Quadratic temperature dependence of electronic heat capacities in the κ-type organic superconductors

Abstract

We carried out a systematic measurement and data analysis of low-temperature heat capacities of three BEDT-TTF-based superconductive compounds with [Formula: see text]-type structure, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene so as to compare the character of the quasi-particle excitations reflected in the electronic heat capacity. We used an original relaxation calorimetry cell with much reduced addenda heat capacity as compared with previous works. The three compounds, [Formula: see text]-(BEDT-TTF)2Cu(NCS)2, [Formula: see text]-(BEDT-TTF)2Ag(CN)2H2O, [Formula: see text]-(BEDT-TTF)4Hg[Formula: see text]Br8 show distinct quadratic temperature dependence in their electronic heat capacity obtained by subtracting normal states data obtained by applying magnetic fields from the 0 T data. The line-nodal gap due to [Formula: see text]-wave pairing symmetry is suggested as common phenomena of the superconductivity of the [Formula: see text]-type compounds.