NMR study of the vortex slush phase in organic superconductor $\kappa \text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$

The vortex state in a single crystal of the layered organic superconductor $\kappa \text{−}\left(\mathrm{BEDT}\text{−}{\mathrm{TTF})}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$, where BEDT-TTF (or ET) is bis(ethylenedithio)tetrathiafulvalene, was studied by ${}^1\mathrm{H}\text{−}\mathrm{NMR}$. Under a low field region around $0.75\mathrm{T}$, the vortex-glass-liquid transition was demonstrated by a diverging of the longitudinal nuclear spin relaxation rate and peak broadening in spectra. Under a high field region near the upper critical field $H_{c2}\left(0\right)\simeq 7\mathrm{T}$, the curvature of nuclear spin relaxation curves showed a drastic change at the temperature where the emergence of the quantum vortex slush state was reported. The mechanism in this curvature change was discussed in terms of the fluctuating field produced by fragments of vortex glass.

Figure 1

HT-phase diagram of $\kappa \text{−}\left(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2\mathrm{Cu}\right(\mathrm{NCS}{)}_2$ reported by Sasaki et al. (Ref. 1) and the experimental conditions of temperature range and fields performed in this paper, denoted by two grey thick dotted lines. Each curve shows the boundary between the regions of the normal state, the vortex-liquid phase, the vortex-glass phase, the Bragg-glass phase, and the quantum vortex slush state. Symbols of data points on curves are the same as in Ref. 1.

Figure 2

Temperature dependence of $T_1^{-1}$ and the peak width (FWHM) of ${}^1\mathrm{H}\text{−}\mathrm{NMR}$ on the sample $1$ under the low field of $0.75\mathrm{T}$, which was applied perpendicular to the conducting $bc$ plane. Solid and dashed curves are guides for the eyes. The glass melting temperature $T_M$ and the critical temperature $T_c$ under $H_0$ is shown by solid arrows. The inset shows typical spectra profiles. Horizontal dashed arrows indicate the definition of the linewidth (FWHM).

Figure 3

Typical relaxation curves at high field around $5\mathrm{T}$, (a) for initial part within $60\mathrm{s}$ for the sample 1, and (b) within ${10}^3\mathrm{s}$ of the sample 2. Solid and dashed curves indicate the power-law function.

Figure 4

Temperature dependence of $\alpha$, the power-law index of the relaxation curve, the time evolution of which obeys the power law ${\tau }^{\alpha }$. The dashed curves are guides for the eyes.