Mixed-state Hall effect in high-temperature superconductors in small and large magnetic fields

Abstract

In many high-temperature superconductors, the Hall effect in the mixed superconducting state has a sign opposite to that in the normal state. Some highly anisotropic materials exhibit even a double sign reversal of the mixed-state Hall effect. We investigate this anomaly in precisely-patterned thin films of YBa 2Cu 3O 7−δ (YBCO) in a large range of magnetic fields from 32 mT to 13 T. The Hall conductivity appears to decompose into two contributions of different sign. The positive (hole-like) part increases steeply below the critical temperature and indicates that the scattering rate of the quasiparticles in the superconducting state is drastically reduced. The negative contribution is present around the critical temperature and becomes important in low fields only. The delicate interplay between these two contributions, together with the cutoff of the experimental window by pinning, leads to the various experimental observations, and eventually to a double sign change in fields below 0.25 T that has not been observed previously in YBCO.