High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)(2)-Cu[N(CN)(2)]Br in the superconducting ( B = 0) and normal ( B = 14 T) states show a clearly resolvable anomaly at T(c) = 11.5 K and an electronic contribution, C(es), which can be reasonably well described by strong-coupling BCS theory. Most importantly, C(es) vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.

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The magnetic-field, temperature, and angular dependence of the interlayer magnetoresistance of two different quasi-two-dimensional ͑2D͒ organic superconductors is reported. For -(BEDT-TTF) 2 I 3 , where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, we find a well-resolved peak in the angle-dependent magnetoresistance at ⌰ϭ90°͑field parallel to the layers͒. This clear-cut proof for the coherent nature of the interlayer transport is absent for ␤Љ-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 . This and the nonmetallic behavior of the magnetoresistance suggest an incoherent quasiparticle motion for the latter 2D metal.

The quasi-two-dimensional organic superconductor β ′′ -(BEDT-TTF)2SF5CH2CF2SO3 (Tc ≈ 4.4 K) shows very strong Shubnikov-de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, R b . Comparison with de Haas-van Alphen oscillations allow a reliable estimate of R b which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohler's rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.PACS numbers: 74.70. Kn, 71.30.+h, 72.15.Gd The electrical transport in metals can usually be described by the coherent motion of electrons in Bloch states with well-defined wave vectors. A common approach to this problem is the Boltzmann transport theory which works well for most metals and semiconductors. There are, however, a number of cases where a more complex transport mechanism is involved and where the simple approach fails [1,2]. Prominent examples are the cuprate superconductors [3] and organic metals [4] which reveal unusual normal-state properties. A central issue for these layered materials is whether the electronic conduction can be described by the coherent motion of Bloch electrons with well-defined wave vectors or whether the interlayer transport is caused by an incoherent diffusive motion of the electrons between the layers.With the assumption of a constant scattering time τ s for all charge carriers the semiclassical transport theory predicts a universal temperature and field dependence of the magnetoresistance which can be described as R(B, T )/R(0, T ) = f (B/R(0, T )), where f (x) is a universal function. This is known as Kohler's rule which holds for many metals regardless of the Fermi-surface topology [5]. Furthermore, for B parallel to the current, no magnetoresistance is expected semiclassically. Deviations from this behavior are known to occur for the interlayer transport in some organic conductors [2,4]. This was taken as an indication for incoherent transport. Other evidence for the failure of conventional transport theory is the very large low-temperature normal-state resistivity (a few Ωcm) which would correspond to meanfree paths much shorter than interatomic distances.For the quasi-two-dimensional (2D) organic metals one can assume that the interlayer transport is caused by uncorrelated tunneling events between the layers [1]. Thereby the transport is incoherent because the electrons are scattered many times within the layer before a tunneling event takes place. This may occur when the time it takes for an electron to hop between the layers is much larger than τ s , i.e.,h/t c ≫ τ s , where t c is the interlayer hopping integral. In case the intralayer momentum is conserved during the tunneling process and an interference between the wave functions on adjacent layers is possible, McKenzie and Moses [1] ...