Influence of internal disorder on the superconducting state in the organic layered superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br

TL;DR

This study investigates how internal disorder affects the superconducting properties of the layered organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br, revealing that cooling procedures and sample origin influence the observed order parameter symmetry.

Contribution

It demonstrates that controlled cooling and sample history critically impact the superconducting state, reconciling previous contradictory findings regarding the order parameter symmetry.

Findings

Optimal cooling procedures depend on sample origin.

Ground state exhibits unconventional d-wave behavior.

Disorder influences the temperature dependence of penetration depth.

Abstract

We report high-sensitivity AC susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br. We have studied nominally pure single crystals from the two different syntheses and employed controlled cooling procedures in order to minimize intrinsic remnant disorder at low temperatures associated with the glass transition, caused by ordering of the ethylene moieties in BEDT-TTF molecule at T_G = 75 K. We find that the optimal cooling procedures (slow cooling of -0.2 K/h or annealing for 3 days in the region of T_G) needed to establish the ground state, depend critically on the sample origin indicating different relaxation times of terminal ethylene groups. We show that, in the ground state, the behavior observed for nominally pure single crystals from both syntheses is consistent with unconventional d-wave order parameter. The in-plane penetration depth lambda_in(T) is strongly linear, whereas the out-of-plane component lambda_out(T) varies as T^2. In contrast, the behavior of single crystals with long relaxation times observed after slow (-0.2 K/h) cooling is as expected for a d-wave superconductor with impurities (i.e. lambda_in(T) propto lambda_out(T) propto T^2) or might be also reasonably well described by the s-wave model. Our results might reconcile the contradictory findings previously reported by different authors.