Intrinsic vs. extrinsic inelastic scattering contributions in κ-(BEDT-TTF)2Cu[N(CN)2]Br – Transport measurements under hydrostatic pressure

Interlayer-resistivity measurements have been performed on a variety of single crystals of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. These crystals, which have been synthesized along two somewhat different routes, reveal strongly sample-dependent resistivity profiles: while the majority of samples shows a more or less pronounced ρ(T) maximum around 90 K with a semiconducting behaviour above, some crystals remain metallic at all temperatures T ≤ 300 K. In the absence of significant differences in the crystals' structural parameters and chemical compositions, as proved by high-resolution X-ray and electron-probe-microanalysis [C. Strack et al., Phys. Rev. B 72 (2005) 054511], these results indicate that real structure phenomena, i.e. disorder and/or defects, may strongly affect the inelastic scattering. Comparative resistivity measurements under He-gas pressure on two crystals with strongly differing ρ(T) profiles indicate that these additional, sample-dependent scattering contributions are characterized by an extraordinarily strong pressure response which is highly non-monotonous as a function of temperature. No correlations have been found between the strength of these scattering contributions and other characteristic properties such as the glass transition at Tg = 77 K, the temperature T∗ ≈ 40 K, where the temperature dependence of the resistivity changes rather abruptly, or the superconducting transition temperature Tc.