Evidence for the presence of the Fulde-Ferrell-Larkin-Ovchinnikov state in CeCu$_2$Si$_2$ revealed using $^{63}$Cu NMR

TL;DR

This study provides NMR evidence for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in CeCu$_2$Si$_2$, showing enhanced relaxation rates near the upper critical field, indicating inhomogeneous superconductivity with high-density Andreev bound states.

Contribution

First direct NMR evidence of the FFLO state in CeCu$_2$Si$_2$, revealing inhomogeneous superconductivity near pair-breaking fields.

Findings

Enhanced $1/T_1T$ near $H_{c2}$ suggests FFLO phase.

Decreased Knight shift indicates spin-singlet pairing.

Observation of high-density Andreev bound states in the inhomogeneous SC region.

Abstract

Nuclear magnetic resonance measurements were performed on CeCu$_{2}$Si$_{2}$ in the presence of a magnetic field close to the upper critical field $\mu_{0} H_{\rm c2}$ in order to investigate its superconducting (SC) properties near pair-breaking fields. In lower fields, the Knight shift and nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreased below the SC transition temperature $T_{\rm c}(H)$, a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, $1/T_1T$ was enhanced just below $T_{\rm c}(H)$ and exhibited a broad maximum when magnetic fields close to $\mu_0H_{\rm c2}(0)$ were applied parallel or perpendicular to the $c$ axis, although the Knight shift decreased just below $T_{\rm c}(H)$. This enhancement of $1/T_1T$, which was recently observed in the organic superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.