High Transition Temperature Driven by Type-II Dirac Fermions in Topological Superconductor B₇Be₂B₇ Nanosheet

Topological superconductors (TSCs) offer a promising avenue for delving into exotic states of matter and fundamental physics. We propose a strategy for realizing high transition temperatures (high-<i>T</i>_c) in TSCs by leveraging nontrivial topology alongside a high carrier density near the Fermi level in metal-doped borophenes. We identified 39 candidates with exceptional thermodynamic stability from thousands of Be-intercalated borophenes (Be_1-<i>x</i>B_<i>x</i>) via extensive structural searches. Seven candidates exhibit high carrier densities, with B₇Be₂B₇ emerging as a particularly promising candidate. This nanosheet displays both type-I and type-II Dirac fermions, indicative of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Z</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math> topological metals, thereby positioning it as an ideal platform for high-<i>T</i>_c TSCs. The high-density π electrons of B₇Be₂B₇ originating from type-II Dirac fermions, coupled with the out-of-plane vibrations of B and Be atoms, significantly enhance the electron-phonon coupling (λ = 1.42), resulting in a substantially high-<i>T</i>_c of 31.5 K. These findings underscore the potential of metal-doped borophenes as a cutting-edge material platform for achieving high-<i>T</i>_c TSCs.