Structure-dependent Electron–Phonon Coupling and Superconductivity in Two-dimensional MB7 (M = Cr, Mn, Fe) Monolayers

Abstract

We present a systematic first-principles study of two-dimensional MB7 (M = Cr, Mn, Fe) monolayers, focusing on their structural, electronic, vibrational, and superconducting properties. CrB7 and FeB7 exhibit transition-metal-stabilized boron frameworks, consisting of a distorted hexagonal boron layer sandwiched between two kagome boron layers, where the structural stability is enhanced by charge transfer from transition-metal atoms. Phonon calculations demonstrate that CrB7 is dynamically stable, whereas FeB7 exhibits only a weak soft mode associated with the intrinsic flexibility of two-dimensional systems.In contrast, the high-symmetry MnB7 structure exhibits phonon instability and spontaneously reconstructs into a lower-energy monoclinic MnB7 phase. Based on the dynamically stable structures and the reconstructed monoclinic phase, we further investigate their phonon-mediated superconducting properties. All investigated stable and reconstructed phases exhibit phonon-mediated superconductivity, with electron–phonon coupling (EPC) primarily originating from low-frequency metal-associated phonon modes. These results reveal the crucial roles of structural stability, electronic states, and phonon characteristics in regulating superconductivity in two-dimensional transition-metal borides, providing insights into the design of low-dimensional boron-based superconducting materials.