spins along the c direction. From the hole-doping dependence, the in-plane correlation lengths of both charge and spin orders are found to give a maximum at x = 0.5. These features are the same with those of x = 0.5 in Lal(1-x)Sri(1+x)MnO(4) (LSMO), a typical checkerboard and spin ordered compound. However, in (H, H, L) plane, we found a magnetic scattering peak at Q = (0.25, 0.25, 0.5) position below T-N. This magnetic peak can not be understood by considering the Co2+ spin configuration, suggesting that this peak is originated from Co3+ spin order. By analyzing these superlattice reflections, we found that they are originated from high-spin state of Co3+ spin order.

DOI： 10.1143/JPSJ.76.114715

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Language：English   Publishing type：Research paper (scientific journal)  

We examined checkerboard charge and spin order in La1.5Ca0.5CoO4 by elastic and inelastic neutron scattering. We discovered the sharp magnetic peaks with l=half-integer and broad magnetic peaks with l=integer in (0.5, 0, l). The origin of magnetic scattering can be explained by the minority and majority stacking arrangement of Co2+/Co3+ spins along the c-axis since the same magnetic reflections arise from the two stacking vectors in La0.5Sr1.5MnO4. Spin-wave dispersion at majority stacking is seen up to 16 meV, in contrast to 50 meV in La1.5Sr0.5NiO4. This indicates a much weaker exchange interaction in the cobaltates than in nickelates. The dispersion at majority stacking has almost a same one with that of minority stacking. © 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.530

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Magnetic properties were investigated for single crystals of La2-xCaxCoO4 (0.5 <= x <= 0.8) and for polycrystalline samples of La2-xBaxCoO4 (0.5 <= x <= 0.9). In the Ca system, the effective moment mu(eff) decreased with increasing x from 4.0 mu(B) (x = 0.5) to 3.0 mu(B) (x = 0.7-0.8), accompanied by a steep reduction of Weiss temperature Theta. These drastic changes of the magnetic properties suggest a spin-state transition of the Co3+ ions from the high-spin state to the intermediate-spin state. On the other hand, the effective moment of La2-xBaxCoO4 was almost 4.0 mu B and there is no change of Weiss temperature Theta. These facts suggest that the intermediate-spin state is related to the Co-O bond length, i.e. crystal fields. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.423

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL INST MATERIALS SCIENCE  

Magnetic and electrical properties were characterized for newly synthesized La1-xSrxOCuSe (0 &lt; x &lt; 0.25) and La1-xBaxOCuTe (0 &lt; x &lt; 0.3). Magnetic measurement showed paramagnetic behavior, but it did not obey the Curie-Weiss law, and an ordered phase was not observed down to 1.8 K in the Se and Te systems. The electrical properties of La1-xSrxOCuSe indicated a metal-semiconductor transition at approximately x=0.05 and a metallic state of a higher concentration range x. The electrical conductivity of La0.8Sr0.2OCuSe was 209 S cm(-1) at room temperature. In La1-xBaxOCuTe, electrical resistivity systematically decreased with increasing x and semiconductive behavior was observed for all x values. Metal-semiconductor transition is probably caused by the degree of orientation and difference of acceptor levels in this system. (c) 2005 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.stam.2005.11.013

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