Zero Cu valence and superconductivity in high-quality CuxBi2Se3 crystal

Abstract

In this work, the role of Cu dopants in the development of superconductivity in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ is investigated. A series of $\mathrm{C}{\mathrm{u}}_{x}\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ ($x=0--0.3$) crystals is grown using the Bridgman method and electrochemical techniques. Based on the observable lattice increases along the $c$ axis, the existence of Cu atoms intercalated in the van der Waals (vdW) gaps in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ is verified by x-ray diffraction, scanning electron microsopy--energy dispersive spectroscopy, and transmission electron microscopy analysis. Furthermore, the chemical state of the Cu is found to be zero valence by characterization of the x-ray photoelectron and Auger electron spectra. The absence of $\mathrm{C}{\mathrm{u}}^{1+}$ and $\mathrm{C}{\mathrm{u}}^{2+}$ in the electron energy-loss spectroscopy near-edge fine structure is confirmed as well. Meanwhile, our Raman data also show the same result: the intercalation of Cu in the vdW gap which weakens the binding of the quintuple layers in the $\mathrm{C}{\mathrm{u}}_{0.1}\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ crystal. The electric resistance and magnetic susceptibility show a superconducting transition near 3--3.4 K in the series of Cu-doped $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. A sharp superconducting transition with the highest value of ${T}_{C}=3.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and the largest magnetic shielding fraction of 84% is observed in the optimized 10% Cu-doped $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. These results imply that the formation of superconducting quasiparticles is not related to the charge transfer of Cu, but is supported by the internal stress of Cu intercalated in the van der Waals gap. The superconducting transition observed in the specific heat implies that the superconductivity in $\mathrm{C}{\mathrm{u}}_{x}\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ is unconventional.