# Absence of the Pauli-Paramagnetic Limit in a Superconducting U$_6$Co

## TL;DR

This study uses NMR measurements to show that U$_6$Co superconductors do not exhibit the Pauli-paramagnetic limit, supporting a full-gap s-wave pairing mechanism with negligible spin susceptibility changes.

## Contribution

It provides the first NMR evidence that U$_6$Co lacks the Pauli-paramagnetic limit, indicating unique superconducting properties related to its small spin susceptibility.

## Key findings

- Negligible change in the spin part of the Knight shift in the SC state.
- Absence of the Pauli-paramagnetic effect in U$_6$Co.
- Exponential behavior of $1/T_1$ at low temperatures supports full-gap s-wave pairing.

## Abstract

We performed $^{59}$Co nuclear magnetic resonance (NMR) measurements of single-crystalline U$_6$Co. There is a small decrease in the Knight shift in the superconducting (SC) state, but this change mainly arises from the SC diamagnetic effect. The negligible change of the spin part of the Knight shift, together with the absence of the Pauli-paramagnetic effect in the SC U$_6$Co, is understood as a consequence of the small spin susceptibility. The nuclear spin-lattice relaxation rate $1/T_1$ is also measured in the SC state under the magnetic field, and exhibits a tiny Hebel-Slichter peak just below the SC transition temperature and exponential behavior at lower temperatures. These behaviors are in agreement with the full-gap s-wave pairing in U$_6$Co.