Vortex liquid properties in optimally doped and oxygen-deficient YBa2Cu3O7-delta single crystals

The electrical anisotropy, gamma = (m(c)/m(ab))(1/2), can easily be varied by changing the oxygen content in single crystals of YBa2Cu3O7-delta (YBCO) ; thus making this system useful as a test system For vortex dynamics in hi gh-temperature superconductors (HTSC). Here, some recent results are review ed. Measurements of the c-axis resistivity with the magnetic field B II ab- plane is shown to be a powerful tool in studying vortex velocity correlatio ns both parallel and perpendicular to the applied field. In optimally doped crystals, we iind evidence that the in-plane vortex lattice melts at T-m i nto a disentangled vortex liquid, which looses its correlation along the c- axis at a higher temperature T-br > T-m. For weakly disordered crystals and magnetic fields B \\ c-axis, a relation between the vortex glass line and the magnetic field dependence of the characteristic energy, U*, in the pinn ed vortex liquid is proposed. This approach is shown to give support to our idea of describing the vortex glass resistivity in the liquid regime by a modified glass correlation Length, xi(g) = xi(0)\k(B)T/U-0 - 1\(-nu) which depends on the average pinning energy, Lr,, in the system [A. Rydh, O. Rapp , M. Andersson, Phys. Rev. Lett. 83 (1999) 1850]. (C) 2000 Elsevier Science B.V. All rights reserved.