Critical phenomenon of Cu doping and its correlation with superconductivity suppression in FeSe_0.4Te_0.6 single crystals

Abstract Elemental doping offers crucial insights into both the superconducting mechanisms of iron-based superconductors (IBSs) and their practical applications. In this study, we synthesized a series of Fe 1− x Cu x Se 0.4 Te 0.6 single crystals (0 ⩽ x ⩽ 0.06) using the Bridgman method, and systematically investigated their structural, electronic and superconducting properties. X-ray diffraction combined with x-ray photoelectron spectroscopy revealed that Cu doping induces anisotropic lattice distortion, featuring a -axis expansion (+0.94%) and c -axis contraction (−0.59%), with concomitant modifications in copper incorporation behavior within the FeSe 0.4 Te 0.6 lattice near the critical composition x = 0.01. Both magnetic susceptibility and transport measurements demonstrated a linear suppression of the superconducting transition temperature ( T c ), decreasing from 14 K ( x = 0) to 4.7 K ( x = 0.02), no superconducting transition was observed down to 4 K at x = 0.03, with extrapolation indicating full suppression of superconductivity at x = 0.034. A metal-to-insulator transition emerged at x ≈ 0.01, which we attributed to enhanced in-plane disorder and impurity scattering. These experimental observations align with previous theoretical studies. These results provide crucial experimental evidence for understanding doping effects in strongly correlated electron systems, particularly in IBSs.