Stepwise behavior of vortex-lattice melting transition in tilted magnetic fields in single crystals of Bi2Sr2CaCu2O8+δ

The vortex-lattice melting transition in Bi2Sr2CaCu2O8+delta single crystals was studied using in-plane resistivity measurements in magnetic fields tilted away from the c axis to the ab plane. In order to avoid the surface barrier effect which hinders the melting transition in the conventional transport measurements, we used the Corbino geometry of electric contacts. The complete H-c-H-ah phase diagram of the melting transition in Bi2Sr2CaCu2O8+delta is obtained for the first time. The c-axis melting field component H-melt(c) exhibits the novel, stepwise dependence on the in-plane magnetic fields H-ab which is discussed on the basis of the crossing vortex-lattice structure. The peculiar resistance behavior observed near the ab plane suggests the change of phase transition character from first to second order.