Superconductivity in CaBi₂

Abstract

and 124 Oe. Results of electronic structure calculations are reported and charge densities, electronic bands, densities of states and Fermi surfaces are discussed, focusing on the effects of spin-orbit coupling and electronic property anisotropy. We find a mixed quasi-2D + 3D character in the electronic structure, which reflects the layered crystal structure of the material. Keywords: Type-I superconductivity, 2 I IntroductionMuch effort has recently been devoted to Bi-based candidates for topological insulators [1,2,3]. The presence of the heavy element bismuth provides the strong spin-orbit coupling that is essential for formation of the topologically-nontrivial band structure of these materials. To the best of our knowledge, no report on the physical properties of CaBi 2 has been previously published. CaBi 2 crystallizes in an orthorhombic lattice in non-symmorphic space group Cmcm (no. 63) [19], and is isostructural with ZrSi 2 [21,22]. In order to study the electronic properties of CaBi 2 , we employed a self-flux-based single crystal growth method [32], and the physical properties of the resulting crystals were analyzed. The electronic structure of the system was next calculated, using density functional theory methods. Electronic bands, densities of states, Fermi surfaces and charge densities are described here in addition to the electronic properties of the material; the analysis focuses on spin-orbit coupling effects and the anisotropy of the electronic states. II Materials and MethodsTo grow the CaBi 2 crystals, calcium granules (Alfa Aesar, 99.5%) and bismuth pieces (Alfa Aesar, 99.99%) in a 3:17 molar ratio (15 at% of Ca) were put in a carbon-coated quartz tube inside an Ar-filled glovebox. A plug of quartz wool was then inserted, and the tube was subsequently evacuated and sealed without exposing the Ca metal to air. The ampule was heated to 550ºC, kept at that temperature for 8 hours, and then slowly cooled (3ºC per hour) to 310ºC at which temperature the excess Bi was spun off with the aid of a centrifuge (3000 rpm, Heat capacity and electrical resistivity measurements were performed using a 3 He-refrigerator equipped Quantum Design PPMS system. Electrical contacts were glued to the sample surface using silver paste. A standard relaxation method was used for the heat capacity measurements.Magnetic susceptibility measurements were carried out in a Quantum Design MPMS-XL SQUID magnetometer equipped with an iQuantum 3 He refrigerator.Electronic band structure calculations were performed using the plane-wave pseudopotential III ResultsThe EDS results yielded the Ca:Bi ratio of 1:2, confirming the stoichiometry of the grown crystals. Additionally, some elemental Bi spots on the surface were found. They may originate either from remaining flux material that was not removed during the centrifugation process, or arise from CaBi 2 decomposition in contact with air and moisture. The room temperature PXRD pattern of crushed crystals is presented in Fig. 2 (the increased background in the low 2θ range is due...

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