We present ultra-high-resolution dilatometric studies in magnetic fields on a quasi-two-dimensional organic conductor κ-(D8-BEDT-TTF) 2 Cu[N(CN) 2 ]Br, which is located close to the Mott metal-insulator (MI) transition. The obtained thermal expansion coefficient, α(T ), reveals two remarkable features: (i) the Mott MI transition temperature T MI = (13.6 ± 0.6) K is insensitive to fields up to 10 T, the highest applied field; (ii) for fields along the interlayer b axis, a magnetic field induced (FI) phase transition at T FI = (9.5 ± 0.5) K is observed above a threshold field H c ∼ 1 T, indicative of a spin reorientation with strong magnetoelastic coupling.
Interlayer-resistivity measurements have been performed on a variety of single crystals of the quasi-two-dimensional organic superconductor k-(BEDT-TTF) 2 Cu[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 r(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 r(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 T g ¼ 77 K, the temperature T* z 40 K, where the temperature dependence of the resistivity changes rather abruptly, or the superconducting transition temperature T c . To cite this article: C.
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