Multi-band superconductivity driven by a site-selective mechanism in Mo$_8$Ga$_{41}$

Abstract

The family of the endohedral gallide cluster compounds recently emerged as a new family of superconductors which is expected to host systems displaying unconventional physics. Mo$_8$Ga$_{41}$ is an important member of this family which shows relatively large $T_c \sim$ 10 K and has shown indications of strong electron-phonon coupling and multi-band superconductivity. Here, through direct measurement of superconducting energy gap by scanning tunneling spectroscopy (STS) we demonstrate the existence of two distinct superconducting gaps of magnitude 0.85 meV and 1.6 meV respectively in Mo$_8$Ga$_{41}$. Both the gaps are seen to be conventional in nature as they evolve systematically with temperature as per the predictions of BCS theory. Our band structure calculations reveal that only two specific Mo sites in an unit cell contribute to superconductivity where only $d_{xz}$/$d_{yz}$ and $d_{x^2-y^2}$ orbitals have strong contributions. Our analysis indicates that the site-elective contribution govern the two-gap nature of superconductivity in Mo$_8$Ga$_{41}$.