Anomalous superconducting state in LiFeAs implied by the $^{75}$As Knight shift measurement

TL;DR

This study uses $^{75}$As NMR to reveal an unusual superconducting state in LiFeAs, characterized by a constant Knight shift over a temperature range, suggesting proximity to a critical instability influences its superconducting properties.

Contribution

The paper reports the discovery of an anomalous superconducting state in LiFeAs with a constant Knight shift, indicating a novel behavior linked to its critical instability proximity.

Findings

Identification of two superconducting transition temperatures.

Observation of a constant Knight shift in the superconducting state.

Association of the anomalous state with LiFeAs's proximity to critical instability.

Abstract

$^{75}$As NMR investigation of a single crystal of superconducting LiFeAs is presented. The Knight shift and the \textit{in situ} ac susceptibility measurements as a function of temperature and external field are indicative of two superconducting (SC) transition temperatures, each of which is associated with its own upper critical field. Strikingly, the Knight shift maintains its normal state value over a temperature range in the SC state before it drops abruptly being consistent with spin-singlet pairing. Together with our previous NMR study, the anomalous SC state featured by the constant Knight shift is attributed to the extremely sensitive SC properties of LiFeAs, probably stemming from its proximity to a critical instability.