Superconductivity in topological insulator β-PdBi₂ under pressure

Abstract

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.

Superconducting Properties of Pd1–xPtxBi2 over a Wide Pressure Range

“…The plots of M / H – T of Pd 1– x Pt x Bi 2 (nominal x = 0.2) at zero-field cooling (ZFC) and field cooling (FC) are shown in Figure b, which demonstrate the superconducting transition with a T c value of 4.35 K and an onset transition temperature ( T c onset ) of 4.85 K. The T c value was different from that of α-PdBi 2 (1.6–1.7 K) , but similar to that of β-PdBi 2 (4.1–5.4 K). − The shielding fraction at 2.0 K was determined to be 37%, the value of which was assigned to the fraction of the β phase of Pd 1– x Pt x Bi 2 (nominal x = 0.2) because of no observation of diamagnetic signals due to the α phase down to 2.0 K, as seen in Figure b. The EDX spectrum exhibited peaks corresponding to Pd, Pt, and Bi only (not shown), indicating the stoichiometry of Pd 0.88(6) Pt 0.12(6) Bi 2.2(2) .…”

“…The lattice constants a and c obtained were 3.3568(2) Å and 13.211(3) Å, respectively, for the tetragonal phase (β phase) of Pd 0.88(6) Pt 0.12(6) Bi 2.2(2) . The lattice constants of the α phase of Pd 0.88(6) Pt 0.12(6) Bi 2.2(2) were also similar to those of α-PdBi 2 ( a = 12.74 Å, b = 4.25 Å, c = 5.665 Å, and β = 102.58°), whereas the lattice constants of the β phase were similar to those of β-PdBi 2 ( a = 3.3503(4) Å and c = 12.923(2) Å) . The lattice constants of Pd 0.88(6) Pt 0.12(6) Bi 2.2(2) were slightly larger than those of PdBi 2 because the ionic radius of Pt 2+ (0.94 Å for six coordination) was also slightly larger than that of Pd 2+ (0.86 Å for six coordination) .…”

“…NaCl was used as a pressure medium to provide hydrostatic pressure. The equipment and methods used for the XRD and R measurements have been fully described in previous reports. , …”

“… , α-PdBi 2 was converted into β-PdBi 2 under pressure: an irreversible transformation from α-PdBi 2 to β-PdBi 2 occurred above 8 GPa. A comprehensive T c – p behavior of PdBi 2 was recently reported by our group . In particular, the superconducting properties of β-PdBi 2 were explored over a wide pressure range.…”

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

“…By contrast, the RbBi 2 samples showed an almost constant T c onset -p behavior, similar to KBi 2 . The constant or slight increase in T c onset for KBi 2 and RbBi 2 against pressure is remarkable, that is, the observed robustness of T c onset against pressure is common for Bi-based compounds, such as BaBi 3 and PdBi 2 , 37,48 indicating unconventional electronic structures, such as the Fermi surface consisting of multiple orbitals or strong SOC of Bi. The temperature dependence of the reduced critical field of KBi 2 and RbBi 2 under pressure suggests a deviation from the swave dirty limit model for the Cooper pair symmetry.…”