Anisotropic superconducting gap probed by $^{125}$Te NMR in noncentrosymmetric Sc$_6M$Te$_2$ ($M$ = Fe, Co)

TL;DR

This study uses $^{125}$Te NMR to investigate the anisotropic superconducting gap in noncentrosymmetric Sc$_6M$Te$_2$ ($M$ = Fe, Co), revealing unconventional pairing characteristics and weak electron correlations.

Contribution

It provides the first NMR evidence of anisotropic or mixed pairing symmetry in noncentrosymmetric Sc$_6M$Te$_2$ superconductors, highlighting residual density of states and weak correlations.

Findings

Suppressed Knight shift below $T_c$ with finite residual value

Power-law behavior of $1/T_1$ without coherence peak

Normal state exhibits Korringa relation indicating weak correlations

Abstract

The superconducting gap symmetry is investigated by $^{125}$Te NMR measurements on Sc$_6M$Te$_2$ ($M$ = Fe, Co) without spatial inversion symmetry. The spin susceptibility obtained from the Knight shift $K$ is suppressed below the superconducting transition temperature, while leaving a finite value down to the lowest temperature ($\simeq 0.4$ K). The nuclear spin-lattice relaxation rate $1/T_1$ follows a power law against temperature $T$ without showing a coherence peak characteristic of the isotropic gap. The result implies a pairing admixture or a residual density of states under magnetic field. The normal metallic state has a Korringa scaling relation between $1/T_1T$ and the Knight shift, reflecting a weak electron correlation.