Exploring Superconductivity in the Close Proximity of Polar-Nonpolar Structural Phase Transition

Depending on the way in which the elements in the periodic table are combined, the compounds represented by MX₂ (M = transition metal and X = pnictogen or chalcogen) exhibit a wide variety of crystal structures. This is because the structure changes depending on the energies and band filling of the d and p states. In this respect, the combination of Pt and Bi is quite exquisite, and thus PtBi₂ alone exhibits various crystal structures such as α-PtBi₂ (Pbca, , No. 61), β-PtBi₂ (, , No. 205), γ-PtBi₂ ( , , No. 147), γ'-PtBi₂ (P31m, , No. 157), and δ-PtBi₂ (Pnnm, , No. 58). This is a remarkable feature because PtP₂, PtAs₂, and PtSb₂ that are composed of elements of the same groups, in contrast, crystallize only in the same structure as β-PtBi₂, suggesting the structural instability of PtBi₂.

In this study, the structural instability of PtBi₂ has been utilized to explore structural phase transitions by chemical substitutions and the associated emergence of new physical properties. Notably, substituting Se or Te for some of the Bi in γ'-PtBi₂ (P31m, , No. 157) with a polar structure stabilizes the nonpolar structure (, , No. 164). This structural phase transition drastically increases superconducting transition temperature T_c from 0.6 K to 2.4 K. T_c increases as the system approaches the polar-nonpolar structural phase boundary.

According to the Bardeen-Cooper-Schrieffer (BCS) theory, when phonons soften in close proximity to a structural phase transition, T_c increases. Indeed, such compounds have been noted. However, in close proximity to a polar-nonpolar structural phase transition, the polar structural instability is crucial. The effect of this on superconductivity remains unclear, and studies to explore superconductivity beyond the BCS theory are currently being actively conducted. The present study reports new compounds in which T_c is enhanced in close proximity to the polar-nonpolar structural phase transition. The systems can provide a unique opportunity to study the relationship between polar-nonpolar structural phase transitions and superconductivity from the viewpoints of the competition between the spin splitting of the Fermi surface and the superconducting gap and the soft-phonons related to the polar structural instability. The results of this study are expected to lead to new guidelines for the future understanding of superconductivity and the efforts to search for new fluctuations to increase T_c.

(written by Kazutaka Kudo on behalf of all authors.)