N. E. Hussey

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-Tc) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+δ in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-Tc materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Quantum oscillations in an overdoped high-Tc superconductor

The nature of the metallic phase in the high-transition-temperature (high-Tc) copper oxide superconductors, and its evolution with carrier concentration, has been a long-standing mystery. A central question is how coherent electronic states, or quasiparticles, emerge from the antiferromagnetic insulator with doping. Recent quantum oscillation experiments on lightly doped copper oxides have shown evidence for small pockets of Fermi surface, the formation of which has been associated with the opening of the pseudogap—an anisotropic gap in the normal state excitation spectrum of unknown origin. As the doping is increased, experiments suggest that the full Fermi surface is restored, although the doping level at which the pseudogap closes and the nature of the electronic ground state beyond this point have yet to be determined. Here we report the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements confirm that, in overdoped superconducting copper oxides, coherence is established at all Fermi wavevectors, even near the zone boundary where the pseudogap is maximal and electronic interactions are strongest; they also firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting phase diagram.

Small Fermi Surface Pockets in Underdoped High Temperature Superconductors: Observation of Shubnikov-de Haas Oscillations in YBa2Cu4O8

A. F. Bangura, J. D. Fletcher, A. Carrington, J. Levallois, M. Nardone, B. Vignolle , P. J. Heard, N. Doiron-Leyraud, D. LeBoeuf, L. Taillefer, S. Adachi, C. Proust and N. E. Hussey H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, United Kingdom. Laboratoire National des Champs Magnétiques Pulsés, UMR CNRS-UPS-INSA 5147, Toulouse, France. Département de physique and RQMP, Université de Sherbrooke, Sherbrooke, J1K 2R1, Canada. and Superconducting Research Laboratory, International Superconductivity Center, Shinonome 1-10-13, Tokyo 135, Japan. (Dated: February 1, 2008)

Direct measurement of the upper critical field in cuprate superconductors

In the quest to increase the critical temperature Tc of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field Hc2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect Hc2 in the cuprates YBa2Cu3Oy, YBa2Cu4O8 and Tl2Ba2CuO6+δ, allowing us to map out Hc2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa2Cu3Oy is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from Hc2, suffers a sudden 20-fold collapse. This reveals that phase competition—associated with Fermi-surface reconstruction and charge-density-wave order—is a key limiting factor in the superconductivity of cuprates.

Electronic ground state of heavily overdoped nonsuperconducting La2-xSrxCuO4

We report detailed thermodynamic and transport measurements for nonsuperconducting

de Haas – van Alphen effect in single crystal MgB 2

{\mathrm{La}}_{1.7}{\mathrm{Sr}}_{0.3}{\mathrm{CuO}}_{4}.

Correlation between the superconducting transition temperature and anisotropic quasiparticle scattering in Tl 2 Ba 2 CuO 6+δ

Collectively, these data support the presence of a highly correlated Fermi-liquid ground state in

Fermi surface and electronic homogeneity of the overdoped cuprate superconductor Tl2Ba2CuO6+\delta as revealed by quantum oscillations

{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}

A detailed de Haas–van Alphen effect study of the overdoped cuprate Tl2Ba2CuO6+δ

beyond the superconducting dome, and imply that charge transport in the cuprates is dominated at finite temperatures by electron-electron scattering.

Fermi-surface reconstruction and two-carrier model for the Hall effect in YBa2Cu4O8

Abstract We present an experimental study of the de Haas–van Alphen effect in single crystals of MgB 2 , using a piezo-resistive torque technique. Three quasi-particle orbits were observed. Two originate from a single warped Fermi surface tube along the c direction, and the third from a cylindrical section of an in-plane honeycomb network. The quasi-particle effective masses on these orbits were determined and compared to band structure calculations. From this we deduce that the electron–phonon coupling strength λ , is a factor ∼3 larger for the c -axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.