| Research Abstract |
This research project had aimed to elucidate the mechanism of superconductivity and magnetism in p electron system (borides, carbides, silicides ). We developed new superconducting materials and investigated the superconductivity in MgB2, NbB2, CaAISi, Y2C3 and magnetism in CaB6 from various experimental methods. The research results are listed as follows;
1. We clarified that MgB2 is a new type strong coupling superconductor of which high energy phonon of light element plays an important role, because unusual temperature dependence of phonon density of states was observed at the energy range, corresponding to in B-plane phonon in MgB2 and Al-doped MgB2 probed by inelastic neutron scattering. From tunneling spectroscopy, we observed multi superconducting gaps which are correlated with each other.
2. From the energy gap measurement by tunneling spectroscopy and MEM/Rietveld analysis using synchrotron X-ray in NbB2 and CaA1Si of which crystal structures are same with MgB2, we elucidate tha
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t these superconductors are considered to be strong coupling superconductors and have some correlation between superconducting properties and crystal structures in each system.
3. We discovered new superconductors W_7Re_<13>X(X=B, C) (T_c=7.1 K, 7.3 K), Mo_7Re_<13>X(X=B, C) (T_c=8.3 K, 8.1 K) and Y_2C_3(T_c=18 K). The T_c of Y_2C_3 is changed from 11 K to 18 K by nominal composition and heat treatment. To elucidate the change of T_c in Y_2C_3 in terms of crystal structure, we performed crystal structure analysis and MEM/Rietveld analysis using synchrotron X-ray in detail. We clearly observed the forming C-C dimmer in Y_2C_3 and the evidence for correlation between Tc and the distance of C-C dimmer. From specific heat, ^<13>C-NMR and tunneling spectroscopy measurements, Y_2C_3 has two gaps in its superconducting state and each gap reveals the s-wave symmetry behavior. Especially, it is considered that the correlation between each gap is relatively small from the temperature dependence of each superconducting gap.
4. We observed some phase transition of weak internal field at 150 K in temperature dependence of asymmetry of CaB_6 in μSR measurement. This behavior can not be ascribed to nuclear dipole field of B atoms. Moreover, a small electron pocket at Γ point was observed by ARPES (Angle Resolved Photo Emission Spectroscopy) using high quality single crystal of CaB_6. It is suggested that an unusual-magnetic behavior in CaB_6 is related to these results observed in μSR and ARPES measurements. Less
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