Dirac surface states, multiorbital dimerization, and superconductivity in Nb- and Ta-based A15 compounds

Abstract

Using first-principle calculations, we investigate the electronic, topological, and superconducting properties of Nb3X (X=Ge, Sn, Sb) and Ta3Y (Y=As, Sb, Bi) A15 compounds. We demonstrate that these compounds host Dirac surface states, which are related to a nontrivial Z2 topological value. The spin-orbit coupling (SOC) splits the highly degenerate R point close to the Fermi level enhancing the amplitude of the spin Hall conductance. Indeed, despite the moderate spin orbit of the Nb-compounds, a large spin Hall effect is also obtained in Nb3Ge and Nb3Sn compounds. We show that the Coulomb interaction opens the gap at the R point thus making the occurrence of Dirac surface states more obvious. We then investigate the superconducting properties by determining the strength of the electron-phonon BCS coupling. The evolution of the critical temperature is tracked down to the 2D limit indicating a reduction of the transition temperature, which mainly arises from the suppression of the density of states at the Fermi level. Finally, we propose a minimal tight-binding model based on three coupled Su-Schrieffer-Heeger chains with t2g Ta and Nb orbitals reproducing the spin-orbit splittings at the R point among the π-bond bands in this class of compounds. We separate the kinetic parameters in π and δ bonds, in intradimer and interdimer hoppings, and discuss their relevance for the topological electronic structure. We point out that Nb3Ge might represent a Z2 topological metal with the highest superconducting temperature ever recorded.

Source-reported materials — not catalogue approval

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