399 research outputs found
Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys
The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering.Comment: 4 pages, 4 figure
Momentum anisotropy of the scattering rate in cuprate superconductors
We examine the momentum and energy dependence of the scattering rate of the
high temperature cuprate superconductors using angle resolved photoemission
spectroscopy. The scattering rate is of the form a + b*w. The inelastic
coefficient b is found to be isotropic. The elastic term, a, however, is found
to be highly anisotropic in the pseudogap phase of optimal doped samples, with
an anisotropy which correlates with that of the pseudogap. This can be
contrasted with heavily overdoped samples, which show an isotropic scattering
rate in the normal state
Renormalization of spectral lineshape and dispersion below Tc in Bi2Sr2CaCu2O8+d
Angle-resolved photoemission (ARPES) data in the superconducting state of
Bi2Sr2CaCu2O8+d show a kink in the dispersion along the zone diagonal, which is
related via a Kramers-Kronig analysis to a drop in the low-energy scattering
rate. As one moves towards (pi,0), this kink evolves into a spectral dip. The
occurrence of these anomalies in the dispersion and lineshape throughout the
zone indicate the presence of a new energy scale in the superconducting state.Comment: New Figure 3 with expanded discussio
Early Evidence of Natal-Habitat Preference: Juvenile Loons Feed on Natal-Like Lakes After Fledging
Many species show natal habitat preference induction (NHPI), a behavior in which young adults select habitats similar to those in which they were raised. However, we know little about how NHPI develops in natural systems. Here, we tested for NHPI in juvenile common loons (Gavia immer) that foraged on lakes in the vicinity of their natal lake after fledging. Juveniles visited lakes similar in pH to their natal lakes, and this significant effect persisted after controlling for spatial autocorrelation. On the other hand, juveniles showed no preference for foraging lakes of similar size to their natal one. When lakes were assigned to discrete classes based on size, depth, visibility, and trophic complexity, both juveniles from large lakes and small lakes preferred to visit large, trophically diverse lakes, which contained abundant food. Our results contrast with earlier findings, which show strict preference for lakes similar in size to the natal lake among young adults seeking to settle on a breeding lake. We suggest that NHPI is relaxed for juveniles, presumably because they select lakes that optimize shortâterm survival and growth. By characterizing NHPI during a poorly studied life stage, this study illustrates that NHPI can take different forms at different life stages
Quasiparticles in the superconducting state of Bi2Sr2CaCu2O8
Recent improvements in momentum resolution by a factor of 32 lead to
qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along
the (pi,pi) direction, where there is a node in the superconducting gap. With
improved resolution, we now see the intrinsic lineshape, which indicates the
presence of true quasiparticles at the Fermi momentum in the superconducting
state, and lack thereof in the normal state. The region of momentum space
probed here is relevant for charge transport, motivating a comparison of our
results to conductivity measurements by infrared reflectivity.Comment: revised paper with new figure
Strong electronic correlations in superconducting organic charge transfer salts
We review the role of strong electronic correlations in
quasi--two-dimensional organic charge transfer salts such as (BEDT-TTF),
(BETS) and -[Pd(dmit)]. We begin by defining minimal
models for these materials. It is necessary to identify two classes of
material: the first class is strongly dimerised and is described by a
half-filled Hubbard model; the second class is not strongly dimerised and is
described by a quarter filled extended Hubbard model. We argue that these
models capture the essential physics of these materials. We explore the phase
diagram of the half-filled quasi--two-dimensional organic charge transfer
salts, focusing on the metallic and superconducting phases. We review work
showing that the metallic phase, which has both Fermi liquid and `bad metal'
regimes, is described both quantitatively and qualitatively by dynamical mean
field theory (DMFT). The phenomenology of the superconducting state is still a
matter of contention. We critically review the experimental situation, focusing
on the key experimental results that may distinguish between rival theories of
superconductivity, particularly probes of the pairing symmetry and measurements
of the superfluid stiffness. We then discuss some strongly correlated theories
of superconductivity, in particular, the resonating valence bond (RVB) theory
of superconductivity. We conclude by discussing some of the major challenges
currently facing the field.Comment: A review: 52 pages; 10 fig
Band-structure trend in hole-doped cuprates and correlation with Tcmax
By calculation and analysis of the bare conduction bands in a large number of
hole-doped high-temperature superconductors, we have identified the energy of
the so-called axial-orbital as the essential, material-dependent parameter. It
is uniquely related to the range of the intra-layer hopping. It controls the Cu
4s-character, influences the perpendicular hopping, and correlates with the
observed Tc at optimal doping. We explain its dependence on chemical
composition and structure, and present a generic tight-binding model.Comment: 5 pages, Latex, 5 eps figure
A Novel 2D Folding Technique for Enhancing Fermi Surface Signatures in the Momentum Density: Application to Compton Scattering Data from an Al-3at%Li Disordered Alloy
We present a novel technique for enhancing Fermi surface (FS) signatures in
the 2D distribution obtained after the 3D momentum density in a crystal is
projected along a specific direction in momentum space. These results are
useful for investigating fermiology via high resolution Compton scattering and
positron annihilation spectroscopies. We focus on the particular case of the
(110) projection in an fcc crystal where the standard approach based on the use
of the Lock-Crisp-West (LCW) folding theorem fails to give a clear FS image due
to the strong overlap with FS images obtained through projection from higher
Brillouin zones. We show how these superposed FS images can be disentangled by
using a selected set of reciprocal lattice vectors in the folding process. The
applicability of our partial folding scheme is illustrated by considering
Compton spectra from an Al-3at%Li disordered alloy single crystal. For this
purpose, high resolution Compton profiles along nine directions in the (110)
plane were measured. Corresponding highly accurate theoretical profiles in
Al-3at%Li were computed within the local density approximation (LDA)-based
Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA)
first-principles framework. A good level of overall accord between theory and
experiment is obtained, some expected discrepancies reflecting electron
correlation effects notwithstanding, and the partial folding scheme is shown to
yield a clear FS image in the (110) plane in Al-3%Li.Comment: 24 pages, 8 figures, to appear in Phys. Rev.
Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping Dependent ARPES Study
Comparing ARPES measurements on Bi2212 with penetration depth data, we show
that a description of the nodal excitations of the d-wave superconducting state
in terms of non-interacting quasiparticles is inadequate, and we estimate the
magnitude and doping dependence of the Landau interaction parameter which
renormalizes the linear T contribution to the superfluid density. Furthermore,
although consistent with d-wave symmetry, the gap with underdoping cannot be
fit by the simple coskx-cosky form, which suggests an increasing importance of
long range interactions as the insulator is approached.Comment: 4 pages, 3 eps figs, manuscript and Fig. 3 significantly revise
The functional role of biodiversity in ecosystems: incorporating trophic complexity
Understanding how biodiversity affects functioning of ecosystems requires integrating diversity within trophic levels (horizontal diversity) and across trophic levels (vertical diversity, including food chain length and omnivory). We review theoretical and experimental progress toward this goal. Generally, experiments show that biomass and resource use increase similarly with horizontal diversity of either producers or consumers. Among prey, higher diversity often increases resistance to predation, due to increased probability of including inedible species and reduced efficiency of specialist predators confronted with diverse prey. Among predators, changing diversity can cascade to affect plant biomass, but the strength and sign of this effect depend on the degree of omnivory and prey behaviour. Horizontal and vertical diversity also interact: adding a trophic level can qualitatively change diversity effects at adjacent levels. Multitrophic interactions produce a richer variety of diversityâfunctioning relationships than the monotonic changes predicted for single trophic levels. This complexity depends on the degree of consumer dietary generalism, tradeâoffs between competitive ability and resistance to predation, intraguild predation and openness to migration. Although complementarity and selection effects occur in both animals and plants, few studies have conclusively documented the mechanisms mediating diversity effects. Understanding how biodiversity affects functioning of complex ecosystems will benefit from integrating theory and experiments with simulations and networkâbased approaches
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