Mixed State Hall Effect in a Twinned YBa2Cu3O7-d Single Crystal

Abstract

Thanks to the 9 contacts deposited on the surface of a high quality YBCO twinned single crystal, we investigate both vortex guided motion along twins and the mixed state Hall effect. Firstly, we clearly identify the vortex phase transition and show that, when the magnetic field is along the twin boundaries (within about 2 degrees), the vortex solid is a Bose glass. Secondly, the effects of twins on the vortex dynamics are studied by measuring the direction and magnitude of the total electric field while rotating the current in the ab plane. We observe a partially guided vortex motion along the dominating twin plane family, already apparent in the vortex liquid, and becoming more and more pronounced as the temperature or the magnetic field is reduced. There is no change related to the vortex phase transition. Finally, the mixed state Hall effect is studied, with particular focus given to the vortex phase transition. We show that the Hall resistivity scaling law rho_xy=A*rho_xx^b remains unchanged in the vortex solid phase. For a magnetic field along the twins, the exponent is b=2, corresponding to a constant Hall conductivity. For a tilted magnetic field, we find b=1.4. In this case, the Hall conductivity changes sharply at the vortex lattice melting, its slope becoming much larger in the vortex solid. This effect is strongly current dependent, and thus demonstrates that the Hall conductivity is pinning dependent, hopefully solving a long term controversy. We also thoroughly review the experimental data on the mixed state Hall effect, and discuss some theoretical models concerning both the anomaly (a sign reversal of the Hall effect) and the scaling law. A novel model for the Hall scaling law is given, directly inspired from the theory of percolation in metallic conductors.