Robust surface superconductivity induced by hexagonal diamond motif stabilized on cubic diamond (111) surface

Surface superconductivity is of scientific interest and technological significance. However, materials exhibiting robust surface superconductivity while preserving structurally stable bulk phases remain scarce. Here, we report an eclipsed configuration on the (111) surface of cubic diamond (ECDS), which resembles a hexagonal diamond motif and is stabilized by a periodic arrangement of boron and carbon atoms on the surface. First-principles calculations reveal that this structure is significantly more stable than the conventional staggered cubic diamond surface (SCDS). The enhanced stability is associated with hyperconjugation effects, resulting from wave-function overlap between surface C--B antibonding orbitals and subsurface C--C bonding orbitals. ECDS exhibits metallic behavior characterized by a two-dimensional hole band derived from the ${p}_{x}$ and ${p}_{y}$ orbitals of surface C and B atoms. Remarkably, it supports strong surface superconductivity with a transition temperature of 40.4 K, driven by the coupling between in-plane electronic states and out-of-plane vibrational modes of C--B bonds at the surface. These findings highlight the potential of chemically engineered diamond surfaces as a platform for achieving surface superconductivity, with promising applications in advanced materials.