Suppression of superconductivity by non-magnetic disorder in organic superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$

TL;DR

This study systematically investigates how non-magnetic disorder suppresses superconductivity in the organic superconductor $$-(BEDT-TTF)$_2$Cu(NCS)$_2$, revealing a linear relation consistent with unconventional pairing in low-disorder regimes.

Contribution

It provides a quantitative analysis of disorder effects on $T_c$ using de Haas-van Alphen measurements, confirming the Abrikosov-Gorkov relation in small-disorder regions and exploring deviations at higher disorder levels.

Findings

Linear suppression of $T_c$ with increasing disorder in low-disorder regime.

Agreement with Abrikosov-Gorkov formula in small-disorder region.

Deviations from the formula observed at high disorder levels.

Abstract

The suppression of superconductivity by nonmagnetic disorder is investigated systematically in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$. We introduce a nonmagnetic disorder arising from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A quantitative evaluation of the scattering time $\tau_{\rm dHvA}$ is carried out by de Haas-van Alphen (dHvA) effect measurement. A large reduction in $T_{\rm c}$ with a linear dependence on $1/\tau_{\rm dHvA}$ is found in the small-disorder region below $1/\tau_{\rm dHvA} \simeq$ 1 $\times$ 10$^{12}$ s$^{-1}$ in both the BMDT-TTF molecule-substituted and X-ray-irradiated samples. The observed linear relation between $T_{\rm c}$ and $1/\tau_{\rm dHvA}$ is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the small-disorder region. This observation is reasonably consistent with the unconventional superconductivity proposed thus far for the present organic superconductor. A deviation from the AG formula, however, is observed in the large-disorder region above $1/\tau_{\rm dHvA} \simeq$ 1 $\times$ 10$^{12}$ s$^{-1}$, which reproduces the previous transport study (J. G. Analytis {\it et al.}: Phys. Rev. Lett. {\bf 96} (2006) 177002). We present some interpretations of this deviation from the viewpoints of superconductivity and the inherent difficulties in the evaluation of scattering time.