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epl draftHall effect and Fermi surface reconstruction via electron pockets in the high-T c cuprates. F--Transport propertiesAbstract -The mechanism by which the Fermi surface of high-Tc cuprates undergoes a dramatic change from a large hole-like barrel to small arcs or pockets on entering the pseudogap phase remains a question of fundamental importance. Here we calculate the normal-state Hall coefficient from the resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang. In this model, reconstruction of the Fermi surface occurs via an intermediate regime where the Fermi surface consists of both hole-and electron-like pockets. We find that the doping (x) dependence of the Hall number transitions from 1+x to x over this narrow doping range. At low temperatures, a switch from a downturn to an upturn in the Hall coefficient signals the departure of the electronlike pockets from the Fermi surface.

We report on the behavior of a high-T c superconducting (HTS) homopolar dynamo which outputs a DC open-circuit voltage when the stator is in the superconducting state, but behaves as a conventional AC alternator when the stator is in the normal state. We observe that this time-averaged DC voltage arises from a change in the shape of the AC voltage waveform that is obtained from a normal conducting stator. The measured DC voltage is proportional to frequency, and decreases with increasing flux gap between the rotor magnet and the HTS stator wire. We observe that the DC output voltage decreases to zero at large flux gaps, although small differences between the normal-conducting and superconducting waveforms are still observed, which we attribute to screening currents in the HTS stator wire. Importantly, the normalised pulse shape is found to be a function of the rotor position angle only. Based on these observations, we suggest that the origin of this unexpected DC effect can be explained by a model first proposed by Giaever, which considers the impact of time-varying circulating eddy currents within the HTS stator wire. Such circulating currents form a superconducting shunt path which "short-circuits" the high field region directly beneath the rotor magnet, at those points in the cycle when the rotor magnet partially overlaps the superconducting stator wire. This reduces the output voltage from the device during these periods of the rotor cycle, leading to partial rectification of the output voltage waveform and hence the emergence of a time-averaged DC voltage.

We analyse fluctuations about Tc in the specific heat of (Y,Ca)Ba2Cu3O 7−δ , YBa2Cu4O8 and Bi2Sr2CaCu2O 8+δ . The mean-field transition temperature, T mf c , in the absence of fluctuations lies well above Tc especially at low doping where it reaches as high as 150K. We show that phase and amplitude fluctuations set in simultaneously and T mf c scales with the gap, ∆0, such that 2∆0/kBT mf c is comparable to the BCS weak-coupling value, 4.3, for d-wave superconductivity. We also show that T mf c is unrelated to the pseudogap temperature, T * .