Epitaxial Stabilization and Emergent Charge Order in Copper Selenide Thin Films

TL;DR

This study reports the epitaxial stabilization of copper selenide thin films in cubic and rhombohedral phases, revealing a charge density wave transition in the cubic phase and demonstrating the potential for tuning electronic phases through epitaxial growth.

Contribution

It introduces a method to stabilize high-temperature phases of Cu$_{2-x}$Se at room temperature and uncovers a new charge density wave in epitaxial thin films.

Findings

Cubic phase stabilized at room temperature outside bulk stability.

Observation of a hysteretic resistivity peak near 140 K indicating CDW transition.

First-principles calculations confirm Fermi surface nesting related to CDW.

Abstract

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.