Inherent surface superconducting phase in the Mo8Ga41 single-gap bulk superconductor as seen by nuclear resonance and tunneling spectroscopy

Abstract

A comprehensive study of the intermetallic superconductor Mo8Ga41 was conducted by means of HRPXRD, ab-initio calculations, NMR and NQR spectroscopy on 69Ga and 71Ga nuclei, as well as tunneling spectroscopy. All experimental methods used demonstrated the presence of an inherent surface superconducting phase in addition to the bulk superconducting phase. For the latter, the s-wave type single-gap superconducting behavior was observed with a slightly increased 2Δ0/kBTc = 3.89 in agreement with the known enhanced electron-phonon coupling in this compound. From nuclear spin-lattice relaxation experiment, the estimated superconducting gap value of the surface phase was 2Δ*0 = 22.2 K, which corresponds to the T*c ≈ 6.3 K in the weak-coupling BCS limit. Meanwhile, the tunneling spectroscopy reveals similar values of T*c ∼ 6.0–7.3 K for this surface phase. Thus, the known evidence of the Mo8Ga41 multi-gap behavior can be explained by the influence of the inherent surface superconducting phase.