Two papers in this issue report notable contributions towards an understanding of high-temperature superconductivity, still an elusive goal after more than 20 years of intensive research. Doiron-Leyraud et al. report the observation of a Fermi surface in a high-temperature superconductor, a phenomenon regarded as the classic signature of a metal. Gomes et al. tackled the long-standing question of whether the gap in the electronic energy spectrum at temperatures above the critical temperature of a high-temperature superconductor is associated with electron pairing. They find that it is. With these two elegant experimental papers adding some solid new data to the mix, it is the turn of the theorists to work out the implications for superconductivity mechanisms. The observation of quantum oscillations in the electrical resistance of YBa2Cu3O6.5, is reported, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides (once superconductivity is suppressed by a magnetic field). The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Despite twenty years of research, the phase diagram of high-transition-temperature superconductors remains enigmatic1,2. A central issue is the origin of the differences in the physical properties of these copper oxides doped to opposite sides of the superconducting region. In the overdoped regime, the material behaves as a reasonably conventional metal, with a large Fermi surface3,4. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected ‘Fermi arcs’5,6. The fundamental question, then, is whether underdoped copper oxides have a Fermi surface, and if so, whether it is topologically different from that seen in the overdoped regime. Here we report the observation of quantum oscillations in the electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5, establishing the existence of a well-defined Fermi surface in the ground state of underdoped copper oxides, once superconductivity is suppressed by a magnetic field. The low oscillation frequency reveals a Fermi surface made of small pockets, in contrast to the large cylinder characteristic of the overdoped regime. Two possible interpretations are discussed: either a small pocket is part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from a topological change at a critical point in the phase diagram. Our understanding of high-transition-temperature (high-Tc) superconductors will depend critically on which of these two interpretations proves to be correct.

Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor.

High magnetic fields: a tool for studying electronic properties of layered organic metals.

Synthesis, physical, magnetic and electrical properties of Al–Ga substituted co-precipitated nanocrystalline strontium hexaferrite

Structural, magnetic and dielectric properties of Zr–Cd substituted strontium hexaferrite (SrFe12O19) nanoparticles

Epitaxial growth of transition-metal silicides on silicon

Below a critical filling factor (v c =0.28) and critical temperature (T c =1.4 K at 26 T) an additional line has been observed in the luminescence spectrum of a GaAs-Al x Ga 1-x As heterojunction; it grows in intensity with decreasing v, dominating the spectrum at v<1/11. The lack of correlation between v c and the disorder-related properties of the system indicates the intrinsic nature of the line which we propose signals the formation of a pinned Wigner solid

Novel magneto-optical behavior in the Wigner-solid regime.

Discontinuities have been observed in the energy dependence on magnetic field of the luminescence line corresponding to 2D electron-hole recombination in GaAs/AlGaAs heterojunctions at filling factors \ensuremath{\nu}=2/3, 1/3, 4/5, 3/5, 2/5, 1/5, 1/7, and 1/9. We associate these energy-position shifts with discontinuous behavior of the chemical potential due to condensation of the 2D electrons into an incompressible Fermi-liquid state, and thereby evaluate the energy gaps at fractional filling factors down to 1/9.

Magneto-optical evidence for fractional quantum Hall states down to filling factor 1/9.

Low-temperature, high-field deHaas--van Alphen measurements are presented which show that the conduction-electron mass in the Kondo lattice ${\mathrm{CeB}}_{6}$ decreases strongly with field. This field dependence is consistent with recent specific-heat results. The geometry of the Fermi surface does not depend on field. Thus we observe a reduction in the many-body enhancement of the electronic density of states at the Fermi energy which is described by a change of the itinerant-electron mass alone; the number of particles and the occupation of states in k space remain unchanged. We argue that ${\mathrm{CeB}}_{6}$ represents a different limit of heavy-fermion behavior as compared to ${\mathrm{UPt}}_{3}$.

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Observation of the Magnetic Field Dependence of the Cyclotron Mass in the Kondo Lattice CeB6

We have performed a detailed anisotropy study of the superconducting properties of untwinned single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for \ensuremath{\delta}\ensuremath{\approxeq}0.1 and \ensuremath{\delta}\ensuremath{\approxeq}0.4 in magnetic fields up to 22 T using a rf method giving critical fields ${\mathrm{H}}^{\mathrm{*}}$ which are determined by the onset of flux-line motion. For \ensuremath{\delta}\ensuremath{\approxeq}0.1 we find the highest ${\mathrm{H}}^{\mathrm{*}}$ along the b axis; the a-b anisotropy amounts to 20% at 85 K. The 60-K superconductivity for \ensuremath{\delta}\ensuremath{\approxeq}0.4 is characterized by the largest ${\mathrm{H}}^{\mathrm{*}}$ along the a axis and pronounced 2D behavior as manifested by a cusp in the angular dependence of ${\mathrm{H}}^{\mathrm{*}}$ and by the extremely steep slope of ${\mathrm{H}}^{\mathrm{*}}$(T) perpendicular to c. We discuss our experimental results with respect to the significance of the Cu-O chains for the high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$.

W. Joss

Papers

Novel magneto-optical behavior in the Wigner-solid regime.

Magneto-optical evidence for fractional quantum Hall states down to filling factor 1/9.

Observation of the Magnetic Field Dependence of the Cyclotron Mass in the Kondo Lattice CeB6

Anisotropic superconducting properties of YBa 2 Cu 3 O 7 − δ ( δ=0.1 and 0.4) untwinned single crystals

Shubnikov-de Haas effect and the fermi surface of χ-(BEDT-TTF)2Cu(NCS)2