VORTEX LATTICE MELTING INTO A DISENTANGLED LIQUID FOLLOWED BY THE 3D-2D DECOUPLING TRANSITION IN YBA2CU4O8 SINGLE-CRYSTALS

A sharp resistance drop associated with vortex lattice melting was obs erved in high quality YBa2Cu4O8 single crystals. The melting line is w ell described by the anisotropic Ginzburg-Landau theory. Two thermally activated Bur flow regions, which were separated by a crossover lint B-cr(T)=1406.5(1 - T/T-c)/T(T-c = 79.0 K, B-cr in unit of Tesla), were observed in the vortex liquid phase. Activation energy for each regio n was obtained and the corresponding dissipation mechanism was discuss ed. Our results suggest that the vortex lattice in YBa2Cu4O8 single cr ystal melts into disentangled liquid, which then undergoes a two- to t hree-dimensional decoupling transition.