New Structure In The Shapley Supercluster

We present new radial velocities for 189 galaxies in a 91 sq. deg region of the Shapley supercluster measured with the FLAIR-II spectrograph on the UK Schmidt Telescope. The data reveal two sheets of galaxies linking the major concentrations of the supercluster. The supercluster is not flattened in Declination as was suggested previously and it may be at least 30 percent larger than previously thought with a correspondingly larger contribution to the motion of the Local Group.Comment: LaTex: 2 pages, 1 figure, includes conf_iap.sty style file. To appear in proceedings of The 14th IAP Colloquium: Wide Field Surveys in Cosmology, held in Paris, 1998 May 26--30, eds. S.Colombi, Y.Mellie

Fermi liquid behavior of the in-plane resistivity in the pseudogap state of YBa_2Cu_4O_8

Our knowledge of the ground state of underdoped hole-doped cuprates has evolved considerably over the last few years. There is now compelling evidence that inside the pseudogap phase, charge order breaks translational symmetry leading to a reconstructed Fermi surface made of small pockets. Quantum oscillations, [Doiron-Leyraud N, et al. (2007) Nature 447:564-568], optical conductivity [Mirzaei SI, et al. (2013) Proc Natl Acad Sci USA 110:5774-5778] and the validity of Wiedemann-Franz law [Grissonnache G, et al. (2016) Phys. Rev. B 93:064513] point to a Fermi liquid regime at low temperature in the underdoped regime. However, the observation of a quadratic temperature dependence in the electrical resistivity at low temperatures, the hallmark of a Fermi liquid regime, is still missing. Here, we report magnetoresistance measurements in the magnetic-field-induced normal state of underdoped YBa_2Cu_4O_8 which are consistent with a T^2 resistivity extending down to 1.5 K. The magnitude of the T^2 coefficient, however, is much smaller than expected for a single pocket of the mass and size observed in quantum oscillations, implying that the reconstructed Fermi surface must consist of at least one additional pocket.Comment: Main + SI : published versio

Correlation between Fermi surface transformations and superconductivity in the electron-doped high-TcT_c superconductor Nd2−x_{2-x}Cex_xCuO4_4

Two critical points have been revealed in the normal-state phase diagram of the electron-doped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$ by exploring the Fermi surface properties of high quality single crystals by high-field magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas effect shows that the weak superlattice potential responsible for the Fermi surface reconstruction in the overdoped regime extrapolates to zero at the doping level $x_c = 0.175$ corresponding to the onset of superconductivity. Second, the high-field Hall coefficient exhibits a sharp drop right below optimal doping $x_{\mathrm{opt}} = 0.145$ where the superconducting transition temperature is maximum. This drop is most likely caused by the onset of long-range antiferromagnetic ordering. Thus, the superconducting dome appears to be pinned by two critical points to the normal state phase diagram.Comment: 9 pages; 7 figures; 1 tabl

Phenomenology of the normal state in-plane transport properties of high-TcT_c cuprates

In this article, I review progress towards an understanding of the normal state (in-plane) transport properties of high-$T_c$ cuprates in the light of recent developments in both spectroscopic and transport measurement techniques. Against a backdrop of mounting evidence for anisotropic single-particle lifetimes in cuprate superconductors, new results have emerged that advocate similar momentum dependence in the transport decay rate $\Gamma$({\bf k}). In addition, enhancement of the energy scale (up to the bare bandwidth) over which spectroscopic information on the quasiparticle response can be obtained has led to the discovery of new, unforeseen features that surprisingly, may have a significant bearing on the transport properties at the dc limit. With these two key developments in mind, I consider here whether all the ingredients necessary for a complete phenomenological description of the anomalous normal state transport properties of high-$T_c$ cuprates are now in place.Comment: 31 pages, 10 figure

Skutterudite Results Shed Light on Heavy Fermion Physics

Only few selected examples among the great diversity of anomalous rare earth skutterudite are reviewed. Focus is first given on PrFe4P12 in comparison with URu2Si2. For PrFe4P12, great progress has been made on determining the nature of the order parameter (OP). A non magnetic order parameter with a multipolar component emerges here while for URu2Si2 the nature of the so-called hidden order remains mysterious. The two systems have several similarities in their temperature--pressure (T, P) and magnetic field--temperature (H, T) phase diagrams, in their spin dynamics, in their nesting character and in their high sensitivity to impurities. Advances on one side must stimulate new views on the other. Besides general considerations on the choice of the OP, a simple basic problem is the treatment of the Kondo coupling in a system with low charge carrier number for the cases of uncompensated and compensated semi-metal. An interesting problem is also the possible decoupling between exciton modes and itinerant carriers.Comment: 8 pages, 10 figures, proceedings of International Conference on "New Quantum Phenomena in Skutterudite and Related Systems

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

Pulsed field measurements of the Hall resistivity and magnetoresistance of underdoped YBa2Cu4O8 are analyzed self-consistently using a simple model based on coexisting electron and hole carriers. The resultant mobilities and Hall numbers are found to vary markedly with temperature. The conductivity of the hole carriers drops by one order of magnitude below 30 K, explaining the absence of quantum oscillations from these particular pockets. Meanwhile the Hall coefficient of the electron carriers becomes strongly negative below 50 K. The overall quality of the fits not only provides strong evidence for Fermi-surface reconstruction in Y-based cuprates, it also strongly constrains the type of reconstruction that might be occurring.Comment: 5 pages, 4 figures, updated after publication in Physical Review B (Rapid Communication

Shubnikov-de Haas oscillations in YBa_2Cu_4O_8

We report the observation of Shubnikov-de Haas oscillations in the underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$ (Y124). For field aligned along the c-axis, the frequency of the oscillations is $660\pm 30$ T, which corresponds to $\sim 2.4$ % of the total area of the first Brillouin zone. The effective mass of the quasiparticles on this orbit is measured to be $2.7\pm0.3$ times the free electron mass. Both the frequency and mass are comparable to those recently observed for ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (Y123-II). We show that although small Fermi surface pockets may be expected from band structure calculations in Y123-II, no such pockets are predicted for Y124. Our results therefore imply that these small pockets are a generic feature of the copper oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review Letters. Only minor changes to the text and reference

Evidence for a small hole pocket in the Fermi surface of underdoped YBa2Cu3Oy

The Fermi surface of a metal is the fundamental basis from which its properties can be understood. In underdoped cuprate superconductors, the Fermi surface undergoes a reconstruction that produces a small electron pocket, but whether there is another, as yet undetected portion to the Fermi surface is unknown. Establishing the complete topology of the Fermi surface is key to identifying the mechanism responsible for its reconstruction. Here we report the discovery of a second Fermi pocket in underdoped YBa2Cu3Oy, detected as a small quantum oscillation frequency in the thermoelectric response and in the c-axis resistance. The field-angle dependence of the frequency demonstrates that it is a distinct Fermi surface and the normal-state thermopower requires it to be a hole pocket. A Fermi surface consisting of one electron pocket and two hole pockets with the measured areas and masses is consistent with a Fermi-surface reconstruction caused by the charge-density-wave order observed in YBa2Cu3Oy, provided other parts of the reconstructed Fermi surface are removed by a separate mechanism, possibly the pseudogap.Comment: 23 pages, 5 figure