Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-2D organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br: quest for instrinsic scattering contributions

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 90 K, is sample dependent, i.e., extrinsic in nature; (ii) the abrupt change in ρ(T) at T* ≈ 40 K from a strongly temperature-dependent behavior at T>T* to an only weakly T-dependent ρ(T) at T2 dependence at low temperatures, i.e., for T_c≤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.