TEMPERATURE-DEPENDENCE OF THE HALL-EFFECT IN HIGH-TEMPERATURE SUPERCONDUCTORS ABOVE AND BELOW T(C)

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