Volovik Effect and Fermi-Liquid Behavior in the s-Wave Superconductor CaPd2As2: 75As NMR-NQR Measurements

TL;DR

This study uses $^{75}$As NMR and NQR measurements to investigate the electronic and magnetic properties of CaPd$_2$As$_2$, revealing conventional s-wave superconductivity and the Volovik effect through detailed relaxation rate and Knight shift analysis.

Contribution

It provides the first clear experimental evidence of the Volovik effect in an s-wave superconductor using NMR/NQR techniques.

Findings

Absence of magnetic correlations in the normal state.

Observation of the Hebel-Slichter peak below T_c.

Suppression of the peak under magnetic field indicating the Volovik effect.

Abstract

The electronic and magnetic properties of the collapsed-tetragonal CaPd$_2$As$_2$ superconductor (SC) with a transition temperature of 1.27 K have been investigated by $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1/$T_1$) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, 1/$T_1$ measured by $^{75}$As NQR shows a clear Hebel-Slichter (HS) peak just below $T_{\rm c}$ and decreases exponentially at lower $T$, confirming a conventional $s$-wave SC. In addition, the Volovik effect, also known as Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.