Specific Heat Studies on Metal-Insulator Transition of Strongly Correlated Organic Systems

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The aim of this project is to clarify the effects of electron correlation and the nature of Metal-Insulator transition in organic system based on BEDT-TTF and similar molecules through low-temperature specific heat measurements, especially focusing on the materials which have kappa-type of crystal structure. Using the financial support of this project, the precision of the calorimeter equipped with the 3He refrigerator and the dilution refrigerator was improved to perform the experiments. In addition, we have also performed calorimetric calibration under several magnetic fields.For insulating material of kappa- (BEDT-TTF) <@D22@>D2X with X=Cu [N (CN) <@D22@>D2] CI,d<@D28@>D2Cu [N (CN) <@D22@>D2] Br, we have found that the electronic specific heat coefficients, gamma is almost zero within the deviation of(]SY.+-。[)0.1mJ/molK2, which demonstrates the existence of distinct charge gap structure around the Fermi level due to the correlation effect of electrons.In the superconducting salts of X=Cu (NCS) _2, Cu [N (CN) _2] Br, the electronic specific heat dominated by the quasi-particle excitations was observed as the quadratic temperature dependence. Under weak magnetic field below 1T,the electronic specific heat recovers rapidly with a square root dependence of magnetic field in both materials. This fact is consistently understood with the temperature dependence results in terms of the different nature of usual BCS type superconductors. The possibility of the d-wave type superconductivity coupled with the electron correlation effect is strongly suggested.