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We report an observation of a sign change in the Hall resistivity p"~i n the superconducting state of the n-type superconductor Ndl 8&Ceo»Cu04~. This anomaly in other superconductors has widely been attributed to extrinsic effects, such as pinning or thermoelectric effects, or else to complicated band structures. However, the behavior of the Hall effect in the n-type cuprate Ndl 8,Ceo»Cu04 y and the systematics of the anomaly in other superconducting materials together provide strong evidence against such models. The data instead indicate that p"~r eveals an intrinsic property of vortex motion.

In both high-T, and low-T, superconductors, the Hall effect in the mixed state often shows a sign reversal below T, that contradicts standard theories for flux-vortex motion. We have measured the mixed-state Hall eA'ect in high-quality superconducting films of T12Ba2CaCu20&, and find that the Hall angle a below T, consists of two components: A field-independent component exhibits a complicated temperature dependence, including a sign change near T"while a second component with very weak temperature dependence is linear in field and resembles the normalstate Hall angle. The field independence of the sign-reversing component is evidence for vortex motion, and can be understood in terms of simple drag forces acting on vortices.

The thermal conductivity, both in-plane (x,b) and out-of-plane (s,), has been measured in ",90-K" crystals of YBa2Cu307y from 10 to 330 K. x,b displays an 18 to 40% increase below the transition temperature T, (as in ceramic data). x"however, is four to five times smaller and shows no anomaly at T,. Comparing these results to x,b of insulating crystals, we find evidence for very strong but highly anisotropic electron-phonon coupling in the normal state. Our results are consistent with a phonon origin for the peak in~,b.

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