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

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 $\hat{I}{¼}_0{H}_{c2}$ 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 $1/T_{1}T$ 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 $\hat{I}{¼}_0{H}_{c2}(0)$ 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 $\hat{I}º$-(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.

5 pages, 3 figures