Clarification of mechanism of superconductivity (in press)ure-induced superconductor,Cs_3C<60>

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Structures and physical properties of Cs_3C_60 were studied in a pressure region from 1 bar to 6 GPa in order to clarify the mechanism of superconductivity in a pressure-induced superconductor Cs_3C_60. Two phases of body-centered orthorhombic (Immm) and simple cubic (A15 : P<m3>^^^-n) phases exist in Cs_3C_60 at 1bar. Further, we tried to prepare the intermediate phases of Cs_3C_60 and Cs_4C_60 because the Cs atom takes a fractional occupancy of 0.75 in Cs_3C_60, and their structures and physical properties were studied. Only an A15 phase was observed in Cs_<3.00(6)>C_60 and Cs_<35(1)>C60 above 30 kbar. We have also succeeded in preparing the sample of Cs_3C_60 containing 50 % of A15 phase. The Pauli paramagnetic susceptibility was observed in the ESR of Cs_<3.00(6)>C_60 and the intermediate phase Cs_<3.3(1)>C_60. However, the plots of ΔH_pp/(Δg)^2 vs T did not follow the Elliot-Yafet relationship, and the plots of resistivity, p, vs T did not show a normal metallic behavior. The plot … More s suggest a hopping or a fluctuation-induced tunneling conductivity. The AC susceptibilities of the intermediate phases showed no superconducting transition above 1.5 K below 1 0 kbar. This fact suggests that the intermediate phase is not a pressure-induced superconductor. The structures and physical properties of 2D polymeric phase of Na_4C_60 and metallofullerenes were studied in a wide temperature and pressure range in order to search the pressure-induced superconducting phase other than Cs_3C_60. The ESR suggests a metallic behavior for Na_4C_60, while the plots of p vs T shows a semi conducting behavior with a gap energy, E_g, of 0.5 - 0.8 eV. It has been found that the 2D polymeric structure tends to change to a 3D polymeric structure. The structural phase transition was observed from a simple cubic (P03) to a face-centered cubic phase in Dy@C_82 isomer I around 310 K. The plots of p vs T show a semi conducting behavior with a small E of 0.2 eV. This suggests an appearance of novel physical properties such as semiconductor-metal transition and superconductivity under high pressure. Less