Measurement of electronic structure and surface reconstruction in the superionic Cu 2 − x Te

Recently, layered copper chalcogenides ${\mathrm{Cu}}_{2}X$ family $(X=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ has attracted tremendous research interests due to their high thermoelectric performance, which is partly due to the superionic behavior of mobile Cu ions, making these compounds ``phonon liquids.'' Here, we systematically investigate the electronic structure and its temperature evolution of the less studied single crystal ${\mathrm{Cu}}_{2\ensuremath{-}x}\mathrm{Te}$ by the combination of angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscope/spectroscopy (STM/STS) experiments. While the band structure of the ${\mathrm{Cu}}_{2\ensuremath{-}x}\mathrm{Te}$ shows agreement with the calculations, we clearly observe a $2\ifmmode\times\else\texttimes\fi{}2$ surface reconstruction from both our low temperature ARPES and STM/STS experiments which survives up to room temperature. Interestingly, our low temperature STM experiments further reveal multiple types of reconstruction patterns, which suggests the origin of the surface reconstruction being the distributed deficiency of liquidlike Cu ions. Our findings reveal the electronic structure and impurity level of ${\mathrm{Cu}}_{2}\mathrm{Te}$, which provides knowledge about its thermoelectric properties from the electronic degree of freedom.