Superconductivity in zerovalent copper intercalated Bi₂Se₃ crystals

In order to understanding of the role of Cu atoms to the formation of superconducting quasiparticles. A series of Cu_xBi₂Se₃ (x= 0~ 0.3) crystals prepared with Bridgman method followed by an electrochemical technique (EC method) at room temperature. The existence of copper confirmed by means of XRD, SEM and TEM EDS mapping as well. Where the obvious increasing on lattice c confirm the intercalating of copper atoms rather than the substitution. The further study on temperature dependent electrical resistance and magnetic susceptibility revealed a superconducting transition at about T_C = 3 ~ 3.5 K. As an illustration, optimized 10 at% of Cu, a clear superconducting transition at T_C = 3.4 K with large shielding fraction (volume susceptibility) of ~ 95% is observed. The characteristic peaks of the zerovalent chemical state of copper confirmed by x-ray photoelectron spectra (XPS) and Auger electron spectra (AES). By the electron energy loss spectroscopy (EELS), near-edge fine structure revealed the absence of Cu¹⁺ and Cu²⁺ characterization peaks, which confirmed the zerovalent of intercalated copper atom. The results of this work suggested that the formation of superconducting quasiparticles do not relate to the charge transfer of copper.