Moiré superlattice on the surface of Sm films driven by surface valence transition

Rare earth (RE) metals exhibit unique mixed-valence behavior due to their 4f electronic configurations. In this study, we investigate the surface electronic structure and valence-mixing phenomena in samarium (Sm) films using molecular beam epitaxy (MBE), scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). A natural moiré superlattice emerges on the Sm film surface as a result of lattice mismatch between the surface divalent Sm²⁺ and bulk trivalent Sm³⁺ layers. ARPES reveals the flat f electron bands corresponding to Sm³⁺ and Sm²⁺ states, particularly demonstrating the partial coexistence of bulk Sm²⁺ flat bands. The dispersive s–d electron bands and surface-condition-induced band shifts are observed and analyzed through both STM and ARPES measurements. Our work provides direct evidence for the formation of a moiré pattern on the surface of Sm films due to its unique surface valence transition, thus paving the way for a new method to generate moiré superlattices in correlated 4f-electron systems.