Low-temperature structural and transport anomalies inCu2Se

Through systematic examination of symmetrically nonequivalent configurations, first-principles calculations have identified a new ground state of ${\mathrm{Cu}}_{2}\mathrm{Se}$, which is constructed by repeating sextuple layers of Se-Cu-Cu-Cu-Cu-Se. The layered nature is in accord with electron and x-ray diffraction studies at and below room temperature and also is consistent with transport properties. Magnetoresistance measurements at liquid helium temperatures exhibit cusp-shaped field dependence at low fields and evolve into quasilinear field dependence at intermediate and high fields. These results reveal the existence of weak antilocalization effect, which has been analyzed using a modified Hikami, Larkin, and Nagaoka model, including a quantum interference term and a classical quadratic contribution. Fitting parameters suggest a quantum coherence length $L$ of 175 nm at 1.8 K. With increasing temperature, the classical parabolic behavior becomes more dominant, and $L$ decreases as a power law of ${T}^{\ensuremath{-}0.83}$.