David LeBoeuf

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

Despite twenty years of research, the phase diagram of high-transition-temperature superconductors remains enigmatic. 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 surface. The underdoped regime, however, is highly anomalous and appears to have no coherent Fermi surface, but only disconnected ‘Fermi arcs’. 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.

Electron pockets in the Fermi surface of hole-doped high-Tc superconductors

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping. The underlying evolution of the electron system in the absence of superconductivity is still unclear, and a question of central importance is whether it involves any intermediate phase with broken symmetry. The Fermi surface of the electronic states in the underdoped ‘YBCO’ materials YBa2Cu3Oy and YBa2Cu4O8 was recently shown to include small pockets, in contrast with the large cylinder that characterizes the overdoped regime, pointing to a topological change in the Fermi surface. Here we report the observation of a negative Hall resistance in the magnetic-field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like. We propose that these electron pockets most probably arise from a reconstruction of the Fermi surface caused by the onset of a density-wave phase, as is thought to occur in the electron-doped copper oxides near the onset of antiferromagnetic order. Comparison with materials of the La2CuO4 family that exhibit spin/charge density-wave order suggests that a Fermi surface reconstruction also occurs in those materials, pointing to a generic property of high-transition-temperature (Tc) superconductors.

Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor

The nature of the pseudogap phase is a central problem in the effort to understand the high-transition-temperature (high-Tc) copper oxide superconductors. A fundamental question is what symmetries are broken when the pseudogap phase sets in, which occurs when the temperature decreases below a value T*. There is evidence from measurements of both polarized neutron diffraction and the polar Kerr effect that time-reversal symmetry is broken, but at temperatures that differ significantly from one another. Broken rotational symmetry was detected from both resistivity measurements and inelastic neutron scattering at low doping, and from scanning tunnelling spectroscopy at low temperature, but showed no clear relation to T*. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa2Cu3Oy that sets in precisely at T* throughout the doping phase diagram. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO2 planes. We conclude that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry. This narrows the range of possible states considerably, pointing to stripe or nematic order.

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)

Thermodynamic phase diagram of static charge order in underdoped YBa2Cu3Oy

A thermodynamic probe of the recently discovered charge-density-wave order in YBa2Cu3Oy reveals a biaxial modulation in magnetic fields up to 40 T.

Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements

The Hall coefficient RH of the cuprate superconductor YBa 2Cu3Oy was measured in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152 in the underdoped regime. In fields large enough to suppress superconductivity, RH(T) is seen to go from positive at high temperature to negative at low temperature, for p0.08. This change of sign is attributed to the emergence of an electron pocket in the Fermi surface at low temperature. At p<0.08, the normal-state R H(T) remains positive at all temperatures, increasing monotonically as T→0. We attribute the change of behavior across p=0.08 to a Lifshitz transition, namely a change in Fermi-surface topology occurring at a critical concentration pL=0.08, where the electron pocket vanishes. The loss of the high-mobility electron pocket across pL coincides with a tenfold drop in the conductivity at low temperature, revealed in measurements of the electrical resistivity ρ at high fields, showing that the so-called metal-insulator crossover of cuprates is in fact driven by a Lifshitz transition. It also coincides with a jump in the in-plane anisotropy of ρ, showing that without its electron pocket, the Fermi surface must have strong twofold in-plane anisotropy. These findings are consistent with a Fermi-surface reconstruction caused by a unidirectional spin-density wave or stripe order.

Linear temperature dependence of resistivity and change in the Fermi surface at the pseudogap critical point of|[nbsp]|a|[nbsp]|high-Tc superconductor

Transport measurements in a high-temperature superconductor provide evidence that the so-called pseudogap phase ends at a quantum critical point located inside the superconducting dome in the phase diagram of cuprates.

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.

de Haas-van Alphen Oscillations in the Underdoped High-Temperature Superconductor YBa2Cu3O6.5

Cyril Jaudet, David Vignolles, ∗ Alain Audouard, Julien Levallois, D. LeBoeuf, Nicolas Doiron-Leyraud, B. Vignolle, M. Nardone, A. Zitouni, Ruixing Liang, 4 D.A. Bonn, 4 W.N. Hardy, 4 Louis Taillefer, 4 and Cyril Proust † Laboratoire National des Champs Magnétiques Pulsés (CNRS-UPS-INSA), Toulouse, France Département de physique & RQMP, Université de Sherbrooke, Sherbrooke, Canada Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada Canadian Institute for Advanced Research, Toronto, Canada (Dated: May 13, 2008)

Synthesis of Tricyclic Fused 3‐Aminopyridines through Intramolecular CoI‐Catalyzed [2+2+2] Cycloaddition between Ynamides, Nitriles, and Alkynes

Three-ring circus: An expedient route to tricyclic fused 2-trimethylsilyl-3-aminopyridines exhibiting unprecedented skeletons is described. The key step is a very efficient cobalt-catalyzed [2+2+2] cycloaddition of a polyunsaturated compound displaying an ynamide, an alkyne, and a nitrile functionality (see picture). The first [2+2+2] cocyclizations between ynamides, nitriles, and alkynes are reported. They open a new access to unprecedented nitrogen-containing heterocycles of type 2-trimethylsilyl-3-aminopyridines. Such frameworks, which can be found in various compounds of biological interest, are very difficult to prepare by conventional methods. However, using [CpCo(C(2)H(4))(2)] (Cp=cyclopentadienyl) as catalyst, the intramolecular cyclizations could be achieved in up to 100 % yield. The presence of the trimethylsilyl group allowed a rare type of Hiyama cross-coupling: one of the silylated pyridines could be coupled with p-iodoanisole to give a new type of biaryl system.