Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)_2Cu[N(CN)_2]Br: Search for intrinsic scattering contributions

摘要

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 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) ∝ AT~2 dependence at low temperatures, i.e., for T_c ≤ T ≤ T_0, although with strongly sample-dependent coefficients A and upper bounds for the T~2 behavior measured by T_0. 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 T~2 dependence originating from coherent Fermi-liquid excitations.