Impurity Effect on the In-plane Penetration Depth of the Organic Superconductors $\kappa$-(BEDT-TTF)$_2X$ ($X$ = Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br)

TL;DR

This study investigates how cooling rates affect the in-plane penetration depth in organic superconductors, revealing that disorder from faster cooling increases scattering and modifies superconducting properties differently in two related compounds.

Contribution

It provides the first quantitative analysis of impurity effects on penetration depth in these organic superconductors using the London model.

Findings

Faster cooling increases impurity scattering in Cu[N(CN)$_2$]Br.

Penetration depth remains unchanged in Cu(NCS)$_2$ despite cooling rate.

Disorder effects are explained by the local-clean approximation.

Abstract

We report the in-plane penetration depth $\lambda_{\parallel}$ of single crystals $\kappa$-(BEDT-TTF)$_2X$ ($X=$ Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br) by means of the reversible magnetization measurements under the control of cooling-rate. In $X$ = Cu(NCS)$_2$, $\lambda_{\parallel}(0)$ as an extrapolation toward $T$ = 0 K does not change by the cooling-rate within the experimental accuracy, while $T_{\textrm{c}}$ is slightly reduced. On the other hand, in $X$ = Cu[N(CN)$_2$]Br, $\lambda_{\parallel}(0)$ indicates a distinct increase by cooling faster. The different behavior of $\lambda_{\parallel}(0)$ on cooling-rate between the two salts is quantitatively explained in terms of the local-clean approximation (London model), considering that the former salt belongs to the very clean system and the later the moderate clean one. The good agreement with this model demonstrates that disorders of ethylene-group in BEDT-TTF introduced by cooling faster increase the electron(quasiparticle)-scattering, resulting in shorter mean free path.