Novel Electronic States in the Soft Lattice of Organic Conductors from the Control to the Design of Electronic States

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Organic conductors are soft and suffer changes in crystal structure of not only the molecular position but also the molecular orientation. The purpose of this study is to reveal the relationship between changes in crystal structure and electronic properties of organic conductors at low temperature under structural controls by "uniaxial compression". This leads to a control and a design of organic compounds, which have a rich variety in structure and electronic properties.We succeeded to develop the experimental method of uniaxial compression for transport and low-temperature x-ray diffraction measurements. Using this method, we measured electrical resistance, magnetoresistance and low temperature crystal structure of quasi two-dimensional conductors α-(BEDT-TTF)_2XHg(SCN)_4 where X=K and X=NH_4. In relation to superconductivity, we found an increase in the superconducting critical temperature from 1.5K of ambient pressure to 6K under the uniaixial compression parallel to the c-axis. Th … More e electronic structure derived by x-ray structure analyses was found to be semi-quantitatively consistent with theoretical expectation for the critical temperature by BCS theory. This result remonstrated the feasibility of the uniaxial compression method for the first time. We discovered also a novel magnetoresistance phenomenon in studies at hydrostatic pressures and named it "small closed-orbit effect".We measured also uniaxial strain effects in related materials like the β- and θ-type BEDT-TTF series and BO series of organic conductors. Changes in electronic properties were found to occur as expected. However, we discovered some novel phenomena to be investigated in future studies. We tried also a material design using organic acceptors to succeed in synthesizing original organic superconductor (BETS)_2(Cl_2TCNQ). This is the first organic superconductor composed of only organic molecules.The uniaxial strain method developed in this research is expected to be useful in controlling Coulomb correlation between electrons and to be applied also to magnetic and optical measurements. Less