Vortex phase transition under tilted magnetic fields in 90-K YBa2Cu3O7-δ single crystals

To study the vortex phase transition in 90-K YBa2Cu3O7-δ (YBCO) single crystals under tilted magnetic fields from the ab-planes, we measured the resistivity as functions of the temperature and angle phi between the magnetic field and the ab-planes in magnetic fields up to 170 kOe. In high magnetic fields above 100 kOe, a resistivity jump due to a first-order transition was observed in angles between phi=0 deg. and a certain critical angle phiucr. In intermediate magnetic fields between 30 and 90 kOe, the resistivity jump appears in all angles. At 10 kOe, the angular dependence of the transition is opposite to that for high magnetic fields. We find that the angular dependence of the first-order transition temperature is well described by the effective mass model and that the effective magnetic field at phiucr corresponds to the upper critical point of the first-order melting transition line for H parallel c. These findings lead us to conclude that a conventional tilted vortex lattice exists under inclined magnetic fields in 90-K YBCO, and that the mechanism of its transition is the same as that for H parallel c