Superconductivity in a Ternary Silicide Ca2Pt3Si5

Abstract

Superconductivity in the non-centrosymmetric crystal structure has been attracting a considerable interest since Bauer et al. reported superconductivity in CePt3Si which lacks inversion symmetry in the tetragonal crystal structure. Recently the pressure-induced superconductivity was observed in some uraniumand cerium-based compounds with non-centrosymmetric crystal structures. Many unconventional superconducting properties were found in these heavy fermion superconductors. It is, however, still unclear how the strong antisymmetric spin–orbit interaction (ASOI) due to the lack of inversion symmetry in the crystal affects on superconductivity. In order to elucidate the intrinsic effect of the strong ASOI on superconductivity, it is desirable to develop new non-centrosymmetric superconductors with weakly correlated electrons. Very recently, Oikawa et al. reported new ternary silicide superconductors ATSi3 (A: Ca, Sr and Ba, and T: transition metal) with the BaNiSn3-type tetragonal crystal structure. 5) For example, polycrystalline CaPtSi3 was found to show superconductivity below 2.1K. Since we succeeded in growing single crystals of CeTX3 (X: Si and Ge) compounds with the same BaNiSn3-type crystal structure by the flux method, the same method was applied to ATSi3 compounds. We could not, however, grow single crystals of CaPtSi3, but succeeded in growing single crystals of Ca2Pt3Si5, which were found to be a superconductor. The existence of Ca2Pt3Si5 was clarified in the present paper. Single crystals of Ca2Pt3Si5 were grown by the Sn-flux method. The starting elements were 4N (99.99% pure)-Ca, 4N-Pt, and 5N-Si. These materials with a stoichiometric composition of Ca : Pt : Si : Sn 1⁄4 1 : 1 : 3 : 20 for CaPtSi3 were inserted in an alumina crucible and sealed in a quartz tube with a partial pressure of argon gas. The temperature of the furnace was raised to 1050 C. After homogenization for about 24 h, the furnace was cooled down to 600 C over a period of two weeks and finally cooled down to room temperature at a faster rate. After removing the excess Sn flux by centrifuging, we found many single crystals with a characteristic shape and size of the crystal, as shown in Fig. 1. We randomly chose the crystal and measured the electrical resistivity. All the measured crystals showed superconductivity at the same temperature, as shown later. The obtained single crystals were not CaPtSi3 but Ca2Pt3Si5 with the flat plane of (001) or c-plane, as shown in Fig. 1. Structural parameters of Ca2Pt3Si5 were analyzed by the single crystal X-ray diffraction experiment with Mo K radiation. We confirmed that Ca2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic crystal structure (Ibam #72). Atomic coordinates are shown in Table I. Note that this crystal structure has an inversion center. The lattice parameters were determined as a 1⁄4 10:0676 A, b 1⁄4 11:5469 A, and c 1⁄4 5:9237 A. We measured the electrical resistivity for the current along the [010] direction or the b-axis and the specific heat C at low temperatures, and found that Ca2Pt3Si5 shows a superconducting transition at Tc 1⁄4 1:3K, as shown in Fig. 2. Here, the specific heat was measured by using ten single crystals by means of the quasi-adiabatic heat pulse method. Ca2Pt3Si5