Electronic Structure of Metastable bcc-Cu Ultra-thin Films Grown on FeCo(100) Probed by Angle-resolved Photoemission Spectroscopy

We report an experimental investigation of the electronic band structure of metastable body-centered cubic (bcc)-Cu ultra-thin films epitaxially grown on FeCo(100) substrates. High-quality bcc-Cu films were fabricated by molecular beam epitaxy, enabling in-situ synchrotron angle-resolved photoemission spectroscopy measurements with sufficient energy and momentum resolution. The Fermi surface morphology of bcc-Cu was distinguished from that of conventional face-centered cubic-Cu and showed characteristic features consistent with the bcc structure. Overall, the experimentally observed band dispersions exhibit good qualitative agreement with density functional theory (DFT) calculations. However, the Cu-derived bands are systematically shifted toward higher binding energies compared to bulk-based DFT predictions. These deviations are likely caused by a combination of intrinsic limitations of the DFT approximation and interface-induced effects such as charge transfer at the Cu/FeCo heterointerface. The present results highlight the importance of experimentally determining the electronic structure of metastable ultra-thin films and demonstrate that interface effects must be considered when discussing Fermi surface matching and transport properties in Cu/FeCo heterostructures.