σ-electron-driven superconductivity in two-dimensional B6S4 stabilized by structural resonance

Abstract

Two-dimensional (2D) boron-based materials are promising candidates for high-temperature superconductors. Authors of most studies have focused on stabilizing honeycomb and kagome boron layers by intercalating metals to tune σ- and π-electron states at the Fermi level. However, the specific roles of these electrons in superconductivity remain unclear. Here, we propose a structural resonance strategy to stabilize 2D boron-based materials, exemplified by B6X4 (X = S, Se, and Te) monolayers. These structures, composed of B6 hexagonal rings and B3X2 trigonal bipyramidal units, enforce exclusive σ bonding within B6 rings due to the tetrahedral bonding configuration of boron. Meanwhile, variations in the electronegativity of X elements modulate the electron occupancy of B–B bonds, thereby altering the Fermi-level electronic states. First-principles calculations confirm that these structures exhibit high cohesive energy and strong dynamical stability. Among them, B6S4, which has the largest B–B σ-orbital electron contribution at the Fermi level, displays the highest superconducting transition temperature (Tc≈33 K). This sharply contrasts with the lower Tc values of B6Se4 (24 K) and B6Te4 (1.71 K), which correspond to a reduced B–B σ-electron contribution. Additionally, B6S4 exhibits single-gap superconductivity dominated by B-B σ electrons, which is different from multigap boron-based superconductors that rely on both σ and π electrons. These findings highlight the critical role of B–B σ-orbital electrons in superconductivity and provide insights into the design of 2D boron-based superconductors with tunable electronic properties.

Source-reported materials — not catalogue approval

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