Hall and Magneto-Resistivity Studies on Yttrium BARIUM(2)COPPER(3)OXYGEN(7-DELTA) and Doped Yttrium BARIUM(2)COPPER(3)OXYGEN(7 - in the Mixed State and Normal State

In this thesis, the Hall and magneto-resistivity were studied in YBa_2Cu _3O_{7-delta} crystals both in the mixed state and in the normal state. In the mixed state (H > H_{c1 },T < T_ c-) of type-II superconductors, rho_{xx} and rho_{xy} are induced by the motion of the flux lines. In YBa_2Cu _3O_{7-delta }, the rho_{xx} first shows an activated behavior when rho_{xx}<~ 0.1 times rho_{normal}, then it increases roughly linearly with field until a characteristic field strength H_ k is reached. At higher temperatures the Hall resistivity rho _{xy} shows a negative peak below H_ k, then it approaches the positive normal state rho_{xy} at higher field. It is argued that the flux flow data can not be explained under conventional models, and the negative Hall anomaly is an intrinsic effect. The microscopic mechanism governing the flux dynamics in the high temperature superconductors (HTSC) is still poorly understood, although the main causes are closely related to their short coherence lengths and high temperatures. At last a phase diagram of the YBa_2Cu_3 O_{7-delta} flux phases is presented. In the normal state, rho_ {xx} and rho_{xy } are a probe of the electronic structures of the HTSC. The rho_{xy} and rho_{xx} data show anomalous temperature dependence in pure YBa _2Cu_3O _{7-delta} crystals, viz. rho_{xx} ~ T and rho_{xy} = R_{H}B ~ 1/T, where T is the temperature. It is argued that multi-band effect and conventional magnetic skew scattering can be ruled out as possible explanations, the unusual temperature dependence appears inherently related to the unconventional normal state of the HTSC. The rho_ {xx} and rho_{xy } were also studied in YBa_2 Cu_{3-x}Zn _ xO_{7-delta} and YBa_2Cu_ {3-x}Ni_ xO _{7-delta} crystals with different doping concentration x. By doping with Zn or Ni, the 1/R_{H} is suppressed and the temperature dependence of 1/R_{H } ~ T changes to a weaker temperature dependence. More strikingly, cottheta_ H = rho_{xx}/rho _{xy} = 1/omega_ ctau_ c ~ C(x) + alpha T^2 with alpha a universal constant. Such normal state transport properties are not well understood at present time. It is generally believed that a doped Mott insulator is the right microscopic model for the HTSC. Large on-site coulomb repulsion U leads to a RVB state in which gauge interaction can give rise to linear resistivity and 1/R_ H.