Sign reversal of the flux flow Hall effect in oxygen deficient YBa 2Cu 3O x films

The Hall effect of high temperature superconductors has been reported to slow in some cases a striking sign reversal near T _c, the origin of which is not yet fully understood. We report on measurements of the Hall effect in the mixed state of oxygen deficient YBa ₂Cu ₃O _x thin films, for 7≥×≥6.7. Although no sign reversal of the Hall resistivity ϱ _xy is present for the as-prepared film, it is clearly observed once the oxygen content of the film is reduced to x=6.9 and 6.85. However, by further reducing the oxygen content to x=6.7, the sign reversal vanishes again. The latter might be a consequence of the increase of the resistivity with decreasing oxygen content, since it has been suggested that the amplitude of the reversed Hall resistivity may be related to the mean free path.