Recent years, the search for non-trivial topological materials has become a significant hot topic in condensed matter physics. Since Hor et al firstly 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 T_c 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 T_c 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 T_c of the as-grown crystals with x=0.09 could reach 4.18 K, which was the highest one among reports on Cu_xBi₂Se₃ system 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 T_c 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/k_BT_C was found obviously larger than the standard BCS value 1.764, suggesting the Cu_0.09Bi₂Se₃ a strong-coupling superconductor. Ratios of both T_c/T_F^2D and T_c/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' T_c 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 T_c in Cu_xBi₂Se₃ could be further achieved by gating-technique or high pressure technique, as realized in iron-selenides superconductors.

(a). Temperature dependence of magnetic susceptibility under 0.3 T for H//c & H//ab; (b). Ratio of T_c/T_F^2D provides the experimental evidence of Cu_xBi₂Se₃ as an unconventional superconductor. (Image by SIOM)