Non-resonant rf absorption evidence for reentrant melting of vortex lattice in Bi2Sr2CaCu2O8 single crystals

We have studied the magnetic field dependent rf (20 MHz) losses in Bi2Sr2CaCu2O8 single crystals in the low field and high temperature regime. Above HCl the dissipation begins to decrease as the field is increased and exhibits a minimum at HM>HCl. For H>HM the loss increases monotonically. We attribute the decrease in loss above HCl to the stiffening of the vortex lines due to the attractive electromagnetic interaction between the 2D vortices (that comprise the vortex line at low fields) in adjacent CuO bilayers. The minimum at HM implies that the vortex lines are stiffest and hence represents a transition into vortex solid state from the narrow vortex liquid in the vicinity of HCl. The increase in loss for H>HM marks the melting of the vortex lattice and hence a second transition into vortex liquid regime. We discuss our results in the light of recent theory of reentrant melting of the vortex lattice by G. Blatter et al. (Phys. Rev. B 54, 72 (1996)).