Thermodynamic Investigation of the Gap Structure and Electron Pair-Symnetry in Organic Superconductors

This research project aims at clarifying the structure of energy gap in the superconductive states of BEDT-TTF based organic superconductors through analyses of specific heat data。To get detail information on the ground state and the low-energy excitations from it，we performed the experiment at extremely low-temperature region below IK.We have improved a sample cell for the thermal-relaxation type calorimeter equipped with the dilution refrigerator at Institute for Molecular Science。As a consequence，we have succeeded to reduce the heat capacity of addenda part down to about0.1µJ/K at1K，which is satisfactory small to get enough resolution.For the system ofκ-(BEDT-TTF)I D 22 I D 2 X[X=Cu[N(CN)I D 22 I D 2]Br，Cu(NCS)I D 22 ii D 2]，we have discussed the temperature dependence of specific heat and magnetic fields dependence of electronic specific-heat coefficient，γup to 10 T and also discuss the residγ*term in the superconductive state.The results suggest that the gap-structure of this system can be explained within the frame of line-nodes picture of d-wave superconductors.It is however to be mentioned that theγ(H)curves of both salts show a kink around 2T，which may be related to the vortex states of these materials。We are going to further inquire into the magnetic fields dependence of thisγin order to clarify the pairing nature and vortex state in detail.

This research project aims at clarifying the structure of energy gap in the superconductive states of BEDT-TTF based organic superconductors through analyses of specific heat data. To get detail information on the ground state and the low-energy excitations from it, we performed the experiment at extremely low-temperature region below IK. We have improved a sample cell for the thermal-relaxation type calorimeter equipped with the dilution refrigerator at Institute for Molecular Science. As a consequence, we have succeeded to reduce the heat capacity of addenda part down to about 0.1μJ/K at 1K, which is satisfactory small to get enough resolution. For the system of κ-(BEDT-TTF)ィイD22ィエD2X [X=Cu[N(CN)ィイD22ィエD2]Br, Cu(NCS)ィイD22ィエD2] ,we have discussed the temperature dependence of specific heat and magnetic fields dependence of electronic specific-heat coefficient, γ up to 10T and also discuss the residual γ* term in the superconductive state. The results suggest that the gap-structure of this system can be explained within the frame of line-nodes picture of d-wave superconductors. It is however to be mentioned that the γ(H) curves of both salts show a kink around 2T, which may be related to the vortex states of these materials. We are going to further inquire into the magnetic fields dependence of this γ in order to clarify the pairing nature and vortex state in detail.