SIGN REVERSAL OF THE HALL-EFFECT IN UNTWINNED SINGLE-CRYSTAL SUPERCONDUCTING YBA2CU3O7-DELTA

For untwinned single-crystal YBa2Cu3O7- in a magnetic field H, we find that the in-plane Hall conductivity xy just below the superconducting transition temperature Tc is the sum of two terms, C1/H and C2H, where C1 and C2 are field independent, in agreement with recent theoretical predictions of Dorsey et al. and Kopnin et al. C1 and C2 have opposite signs, as required by the observation that the Hall effect changes sign as the applied field is varied. C1 is proportional to 2, and C2 is approximately proportional to , where =(Tc-T)/Tc, so the crossover field H0, where xy=0, is proportional to 1/2. © 1995 The American Physical Society.