Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero effect along the in-plane c axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end point of the first-order transition line enabling us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-0.15 at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.

We present a detailed structural investigation of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br at temperatures T from 9 to 300 K. Anomalies in the T dependence of the lattice parameters are associated with a glass-like transition previously reported at Tg = 77 K. From structure refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space group Pnma, is established at all temperatures. Further, we extract the T dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %, with an increase to 97(3) % at 9 K. We conclude that the glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups.Due to their exotic superconducting-and normal-state properties, which resemble those of the high-T c cuprates, the organic charge-transfer salts κ-(BEDT-TTF) 2 X, X = Cu(NCS) 2 , Cu[N(CN) 2 ]Br and Cu[N(CN) 2 ]Cl have been intensively studied in recent years 1,2,3,4,5,6 . Resulting from their layered crystal structure, the electronic properties of these materials are quasi-twodimensional. This leads to electronic ground state properties for the series of materials which are commonly summarized within a conceptual phase diagram, with the antiferromagnetic insulator X = Cu[N(CN) 2 ]Cl and the correlated metals X = Cu[N(CN) 2 ]Br, Cu(NCS) 2 on opposite sites of a bandwidth-controlled Mott transition 7 .

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. According to 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, (iii) both variants reveal a ρ(T) ∝ AT 2 dependence at low temperatures, i.e. for Tc ≤ T ≤ T0, although with strongly sample-dependent coefficients A and upper bounds for the T 2 behavior measured by T0. The latter result is inconsistent with the T 2 dependence originating from coherent Fermi-liquid excitations.

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To resolve a superstructure formation previously reported for the organic superconductor κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, we present synchrotron x-ray diffraction experiments carried out at the MAGS beamline at BESSY, HZB. Surprisingly, in our low temperature (28K) experiments, when searching kspace at (h 0 3.5), h = 7, 8 and (h 0 0.5), h = 5, 7, for none of these spots we could detect scattering intensity associated to a superstructure formation, in contradiction to previous reports. Our data suggest that details of the structural properties of κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br-such as superstructure formation-sensitively depend on sample handling, e.g., cooling rates (in our case 4K/min), or thermal cycling. A direct relationship between superstructure formation and terminal ethylene group ordering cannot be verified, disproving proposals put forth previously.

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