Studies of the Anomaly of Hall Coefficient in High T_c Superconducting Thin Films

Measurements of the Hall coefficient (R_H) were carried out for a polycrystalline YBa₂Cu₃O_7-y film, a well oriented YBa₂Cu₃O_7-y film and an oriented GdBa₂Cu₃O_7-y film. Sign change of RH was observed for the two YBa₂Cu₃O_7-y films near T_c, but not for the GdBa₂Cu₃O_7-y film which contains partly the component of (110) orientation. The R_H anisotropy model proposed previously turns out to be incorrect. Why the YBa₂Cu₃O_7-y films experience sign change of R_H while the GdBa₂Cu₃O_7-y film does not is tentatively investigated.