Epitaxial Stabilization and Emergent Charge Order in Copper Selenide Thin Films

We demonstrate epitaxial growth of copper selenide (Cu$_{2-x}$Se) thin films in both cubic and rhombohedral phases, achieved via molecular beam epitaxy on Al$_2$O$_3$ (001) substrates. Remarkably, the high-temperature cubic phase -- which in bulk transforms into the rhombohedral structure below 400 K -- is stabilized at room temperature and below, well outside its bulk equilibrium stability range. In the cubic phase films, temperature-dependent electrical transport reveals a pronounced, hysteretic resistivity peak near 140 K, accompanied by unit cell doubling along the [111] direction, as observed by x-ray diffraction, which are hallmarks of a charge density wave (CDW) transition. First-principles calculations show strong Fermi surface nesting in the cubic phase, consistent with the observed CDW instability. In contrast, the rhombohedral films exhibit suppressed nesting and no structural modulation. These results not only unambiguously identify a previously unreported CDW in Cu$_{2-x}$Se thin films, but also establish an epitaxial platform for tuning emergent electronic phases via strain and interface engineering.