Influence of internal disorder on the superconducting state in the organic layered superconductorκ−(BEDT−TTF

Abstract

We report high-sensitivity ac susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor -(BEDT-TTF) 2 Cu͓N(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 ϭ75 K. We find that the optimal cooling procedures ͑slow cooling of Ϫ0.2 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 in (T) is strongly linear, whereas the out-of-plane component out (T) varies as T 2 . In contrast, the behavior of single crystals with long relaxation times observed after slow (Ϫ0.2 K/h) cooling is as expected for a d-wave superconductor with impurities ͓i.e., in (T)ϰ out (T)ϰT 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.

In-plane superfluid density and microwave conductivity of the organic superconductorκ-(BEDT-TTF)2Cu[N(CN)2]Br: Evidence forMilbradt1, Bardin2, Truncik3 et al. 2013Phys. Rev. B354461View full textAdd to dashboardCite

“…We observe upward curvature of ρ s (T ) in this vicinity, consistent with the superfluid density becoming frequency dependent as the sample enters the vortex-plasma regime, an effect that is prominent in cuprates. 60,61 A similar analysis carried out on previously published data, [26][27][28][29][47][48][49][50] in which ρ s (T ) is 4 to 40 times smaller, predicts vortex unbinding in the range 3 to 8 K. This is not observed. Finally, our data have recently been shown to be consistent 62 with the Homes scaling law 63 relating superfluid density to the product of normal-state conductivity and T c .…”

“…[26][27][28][29][47][48][49][50] Since this amounts to a substantial revision, it is critical that we check its validity. The principle measurement uncertainties in λ arise from the surface impedance scale factor and the normal-state matching technique used to determine absolute reactance.…”

Interplay between the glassy transition and granular superconductivity in organic materials

“…Some of these properties were ascribed to the appearance of a magnetic transition at low temperature (10-20 K), the meaning of which is unclear: Several mechanisms have been proposed including spin-density waves 13 , spin canting 12 , or the suppression of superconductivity by dispersed magnetic ions associated with the persistent disorder 14 . Also, some authors 7,8 3 ascribed the decrease of the apparent superconducting volume to an increase in λ (the inplane penetration depth) through mean-free-path effect 7 .…”

“…To account for such a decrease, one is led to assume that λ can reach unphysical values, as high as 0.1 mm [8] (and much more if applied to the present results). 3) Despite these large changes in the apparent j c and λ , T c stays almost constant.…”

“…Some of these properties were ascribed to the appearance of a magnetic transition at low temperature (10-20 K), the meaning of which is unclear: Several mechanisms have been proposed including spin-density waves 13 , spin canting 12 , or the suppression of superconductivity by dispersed magnetic ions associated with the persistent disorder 14 . Also, some authors 7,8 3 ascribed the decrease of the apparent superconducting volume to an increase in λ (the inplane penetration depth) through mean-free-path effect 7 .However none of these mechanisms is able to explain, even qualitatively, the following experimental facts: 1) The fact that the apparent magnetic critical current density j c (as deduced from the Bean model) always decreases as the degree of quenched disorder increases.2) The low-field susceptibility also decreases in the same experimental conditions.To account for such a decrease, one is led to assume that λ can reach unphysical values, as high as 0.1 mm [8] (and much more if applied to the present results). 3) Despite these large changes in the apparent j c and λ , T c stays almost constant.…”