Superconductivity in topological insulator β-PdBi2under pressure.

The topological insulator PdBi2exhibits two different crystal phases at ambient pressure, i.e., "α-PdBi2" and "β-PdBi2." The pressure dependence of crystal structure and superconductivity of α-PdBi2has been fully elucidated thus far. However, the physical properties of β-PdBi2crystals under pressure have not been sufficiently investigated. In this study, we fully investigate the crystal structure and superconductivity of β-PdBi2under pressure based on synchrotron X-ray diffraction (XRD) patterns. The temperature dependence of β-PdBi2indicates its superconductivity with a superconducting transition temperature (Tc) as high as 4.10 K, and its crystal structure is tetragonal (space group ofI4/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-PdBi2transformed to β-PdBi2under pressure. This suggests that β-PdBi2is stable under pressure. The superconductivity is clearly observed at 0 - 11.8 GPa, and the value ofTcis almost constant at ~4.4 K. The temperature dependence of the upper critical field at ambient pressure and 10.7 GPa indicate that the superconductivity is not attributed to a simples-wave dirty limit but ans-wave clean orp-wave polar model. This is the first systematic study of superconductivity of topological insulator β-PdBi2under pressure.