Superconducting Properties of Pd_1–xPt_xBi₂ over a Wide Pressure Range

The topological insulator, PdBi₂, exhibits two different crystal phases (i.e., α-PdBi₂ and β-PdBi₂) at ambient pressure. Here, we prepared new superconducting materials, Pd_1–xPt_xBi₂, via Pt doping of PdBi₂ and investigated their superconducting properties. Pt doping of PdBi₂ may be anticipated to show higher superconducting transition temperature T_c than pure PdBi₂ because of the example that Pt doping of PdTe₂ (T_c = 2.0 K) leads to higher T_c (T_c = 3.2 K for Pd_0.25Pt_0.75Te₂) via reconstruction of Fermi surface topology. The Pd_1–xPt_xBi₂ samples prepared in this study contained both the α and β phases, as seen in the XRD patterns. The x dependence of T_c was investigated for Pd_1–xPt_xBi₂; the highest T_c value (4.35 K) was observed at x = 0.12, which was probably due to the β phase. In this study, the crystal structure and superconducting transition temperature of Pd_0.88(6)Pt_0.12(6)Bi_2.2(2) are fully investigated over a wide pressure (p) range. Both α phase and β phase remain up to a pressure of approximately 20 GPa; however, the amount of the α phase gradually decreased with an increase in pressure. The pressure dependence of unit cell volume V of the α and β phases resulted in smooth shrinkage of the lattice. The temperature (T) dependence of the electrical resistance (R) at different pressures showed enhancement of T_c of the α phase with pressure and almost constant T_c in the β phase, with a trend that is similar to that of α-PdBi₂ and β-PdBi₂. The temperature dependence of the upper critical field at 10.5 GPa suggested that it deviates from the simple s-wave dirty/clean limit model and follows the p-wave polar model, which causes the topologically nontrivial nature of superconductivity.