ANOMALOUS HALL-EFFECT OF YBA2CU3O7-DELTA IN THE LOW-MAGNETIC-FIELD LIMIT

The Hall effect of YBa2Cu3O7 - delta epitaxial films in the vicinity o f superconducting transition was studied under ultra low magnetic fiel ds down to 10(-3) T where some field-induced extrinsic effects, such a s the reduction and broadening of the transition temperature and the e nhancement of fluctuation, can be neglected. The Hall sign reversal wa s still observed down to the lowest field we arrived, which shows that any mechanism connected with above mentioned extrinsic changes cannot be responsible for the effect. The Hall resistivity rho(xy), Hall ang le tan alpha and Hall conductivity sigma(xy) are all simply proportion al to the magnetic field, resembling the behaviors of the ordinary Hal l effect in the low-magnetic-field limit. The temperature range with o bservable negative Hall resistivity coincides with the critical fluctu ation region. These behaviors shows that the negative Hall effect cann ot be understood in the picture of flux dow.