Breakdown of Fermi-liquid theory in a cuprate superconductor

The behaviour of electrons in solids is remarkably well described by Landau's Fermi-liquid theory, which says that even though electrons in a metal interact they can still be treated as well-defined fermions, called ``quasiparticles''. At low temperature, the ability of quasiparticles to transport heat is strictly given by their ability to transport charge, via a universal relation known as the Wiedemann-Franz law, which no material in nature has been known to violate. High-temperature superconductors have long been thought to fall outside the realm of Fermi-liquid theory, as suggested by several anomalous properties, but this has yet to be shown conclusively. Here we report on the first experimental test of the Wiedemann-Franz law in a cuprate superconductor, (Pr,Ce)$_2$CuO$_4$. Our study reveals a clear departure from the universal law and provides compelling evidence for the breakdown of Fermi-liquid theory in high-temperature superconductors.Comment: 7 pages, 3 figure

Thin-film superconducting shields

To date, magnetic shields utilizing high-temperature superconductor materials mostly comprise bulk materials, usually shaped into a cylinder and then heat treated. We analyze the potential of thin-film technology, whereby a layer of superconductor is deposited on a substrate, to fabricate magnetic shields. Films deposited on a curved surface are inevitably granular, leading to reduced critical current density compared with crystalline films. This limits the maximum field that can be screened. Granular films also have an enhanced penetration depth compared with the bulk value, which determines the maximum possible screening at applied fields below the maximum. We demonstrate the utility of a mutual inductance measurement to evaluate small flat test samples of a granular film for their potential as a shielding material in a more complex geometry. This measurement predicts both the maximum magnetic field that can be screened and the residual leakage at fields below the maximum. The tested film was produced by the Electrophoretic deposition of YBCO on a silver substrate. The maximum screening field was comparable with that obtained for a similar film on a cylindrical substrate. We also show comparable results from a crystalline YBCO film