Hall anomaly and vortex charge in Bi2Sr2CaCu2Ox

We present a systematic study of the Hall conductance in Bi2Sr2CaCu2Ox thin films over a large range of doping. We find that, in a large part of the phase diagram, the Hall coefficient changes sign as a function of temperature in the flux-flow regime. By comparing data from many samples, we show that the sign reversal is tied to the superconducting transition and is not a result of a competing order. We then compare our data to the predictions of the Bardeen-Stephan model and show that, in all samples, there is an additional negative contribution to the Hall conductivity. We extract from the negative excess Hall a vortex charge that is found to be strongly doping dependent.