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EuPtSi crystallizes in the cubic chiral structure (P2 1 3, No.198), which is the same as the noncentrosymmetric space group of MnSi with the skyrmion structure. The first-order antiferromagnetic transition occurs at T N = 4.0 K, and EuPtSi is in the helical magnetic state with the propagation vector q = (0.2, 0.3, 0) at low temperatures. We found the skyrmion structure in a closed H-T phase or the so-called "A-phase" for H ∥ [111], together with a new skyrmion-related phase named the "Bphase" for H ∥ [001]. It is characteristic that the A-phase is clearly observed in the magnetization, Hall resistivity, as-susceptibility, magnetostriction, and magnetoresistance. This is due to a small size of the skyrmion, 18 Å, in other words, a large number of the skyrmion density. The A-phase is thus anisotropic, not observed for H ∥ [110]. We also clarified the split Fermi surface properties based on the chiral nature of the crystal structure from the de Haas-van Alphen experiment and energy band calculation. Furthermore, we present the magnetic properties of EuPtGe with T N ≃ 3 K.

EuAu 5 with the hexagonal structure is known to be a ferromagnet with the Curie temperature T C = 13 K or 14 K. We succeeded in growing high-quality single crystals by the Bridgman method using the Mo crucible. The residual resistivity ρ 0 and the residual resistivity ratio RRR were ρ 0 = 0.15 µΩ • cm and RRR = 61 for J ∥ [0001]. It was found that the resistivity at room temperature is extremely small as in a simple metal, ρ RT = 9.2 µΩ • cm for J ∥ [0001], which is compared with ρ RT = 32 µΩ • cm for J ∥ [1120]. We observed two magnetic transitions at T C = 15 K and T m = 13.5 K. The latter magnetic transition is of the first-order like one in the specific heat measurement. The magnetization at 2 K saturates at an extremely low field of 2 kOe for H ∥ [0001], with a saturated Eu 2+-moment of 7 µ B /Eu. On the other hand, the hard-axis magnetization for H ∥ [1120] saturates at a high field of 40 kOe. We also clarified the Fermi surface properties by the de Haas-van Alphen effect and energy band calculations, revealing a quasi-one dimensional wavy plate-like Fermi surface. The similar highly anisotropic magnetic properties were observed in EuCu 5 with T C = 84 K.

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We grew high-quality single crystals of Sn 4 P 3 and clarified the superconducting and Fermi surface properties. The crystal structure is trigonal and is similar to the crystal structure of a topological insulator Bi 2 Se 3. Sn 4 P 3 becomes superconductive below T sc = 1.05 K. We carried out the de Haasvan Alphen experiment to clarify the Fermi surface properties. Fermi surfaces are found to consist of cylindrical Fermi surfaces and spherical ones.