Coexistence of superconductivity and topological aspects in beryllenes

Abstract

By employing first-principles calculations, we theoretically propose that α- and β-beryllenes are two dimensional superconductors with Dirac bands at ambient pressure. Be-p orbitals dominate the electronic structure near the Fermi level, with two-band and three-band Fermi surfaces for α- and β-beryllene, respectively. α-beryllene hosts intrinsic type-I Dirac Fermions with the existence of nontrivial edge states and topological Z2 index, while β-beryllene has both type-I and type-II Dirac cones around the Fermi level and is topologically trivial. Migdal-Eliashberg theory combined with electron-phonon calculations predicts that both α- and β-beryllene are single-gap superconductors with critical temperature Tc = 9.9 K and 12.6 K, respectively. The difference is ascribed to the structure. Specifically, β-beryllene can be viewed as a bilayer of α-beryllene with a resulting increased density of states and enhanced electron-phonon coupling. And additionally, the Fermi surface of β-beryllene is strongly nested leading to Kohn anomalies that enhance the electron-phonon coupling λ. Replacing Be with heavier group-II atoms (Mg, Ca, Sr, and Ba) leads to strongly suppressed Tc due to electronic structure changes and the ensuing lower λ as well as lower phonon frequencies that reduce the prefactor. Our findings suggest beryllenes are promising two-dimensional elemental platforms for studying superconductivity and Dirac band topology.