Emergent high-temperature superconductivity in two-dimensional XB2 (X = Si, Ge, Sn) monolayers via multicenter bonding

Superconductivity in two-dimensional (2D) materials has attracted considerable attention due to their unique physical properties and potential for high-temperature operation. Boron-based 2D compounds are particularly promising, thanks to their structural flexibility and the emergence of strong electron-phonon coupling (EPC) associated with light elements. While most previous studies have focused on stabilizing boron sheets through metal incorporation, we propose an alternative approach based on multicenter bonding enabled by group-IV non-metallic elements (Si, Ge, Sn). The resulting XB2(X = Si, Ge, Sn) monolayers, which adopt a MgB2-like monolayer configuration, are stabilized by a seven-center two-electron (7c-2e) bonding network between the X atoms and the boron honeycomb lattice. This bonding lowers the energy of the B-pzorbitals and enhances lattice stability. The superconducting transition temperature (Tc) increases significantly with the atomic number ofX—from 4.7 K in SiB2to 13.3 K in GeB2and 24.9 K in SnB2—driven by an increased carrier density near the Fermi level (EF) and softening of the high-frequencyE2phonon mode. Furthermore, we design a SnB4monolayer, in which a Sn layer is sandwiched between the two boron layers. This structure enriches in-plane phonon modes and strengthens EPC, yielding aTcof 38 K, close to the McMillan limit. These findings highlight the critical role of multicenter bonding and targeted phonon engineering in enabling high-Tc2D boron-based superconductors.