Thermodynamic Investigation of Electron Correlation Effects in Organic Salts

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This project is proposed to establish a systematic understanding of the various type of insulation mechanism due to electron correlation effects, which appears in organic 2 : 1salts consisting of BEDT-TTF and DCNQI molecules. Throughout the project, we tried to see the nature of the ground state and low-energy excitations from it by means of the thermodynamic measurements.In the fiscal year 1999, we focussed on studying the peculiar properties around the Mott boundary in the phase diagram of κ-(BEDT-TTF)_2X system. Using the chemical pressure effect produed by partial deuteration in BEDT-TTF molecules, we have got the information on the variation of γ-term across the Mott boundary for the first time. The γvalue decrease monotonously with approaching the insulating phase and no Brinkmann-Rice type enhancement was observed. The psuedo-gap picture established in high-Tc oxide seems to be favored in this system.Next year, we have dealt with the non-dimeric quasi-one-dimensional system (DI-DCNQI)_2Ag in terms of charge-ordering due to the long-range Coulomb interaction, V. Although we could not find any thermal anomaly around the charge-order transition, the existence of large one-dimensional spin excitations which is sensitively suppressed by external magnetic fields has been observed. The possibility of field induced spin-gap is suggested in this system.