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Y. Kawamura, S. Deminami, J. Hayashi, K. Takeda, C. Sekine, L. Salamakha, H. Michor, E. Bauer, J. Gouchi, Y. Uwatoko, T. Kawae:
"Superconductivity of Alkalline-Earth-Filled Skutterudites";
Vortrag: SCTE2018, The 21st International Conference on Solid Compounds of Transition Elements, Wien; 25.03.2018 - 29.03.2018.

Filled skutterudites with a general chemical formula AT4X12 (A = Alkaline earth, Rare earth, T = Transition metal, X = Pnictogen) are one of the intensively studied compounds due to their unique physical properties. Some of the superconductor realized in the filled skutterudites have characteristic physical properties such as a unique heavy fermion superconductor in PrOs4Sb12 [1] and a positive pressure dependence of superconducting transition, TC, in LaFe4P12 and YFe4P12[2]. We have recently succeeded in synthesizing new filled skutterudites, SrOs4P12 by high pressure and high temperature technique. As SrOs4P12 was found to be superconductor, we have investigated the physical properties of superconducting states of SrOs4P12 with magnetization, electrical resistivity, and specific heat measurements. The electrical properties were also investigated under pressure and in a magnetic field. We found SrOs4P12 is a bulk superconductor with two superconducting transition at T_C1 ~ 1.5 K and at T_C2 ~ 1.0 K. The specific heat at TC_C1 and T_C2 increases by 15 mJ/molK and 5 mJ/molK, respectively. The entropy released by superconductivity is balanced out with that by the normal conductivity. T_C1 is rapidly suppressed by the magnetic field, while T_C2 is relatively difficult to be suppressed. H_C2 for T_C1 and for T_C2 can be estimated to be 6 kOe and 14 kOe, respectively. The H_C2-T_c relationship from resistivity reflect both TC1 and TC2; TC decreases with field at a rate of 4.0 kOe/K at low field followed by a downward convex at 0.9 K and the T_C decreases rapidly with field at a rate of 16.4 kOe/K at high field. This downward convex is suppressed by applying pressure. In other words, The T_C1 is sensitive to field but insensitive to pressure, while T_C2 is sensitive pressure but insensitive to field. T_C1 has positive pressure dependence at the field of 0 kOe, contrary to the negative dependence predicted by simple model based on BCS theory.
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[1] E. D. Bauer, et al., Phys. Rev. B 65, 100506 (2002)
[2] L. E. DeLong and G. P. Meisner: Solid State Commun. 53, 119 1985); J.-G. Cheng et al., Phys. Rev. B 88, 024514 (2013)