Pressure dependence of superconductivity in alkali-Bi compounds KBi₂ and RbBi₂

The structural and superconducting properties of alkali-Bi-based compounds, KBi₂ and RbBi₂, were investigated over a wide pressure range for the first time. The samples of KBi₂ and RbBi₂ were prepared using a liquid ammonia (NH₃) technique, and demonstrated superconductivity with superconducting transition temperatures, T_c, of 3.50 and 4.21 K at ambient pressure, respectively. The onset superconducting transition temperature, T_c^onset, of KBi₂ decreased slightly; however, it suddenly jumped at 2 GPa and increased gradually with pressure, indicating the presence of two superconducting phases in the low-pressure range. The pressure-dependent X-ray diffraction patterns indicate that the KBi₂ sample decomposed into KBi and Bi at pressures higher than 2.5 GPa. Moreover, a discontinuous change in T_c^onset was observed for KBi₂ at 9 GPa, which reflects the decomposition of KBi₂ into KBi and Bi. By contrast, the value of T_c^onset of RbBi₂ was almost constant over a pressure range of 0-8 GPa. Thus, the superconducting properties and stability of alkali-Bi-based compounds against pressure were comprehensively explored in this study. In addition, the superconducting Cooper pair symmetry was investigated from the magnetic field dependence of T_c of KBi₂ at 0.790 and 2.32 GPa, and of RbBi₂ at 1.17 GPa, indicating the exact deviation from the simple s-wave paring model, which may be due to the complex electronic structure of Bi. The results elucidated the exotic superconducting properties of KBi₂ and RbBi₂ based on the pressure and magnetic field dependence of T_c and verified the chemical stability of KBi₂ under pressure.