Temperature dependence of the Hall effect in high-temperature superconductors above and below Tc

We review the sign reversal of the Hall angle θH seen in many high-temperature superconductors in small magnetic fields. Preliminary data are presented for a tetragonal (and therefore untwinned) single crystal of YBa2Cu3O7−δ with 4% of the Cu atoms replaced by Co; the sign reversal of θH is not seen. For an untwinned single-crystal sample of the undoped compound, the measured constant-pressure Hall-effect data above Tc are converted to constant-volume data, so direct comparison can be made with theory.