Interplay of charge density waves, superconductivity, and band topology in a kagome boron monolayer decorated by dialkaline earth metals

Abstract

The interplay of collective electronic orders represents a central theme in condensed matter physics, and kagome lattices offer fertile ground for studying their competition and coexistence. Although metal-decorated two-dimensional boron kagome lattices can be stabilized, their quantum phases remain largely unexplored. Here, we report a family of inverse-sandwich monolayers, M2B3 (M = Mg, Ca, Sr, Ba), in which a boron kagome sheet is sandwiched between two alkaline earth metal layers, and identify them as charge density wave (CDW) materials. We found that the CDW in Mg2B3 originates from the combined effects of Fermi surface nesting and electron-phonon coupling, whereas in Ca2B3, Sr2B3, and Ba2B3, it is driven by strong electron-phonon coupling between B σ orbitals and in-plane vibrational modes. These instabilities lead to a 2 × 2 CDW ground state in Mg2B3 and 3 × 3 reconstructions in the heavier analogs, all of which are suppressed at room temperature by anharmonic lattice effects. Remarkably, Mg2B3 hosts a coexisting 2 × 2 CDW and superconductivity with a TC of 10.6 K. Under biaxial compressive strain, superconductivity coexists with nontrivial topology in the heavier compounds. Moreover, the nearly unchanged TC between the CDW ground state and the strained kagome phase implies weak competition between CDW and superconductivity. Our results establish kagome-based boron monolayers as a promising class of materials for exploring intertwined CDW, superconducting, and topological states in two dimensions.

Source-reported materials — not catalogue approval

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