Research of Competition and Coexistence of Charge-ordered and Superconducting States in Novel Organic Superconductors

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In order to investigate the relationship between charge ordering and superconductivity for novel organic superconductor beta-(meso-DMBEDT-TTF)_2PF_6, the detailed measurements of electrical resistivity and magnetoresistivity have been carried out.Our results are followings.(1) Determination of superconducting state by measurements of electrical and magnetoresistivity under pressureThe detailed measurements of electrical resistivity under pressure revealed that the resistance drop of observed at Tc(onset) = 4.6 K under relatively low pressure 0.6 kbar, whose superconductivity was confirmed by applying magnetic field. The superconducting phase extends widely to Tc=2.6K under 5.2 kbar.(2) Determination of charge-ordered state by temperature and magnetic field dependences of magnetoresistance under pressure The temperature dependence of resistivity for beta-(meso-DMBEDT-TTF)_2PF_6 is peculiar under 0.6 kbar; the resistivity minimum is observed at 60K, the increase of resistivity by 8 times with lowering temperature, and the sudden drop of resitivity is observed at 4.6K. The raman spectroscopy under pressure demonstrates that the resistivity increase is originated by charge ordering (CO). In this CO region, large magnetoresistance was observed, its increasing rate (ΔR/R(0T)) increases with applying pressure, and in even the metallic state at low temperatures, the magnetoresistance can be observed. Therefore, the illustrated P-T phase diagram reveals that the static and dyanimical CO states exist in neighbouring of superconducting state, which is the first experimental observation among molecular superconductors.