Superconductivity in topological insulator β-PdBi₂ under pressure

The topological insulator PdBi₂ exhibits two different crystal phases at ambient pressure, i.e., ‘α-PdBi₂’ and ‘-PdBi₂’. The pressure dependence of crystal structure and superconductivity of α-PdBi₂ has been fully elucidated thus far. However, the physical properties of β-PdBi₂ crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of β-PdBi₂ under pressure based on synchrotron x-ray diffraction (XRD) patterns. The temperature dependence of β-PdBi₂ indicates its superconductivity with a superconducting transition temperature (T_c) as high as 4.10 K, and its crystal structure is tetragonal [space group of I4/mmm (no. 139)]. The XRD patterns at 0–22.0 GPa indicate no structural phase transitions, and the unit cell volume shrinks monotonically with pressure, unlike the behavior of α-PdBi₂. Furthermore, α-PdBi₂ transformed to β-PdBi₂ under pressure. This suggests that β-PdBi₂ is stable under pressure. The superconductivity is clearly observed at 0–11.8 GPa, and the value of T_c is almost constant at ∼4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicates that the superconductivity is not attributed to a simple s-wave dirty limit but an s-wave clean or p-wave polar model. This is the first systematic study of superconductivity of topological insulator β-PdBi₂ under pressure.