On back bending of the Hall number density as a function of temperature in YBCO high-Tcsuperconductors

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Abstract
Experimental data for the normal state Hall number density, nH varies as a+bT, in (pure and low-level Zn-doped) rather well oxygenated YBCO high-Tc superconductors show an anomalous change of slope ('back bending') above some temperature T0 which varies from nearly 190 K to nearly 250 K depending on sample quality. A possible origin of this anomaly is discussed within a two-fluid model approach. In so doing, a semi-empirical universal relationship is established between the 'back-bending' temperature T0, the superconducting critical temperature Tc and an 'imperfection parameter' alpha which reflects the role of the 'residual' contribution to the Hall number density at zero temperature. The model predictions are found to be in quite reasonable agreement with some representative experimental data on the Hall number density in high-Tc superconductors. This lends support to the proposal that the mechanism for the phenomenon has its origin in an electronic modification of the CuO2 planes mediated by an oxygen-vacancy ordering redistribution in the basal planes.