Unconventional superconductivity in CuxBi2Se3 from magnetic susceptibility and electrical transport

Although the Cu doped Bi2Se3 topological insulator was discovered and intensively studied for almost a decade, its electrical and magnetic properties in normal state, and the mechanism of "high-Tc" superconductivity regarding the relatively low carrier density are still not addressed yet. In this work, we report a systematic investigation of magnetic susceptibility, critical fields, and electrical transport on the nominal Cu0.20Bi2Se3 single crystals with Tc^onset=4.18 K, the highest so far. The composition analysis on the sample yields the Cu stoichiometry of x=0.09(1). The magnetic susceptibility shows considerable anisotropy and an obvious kink at around 96 K was observed in the magnetic susceptibility for the magnetic field parallel to c-axis (H∥c), which indicates a charge density anomaly. The electrical transport measurements indicate the two-dimensional (2D) Fermi liquid behavior at low temperatures with a high Kadowaki-Woods ratio, A/γ^2=30.3*10-5 μΩ cm mol^2 K^2/mJ^2=30.3a0. The lower critical field at 0 K limit was extracted to be 6.0 Oe for the magnetic field perpendicular to c-axis (H∥ab). In the clean limit, the ratio of energy gap to Tc was determined to be ∆0⁄(kB Tc )=2.029+-0.124 exceeding the standard BCS value 1.764, suggesting that Cu0.09Bi2Se3 is a strong-coupling superconductor. The in-plane penetration depth at 0 K was calculated to be 1541.57 nm, resulting in an unprecedented high ratio of Tc/λ^(-2) (0)9.86. Moreover, the ratio of Tc to Fermi temperature (TF) is estimated to be Tc⁄(TF^2D )=0.034. Both ratios fall into the region of unconventional superconductivity according to Uemura's regime, supporting the unconventional superconducting mechanism in CuxBi2Se3. Finally, the enhanced Tc value higher than 4 K is proposed to arise from the increased density of states at Fermi energy and strong electron-phonon interaction induced by the charge density instability. Keywords: topological material, critical parameters, unconventional superconductivity.