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Abstract

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the κ-(BEDT-TTF)2Cu[N(CN)2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. Judging by their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous ρ(T) maximum in standard HR crystals around 90K, is sample dependent, i.e., extrinsic in nature; (ii) the abrupt change in ρ(T) at T∗≈40K from a strongly temperature-dependent behavior at T>T∗ to an only weakly T-dependent ρ(T) at T<T∗ is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, and (iii) both variants reveal a ρ(T)∝AT2 dependence at low temperatures, i.e., for Tc⩽T⩽T0, although with strongly sample-dependent coefficients A and upper bounds for the T2 behavior measured by T0. Provided that there are no differences in the Fermi surface between both variants—the present experiments give no indications for such differences—the latter result is inconsistent with the T2 dependence originating from coherent Fermi-liquid excitations.