  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.
