Study of Vortex Motion by Thermomagnetic Effects in High Temperature Superconductors

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The aim of this project is to investigate vortex motions driven by thermomagnetic effects and to clarify the phase diagram in the vortex state in high Tc superconductors. Results are as follows. 1) Thermomagnetic effect : Ettingshausen and Nernst effects of high Tc superconductors YBa2Cu3Oy, Bi2Sr2CaCu2O8+delta and La2-xSrxCuO4 have been measured for various configurations of the magnetic field with respect to the crystal axis and the direction of the vortex motion and anisotropic properties of the vortex motion and the transport energy of the moving vortex line have been obtained. Relative magnitude of the anisotoropy are explained on the basis of the effective mass model and the quasi 2 dimensional model. However, absolute value of the transport energy is in disagreement with that expected from the type II superconducting theory and the simple flux flow model. This remains to be solved. 2) Phese diagram in the vortex state : Untwinned YBa_2Cu_3O_y single crystals have been prepared by heating twinned single crystals under an uniaxial stress. Magnetization and electrical resistance measurements on untwinned single crystals have been preformed in the magnetic fields up to 30 T using a Hall probe magnetometry. The anomalous second peak on the magnetization hysteresis curve is observed near the critical point of the first order vortex lattice melting transition. This anomalous behavior is attributed to the field-driven transition from the ordered Bragg glass to the disordered vortex glass phases. The field-driven transition is suppressed by increasing disorder introduced by electron radiation.