Step-wise Behavior of Vortex-Lattice Melting Transition in Tilted   Magnetic Fields in Single Crystals Bi2Sr2CaCu2O8+d
  The vortex lattice melting transition in single crystals Bi2Sr2CaCu2O8+d was studied by the 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. For the first time, the complete Hc-Hab phase diagram of the melting-transition in Bi2Sr2CaCu2O8+d is obtained. The c-axis melting field component Hc-melt exhibits the novel, step-wise dependence on the in-plane magnetic fields Hab which is discussed on the base of the crossing vortex lattice structure. The sharp change of resistance behavior observed near the ab-plane suggests transformation from first-order to second-order phase transition. 
J. Mirkovic, S.E. Savel'ev, E. Sugahara and K. Kadowaki