Double sign reversal of the vortex Hall effect in YBa2Cu3O7-delta thin   films in the strong pinning limit of low magnetic fields
  Measurements of the Hall effect and the resistivity in twinned YBa2Cu3O7-delta thin films in magnetic fields B oriented parallel to the crystallographic c-axis and to the twin boundaries reveal a double sign reversal of the Hall coefficient for B below 1 T. In high transport current densities, or with B tilted off the twin boundaries by 5 degrees, the second sign reversal vanishes. The power-law scaling of the Hall conductivity to the longitudinal conductivity in the mixed state is strongly modified in the regime of the second sign reversal. Our observations are interpreted as strong, disorder-type dependent vortex pinning and confirm that the Hall conductivity in high temperature superconductors is not independent of pinning. 
W. Goeb, W. Liebich, W. Lang, I. Puica, Roman Sobolewski, R. Roessler,   J.D. Pedarnig, D. Baeuerle