Vortex liquid properties in optimally doped and oxygen-deficient YBa 2Cu 3O 7− δ single crystals

Abstract

The electrical anisotropy, γ=( m c / m ab ) 1/2, can easily be varied by changing the oxygen content in single crystals of YBa 2Cu 3O 7− δ (YBCO), thus making this system useful as a test system for vortex dynamics in high-temperature superconductors (HTSC). Here, some recent results are reviewed. Measurements of the c-axis resistivity with the magnetic field B∥ ab-plane is shown to be a powerful tool in studying vortex velocity correlations both parallel and perpendicular to the applied field. In optimally doped crystals, we find evidence that the in-plane vortex lattice melts at T m into 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 pinned 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, ξ g= ξ 0| k B T/ U 0−1| − ν , which depends on the average pinning energy, U 0, in the system [A. Rydh, Ö. Rapp, M. Andersson, Phys. Rev. Lett. 83 (1999) 1850].