研究者詳細 - 笠原　成

論文 - 笠原　成

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Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe 査読

J. P. Sun, K. Matsuura, G. Z. Ye, Y. Mizukami, M. Shimozawa, K. Matsubayashi, M. Yamashita, T. Watashige, S. Kasahara, Y. Matsuda, J-Q. Yan, B. C. Sales, Y. Uwatoko, J-G. Cheng, T. Shibauchi

NATURE COMMUNICATIONS 7 12146 2016年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：NATURE PUBLISHING GROUP

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 <= 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

Magnetotransport study of the pressure-induced antiferromagnetic phase in FeSe 査読

Terashima, T., Kikugawa, N., Kasahara, S., Watashige, T., Matsuda, Y., Shibauchi, T., Uji, S.

Physical Review B 93 ( 18 ) 2016年5月

掲載種別：研究論文（学術雑誌）

© 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

Magnetotransport study of the pressure-induced antiferromagnetic phase in FeSe 査読

Taichi Terashima, Naoki Kikugawa, Shigeru Kasahara, Tatsuya Watashige, Yuji Matsuda, Takasada Shibauchi, Shinya Uji

PHYSICAL REVIEW B 93 ( 18 ) 2016年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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 <= 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

Fermi surface reconstruction in FeSe under high pressure

Terashima, T., Kikugawa, N., Kisw, hi, A., Graf, D., Choi, E.-S., Brooks, J.S., Kasahara, S., Watashige, T., Matsuda, Y., Shibauchi, T., Wolf, T., B{\"o}hmer, A.E., Hardy, F., Meingast, C., L{\"o}hneysen, H.V., Uji, S.

Physical Review B 93 ( 9 ) 2016年3月

掲載種別：研究論文（学術雑誌）

© 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

Fermi surface reconstruction in FeSe under high pressure 査読

Taichi Terashima, Naoki Kikugawa, Andhika Kiswandhi, David Graf, Eun-Sang Choi, James S. Brooks, Shigeru Kasahara, Tatsuya Watashige, Yuji Matsuda, Takasada Shibauchi, Thomas Wolf, Anna E. Boehmer, Frederic Hardy, Christoph Meingast, Hilbert V. Loehneysen, Shinya Uji

PHYSICAL REVIEW B 93 ( 9 ) 2016年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Nematic magnetoelastic effect contrasted between Ba(Fe1-xCox)2As2 and FeSe 査読

Hu, Y., Ren, X., Zhang, R., Luo, H., Kasahara, S., Watashige, T., Shibauchi, T., Dai, P., Zhang, Y., Matsuda, Y., Li, Y.

Physical Review B 93 ( 6 ) 2016年2月

Fermi surface reconstruction in FeSe under high pressure 査読

Terashima, Taichi, Kikugawa, Naoki, Kisw, hi, Andhika, Graf, David, Choi, Eun-Sang, Brooks, James S., Kasahara, Shigeru, Watashige, Tatsuya, Matsuda, Yuji, Shibauchi, Takasada, Wolf, Thomas, Boehmer, Anna E., Hardy, Frederic, Meingast, Christoph, Loehneysen, Hilbert V., Uji, Shinya

Physical Review B 93 ( 9 ) 2016年

掲載種別：研究論文（学術雑誌）

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

Spin nematic susceptibility studied by inelastic neutron scattering in FeSe

S. Shamoto, K. Matsuoka, R. Kajimoto, M. Ishikado, Y. Yamakawa, T. Watashige, S. Kasahara, M. Nakamura, H. Kontani, T. Shibauchi, Y. Matsuda

2015年11月

掲載種別：研究論文（学術雑誌）

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.

Publisher's Note: Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields (vol 115, 027006, 2015) 査読

M. D. Watson, T. Yamashita, S. Kasahara, W. Knafo, M. Nardone, J. Beard, F. Hardy, A. McCollam, A. Narayanan, S. F. Blake, T. Wolf, A. A. Haghighirad, C. Meingast, A. J. Schofield, H. V. Loehneysen, Y. Matsuda, A. I. Coldea, T. Shibauchi

PHYSICAL REVIEW LETTERS 115 ( 21 ) 219902 2015年11月

Momentum-dependent sign inversion of orbital order in superconducting FeSe 査読

Y. Suzuki, T. Shimojima, T. Sonobe, A. Nakamura, M. Sakano, H. Tsuji, J. Omachi, K. Yoshioka, M. Kuwata-Gonokami, T. Watashige, R. Kobayashi, S. Kasahara, T. Shibauchi, Y. Matsuda, Y. Yamakawa, H. Kontani, K. Ishizaka

PHYSICAL REVIEW B 92 ( 20 ) 2015年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Enhancement of critical current density and mechanism of vortex pinning in H+-irradiated FeSe single crystal 査読

Yue Sun, Sunseng Pyon, Tsuyoshi Tamegai, Ryo Kobayashi, Tatsuya Watashige, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi, Hisashi Kitamura

APPLIED PHYSICS EXPRESS 8 ( 11 ) 1 - 113102 2015年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：IOP PUBLISHING LTD

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

Nematic magnetoelastic effect contrasted between Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ and FeSe 査読

Yuwen Hu, Xiao Ren, Rui Zhang, Huiqian Luo, Shigeru Kasahara, Tatsuya Watashige, Takasada Shibauchi, Pengcheng Dai, Yan Zhang, Yuji Matsuda, Yuan Li

Phys. Rev. B 93, 060504(R) (2016) 93 ( 6 ) 2015年10月

掲載種別：研究論文（学術雑誌）

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

Critical current density, vortex dynamics, and phase diagram of single-crystal FeSe 査読

Yue Sun, Sunseng Pyon, Tsuyoshi Tamegai, Ryo Kobayashi, Tatsuya Watashige, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi

PHYSICAL REVIEW B 92 ( 14 ) 1 - 144509 2015年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Dichotomy between the Hole and Electron Behavior in Multiband Superconductor FeSe Probed by Ultrahigh Magnetic Fields 査読

M. D. Watson, T. Yamashita, S. Kasahara, W. Knafo, M. Nardone, J. Beard, F. Hardy, A. McCollam, A. Narayanan, S. F. Blake, T. Wolf, A. A. Haghighirad, C. Meingast, A. J. Schofield, H. v. Loehneysen, Y. Matsuda, A. I. Coldea, T. Shibauchi

PHYSICAL REVIEW LETTERS 115 ( 2 ) 1 - 27006 2015年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Pressure-Induced Antiferromagnetic Transition and Phase Diagram in FeSe 査読

Taichi Terashima, Naoki Kikugawa, Shigeru Kasahara, Tatsuya Watashige, Takasada Shibauchi, Yuji Matsuda, Thomas Wolf, Anna E. Boehmer, Frederic Hardy, Christoph Meingast, Hilbert v. Loehneysen, Shinya Uji

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 6 ) 1 - 63701 2015年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

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

Optical conductivity evidence of clean-limit superconductivity in LiFeAs 査読

R. P. S. M. Lobo, G. Chanda, A. V. Pronin, J. Wosnitza, S. Kasahara, T. Shibauchi, Y. Matsuda

PHYSICAL REVIEW B 91 ( 17 ) 2015年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Emergence of Orbital Nematicity in the Tetragonal Phase of BaFe2(As1-xPx)(2) 査読

Tetsuya Iye, Marc-Henri Julien, Hadrien Mayaffre, Mladen Horvatic, Claude Berthier, Kenji Ishida, Hiroaki Ikeda, Shigeru Kasahara, Takasada Shibauchi, Yuji Matsuda

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 84 ( 4 ) 1 - 43705 2015年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：PHYSICAL SOC JAPAN

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 -> 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

Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations 査読

S. F. Blake, M. D. Watson, A. McCollam, S. Kasahara, R. D. Johnson, A. Narayanan, G. L. Pascut, K. Haule, V. Kiryukhin, T. Yamashita, D. Watanabe, T. Shibauchi, Y. Matsuda, A. I. Coldea

PHYSICAL REVIEW B 91 ( 12 ) 2015年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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

Local characterization of superconductivity in BaFe2(As1-xPx)(2) 査読

Y. Lamhot, A. Yagil, N. Shapira, S. Kasahara, T. Watashige, T. Shibauchi, Y. Matsuda, O. M. Auslaender

PHYSICAL REVIEW B 91 ( 6 ) 2015年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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 >= x(opt).

DOI： 10.1103/PhysRevB.91.060504

Structural Origin of the Anomalous Temperature Dependence of the Local Magnetic Moments in the CaFe2As2 Family of Materials 査読

L. Ortenzi, H. Gretarsson, S. Kasahara, Y. Matsuda, T. Shibauchi, K. D. Finkelstein, W. Wu, S. R. Julian, Young-June Kim, I. I. Mazin, L. Boeri

PHYSICAL REVIEW LETTERS 114 ( 4 ) 1 - 47001 2015年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：AMER PHYSICAL SOC

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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