Nuclear magnetic resonance measurements were performed on ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ in the presence of a magnetic field close to the upper critical field ${\mu }_0{H}_{\mathrm{c}2}$ 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 $1/T_{1}T$ abruptly decreased below the SC transition temperature $T_{\mathrm{c}}(H)$, a phenomenon understood within the framework of conventional spin-singlet superconductivity. In contrast, $1/T_{1}T$ was enhanced just below $T_{\mathrm{c}}(H)$ and exhibited a broad maximum when magnetic fields close to ${\mu }_0{H}_{\mathrm{c}2}(0)$ were applied parallel or perpendicular to the $c$ axis; although the Knight shift decreased just below $T_{\mathrm{c}}(H)$. This enhancement of $1/T_{1}T$, which was recently observed in the organic superconductor $\kappa \text{−}(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$, suggests the presence of high-density Andreev bound states in the inhomogeneous SC region, a hallmark of the Fulde-Ferrell-Larkin-Ovchinnikov phase.

• Received 19 May 2018
• Revised 2 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.157004