Phonon-mediated superconductivity in ternary silicides ${X}_{4}\mathrm{CoSi}$ ($X=\mathrm{Nb}, \mathrm{Ta}$)

Phonon-mediated superconductivity in ternary silicides $X_{4} CoSi$ ( $X = Nb, Ta$ )

Jonas Bekaert

Phys. Rev. B 108, 134504 – Published 17 October 2023

Abstract

The superconducting properties of two recently synthesized ternary silicides with unit formula $X_{4} CoSi$ ( $X = Nb, Ta$ ) are investigated through ab initio calculations combined with Eliashberg theory. Interestingly, their crystal structure comprises interlocking honeycomb networks of Nb/Ta atoms. ${Nb}_{4} CoSi$ is found to harbor better conditions for phonon-mediated superconductivity, as it possesses a higher density of states at the Fermi level, fostering stronger electron-phonon coupling. The superconducting critical temperatures ( $T_{c}$ ) follow the same trend, with ${Nb}_{4} CoSi$ having a twice higher value than ${Ta}_{4} CoSi$ . Furthermore, the calculated $T_{c}$ values (5.9 K vs 3.1 K) agree excellently with the experimentally obtained ones, establishing superconductivity in this new materials class as mediated by the electron-phonon coupling. Furthermore, my calculations show that the superconducting properties of these compounds do not simply correlate with the parameters of their honeycomb networks, contrary to proposals raised in the literature. Rather, their complete fermiology and phonon spectrum should be taken into account in order to explain their respective superconducting properties.