We report on the first measurements of thermal conductivity in the superconducting state of (TMTSF)2ClO4. The electronic contribution to heat transport is found to decrease rapidly below Tc, indicating the absence of low-energy electronic excitations. We argue that this result provides strong evidence for a nodeless superconducting gap function but does not exclude a possible unconventional order parameter.74.70. Kn, 74.25.Fy, 72.15.Eb The (TMTSF) 2 X family of quasi-one dimensional conductors (the Bechgaard salts) are a well-known case of competition between superconducting and Spin-DensityWave ground states [1]. At ambient pressure, most of these extremely anisotropic compounds undergo a metalinsulator transition at low temperatures and have a SDW fundamental state. Under moderate pressure, the SDW instability is suppressed and replaced by a superconducting transition at a critical temperature of the order of 1 K [2]. One exception to this scheme is (TMTSF) 2 ClO 4 which is superconducting at ambient pressure. The highfield properties of these compounds-including a particular version of quantum Hall effect [3] and commensurability effects in the angular magnetoresistance [4]-have been intensely studied during the past few years. However, in spite of early speculations on a possible unconventional nature of superconductivity in this context [5], and contrary to the other families of exotic superconductors (i.e. Heavy Fermions and cuprates), the superconducting state has been subject to very few studies. The only attempt to explore the symmetry of superconducting order parameter in a Bechgaard salt is reported by Takigawa, Yasuoka and Saito [6]. These authors detected a T 3 temperature dependence in the nuclear relaxation rate of proton in (TMTSF) 2 ClO 4 and concluded that the superconducting gap function should vanish along lines on the Fermi Surface.Thermal conductivity has proved to be a powerful probe of gap structure in a number of unconventional superconductors. In the case of the heavy-fermion superconductor UPt 3 , thermal conductivity measurements constitute one major source of our current knowledge on the angular distribution of nodes in the gap function [7,8]. In the case of YBa 2 Cu 3 O 6.9 , several convincing signatures of d-wave superconductivity have been reported in a number of heat transport studies [9]. In the Bechgaard salts, measurements of thermal conductivity have been restricted to temperatures well above the superconducting instability [10]. In this letter, we present the first study of heat transport in an organic superconducting system. Our conclusion happens to be rather surprising as we find strong evidence for a nodeless gap. FIG. 1. Thermal conductivity divided by temperature for a relaxed sample of of (TMTSF)2ClO4. Superconductivity is suppressed by applying a small field along the c-axis. The lower insert presents the temperature dependence of the electrical resistance for the same sample. The upper insert shows the field dependence of κ T for two different temperatures.The e...

The first study of thermal conductivity, κ, in a quasi-two-dimensional organic superconductor of the κ-(BEDT-TTF)2X family reveals features analogous to those already observed in the cuprates. The onset of superconductivity is associated with a sudden increase in κ which can be suppressed by the application of a moderate magnetic field. At low temperatures, a finite linear term -due to a residual electronic contribution-was resolved. The magnitude of this term is close to what is predicted by the theory of transport in unconventional superconductors.The superconductors of κ-(BEDT-TTF) 2 X family [1] share a number of similarities with the high-T c cuprates [2]. Both sets of compounds are quasi-two-dimensional with superconductivity confined to conducting planes sandwiched between insulating layers. The metallic state in both families exhibit common features like low carrier densities, strong electronic correlations and proximity of antiferromagnetic insulating state. While Shubnikov-de Haas experiments [3] have established the existence of a well-defined Fermi surface in the κ-(BEDT-TTF) 2 X family, this metallic state presents some more unconventional properties -like a pseudogap in the electronic density of states in κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br [4]-which have been compared to analagous features in underdoped cuprates [2]. As for the symmetry of the superconducting order parameter, it has yet to become the subject of a consensus as nowadays it is the case in the cuprates. While, early penetration-depth studies on κ-(BEDT-TTF) 2 Cu(NCS) 2 led to conflicting results [5], recent NMR [6] and specific heat [7] studies on κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br provided evidence for the presence of nodes in the superconducting gap.In this letter we present the first study of thermal conductivity in a member of this family. According to our results, heat transport in κ-(BEDT-TTF) 2 Cu(NCS) 2 presents features which have already been detected in YBCO and other high-T c cuprates. Notably, the observation of a residual electronic thermal conductivity at very low temperatures provides strong support for presence of nodes in the superconducting order parameter.We measured the thermal conductivity of five κ-(BEDT-TTF) 2 Cu(NCS) 2 single crystals using a conventional four-probe method. Contacts were realized using silver paint on evaporated gold. The heat current was always applied in the basal (highly-conducting) plane. The temperature gradient was measured with two RuO 2 resistance chips which showed small magnetoresistance and a usable sensitivity up to 15K. The resistive heater and the two thermometers were held by small solenoids of 50 µm manganin wire. In this way, we measured the resistance of the sample and the thermometers with minimal heat loss. For temperatures below 0.25 K, we checked our zero-field results by using another device which was designed for very low temperatures and described elsewhere [8]. Our set-up allowed us to measure, in addition to electrical and thermal conductivities, the thermo-electric power of th...