Promising material shows new evidence of unconventional superconductivity

In recent years, the search for non-trivial topological materials has become a hot topic in condensed matter physics. Since Hor et al, first reported the discovery of superconductivity in Cu doped topological material Bi₂Se₃ in 2010, the Cu_xBi₂Se₃ has become one of the most promising materials as topological superconductor due to its unique physical properties and crystal structure. However, the superconducting transition temperature Tc up to 3.8 K in Cu_xBi₂Se₃ is unexpectedly "high" for a low carrier density semiconductor. So far, the mechanism of such anomalous enhanced Tc phenomenon remains unclear despite nearly a decade of extensive research.

In a recent work conducted by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, high quality single crystal of Cu_xBi₂Se₃ was grown by modified Bridgman method. The Tc of the as-grown crystals with x=0.09 could reach 4.18 K, which was the highest one among reports on Cu_xBi₂Se₃ so far.

A systematic study of the magnetic susceptibility, critical fields, and electrical transport on the Cu_0.09Bi₂Se₃ single crystals were conducted to explore the unusually enhanced Tc and its superconducting properties.

Interestingly, a novel kink in the magnetic susceptibility versus temperature was observed at 96 K, which indicated a charge density anomaly, probably charge density wave (CDW) transition.

The analysis of the magnetoelectrical transport at low temperature yielded a high Kadowaki-Woods ratio, which might be enhanced by the charge density instability and/or strong electronic anisotropy.

Based on the lower critical field measurement, the energy gap ratio Δ0/kBTC was found obviously larger than the standard BCS value 1.764, suggesting the Cu_0.09Bi₂Se₃ a strong-coupling superconductor. Ratios of both Tc/TF2D and Tc/λ-2(0) fell into the region of unconventional superconductors according to Uemura's regime, supporting the unconventional superconducting mechanism in Cu_xBi₂Se₃.

Their research proposed that the high Tc in Cu_xBi₂Se₃ arises from the increased density of states at Fermi energy and strong electron-phonon interaction induced by the charge density instability.

The results suggest the higher Tc in Cu_xBi₂Se₃ could be further achieved by gating-technique or high pressure technique, as realized in iron-selenides superconductors.