Simple model of the static exchange-correlation kernel of a uniform electron gas with long-range electron-electron interaction

A simple approximate expression in real and reciprocal spaces is given for the static exchange-correlation kernel of a uniform electron gas interacting with the long-range part only of the Coulomb interaction. This expression interpolates between the exact asymptotic behaviors of this kernel at small and large wave vectors which in turn requires, among other thing, information from the momentum distribution of the uniform electron gas with the same interaction that have been calculated in the G0W0 approximation. This exchange-correlation kernel as well as its complement analogue associated to the short-range part of the Coulomb interaction are more local than the Coulombic exchange-correlation kernel and constitute potential ingredients in approximations for recent adiabatic connection fluctuation-dissipation and/or density functional theory approaches of the electronic correlation problem based on a separate treatment of long-range and short-range interaction effects.Comment: 14 pages, 14 figures, to be published in Phys. Rev.

Disorder-Induced Resistive Anomaly Near Ferromagnetic Phase Transitions

We show that the resistivity rho(T) of disordered ferromagnets near, and above, the Curie temperature T_c generically exhibits a stronger anomaly than the scaling-based Fisher-Langer prediction. Treating transport beyond the Boltzmann description, we find that within mean-field theory, d\rho/dT exhibits a |T-T_c|^{-1/2} singularity near T_c. Our results, being solely due to impurities, are relevant to ferromagnets with low T_c, such as SrRuO3 or diluted magnetic semiconductors, whose mobility near T_c is limited by disorder.Comment: 5 pages, 3 figures; V2: with a few clarifications, as publishe

Conserving Approximations in Time-Dependent Density Functional Theory

In the present work we propose a theory for obtaining successively better approximations to the linear response functions of time-dependent density or current-density functional theory. The new technique is based on the variational approach to many-body perturbation theory (MBPT) as developed during the sixties and later expanded by us in the mid nineties. Due to this feature the resulting response functions obey a large number of conservation laws such as particle and momentum conservation and sum rules. The quality of the obtained results is governed by the physical processes built in through MBPT but also by the choice of variational expressions. We here present several conserving response functions of different sophistication to be used in the calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio

Statistical characterization of the forces on spheres in an upflow of air

The dynamics of a sphere fluidized in a nearly-levitating upflow of air were previously found to be identical to those of a Brownian particle in a two-dimensional harmonic trap, consistent with a Langevin equation [Ojha {\it et al.}, Nature {\bf 427}, 521 (2004)]. The random forcing, the drag, and the trapping potential represent different aspects of the interaction of the sphere with the air flow. In this paper we vary the experimental conditions for a single sphere, and report on how the force terms in the Langevin equation scale with air flow speed, sphere radius, sphere density, and system size. We also report on the effective interaction potential between two spheres in an upflow of air.Comment: 7 pages, experimen

A colonial-nesting seabird shows no heart-rate response to drone-based population surveys

Aerial drones are increasingly being used as tools for ecological research and wildlife monitoring in hard-to-access study systems, such as in studies of colonial-nesting birds. Despite their many advantages over traditional survey methods, there remains concerns about possible disturbance effects that standard drone survey protocols may have on bird colonies. There is a particular gap in the study of their influence on physiological measures of stress. We measured heart rates of incubating female common eider ducks (Somateria mollissima) to determine whether our drone-based population survey affected them. To do so, we used heart-rate recorders placed in nests to quantify their heart rate in response to a quadcopter drone flying transects 30 m above the nesting colony. Eider heart rate did not change from baseline (measured in the absence of drone survey flights) by a drone flying at a fixed altitude and varying horizontal distances from the bird. Our findings suggest that carefully planned drone-based surveys of focal species have the potential to be carried out without causing physiological impacts among colonial-nesting eiders

Heightened heart rate but similar flight responses to evolved versus recent predators in an Arctic seabird

Predator-prey dynamics in the Arctic are being altered with changing sea ice phenology. The increasing frequency of predation on colonial nesting seabirds and their eggs by the polar bear (Ursus maritimus) is a consequence of bears shifting to terrestrial food resources through a shortened seal-hunting season. We examined antipredator responses in a colony of nesting Common Eiders (Somateria mollissima) on East Bay Island, Nunavut, Canada, which is exposed to established nest predators, such as arctic fox (Vulpes lagopus), but also to recent increases in polar bear nest predation due to the bears’ lost on-ice hunting opportunities. Given eiders’ limited eco-evolutionary experience with bears, we aimed to experimentally contrast eider responses to the recent predation pressure by polar bears to those induced by their more traditional mammalian predator, the arctic fox. Our goal was to characterize whether this population of eiders was vulnerable to a changing predator regime. Using simulated approaches of visual stimuli of both predator types, we measured eider heart rate and flight initiation distance as physiological and behavioral metrics, respectively, to characterize the perceived risk of and subsequent response to imminent threat posed by these two predators that differ in historical encounter rates. Eider heart rates were more responsive to impending visual cues of arctic foxes compared to polar bears, but birds responded behaviorally to all simulated threats with similar flight initiation distances. Results suggest eiders may not perceive the full risk that bears pose as egg and adult predators, and are therefore expected to suffer negative fitness consequences from this ongoing and increasing interaction. Eiders may therefore require conservation intervention to aid in their management

Bosonization of interacting fermions in arbitrary dimension beyond the Gaussian approximation

We use our recently developed functional bosonization approach to bosonize interacting fermions in arbitrary dimension $d$ beyond the Gaussian approximation. Even in $d=1$ the finite curvature of the energy dispersion at the Fermi surface gives rise to interactions between the bosons. In higher dimensions scattering processes describing momentum transfer between different patches on the Fermi surface (around-the-corner processes) are an additional source for corrections to the Gaussian approximation. We derive an explicit expression for the leading correction to the bosonized Hamiltonian and the irreducible self-energy of the bosonic propagator that takes the finite curvature as well as around-the-corner processes into account. In the special case that around-the-corner scattering is negligible, we show that the self-energy correction to the Gaussian propagator is negligible if the dimensionless quantities $( \frac{q_{c} }{ k_{F}} )^d F_{0} [ 1 + F_{0} ]^{-1} \frac{\mu}{\nu^{\alpha}} | \frac{ \partial \nu^{\alpha} }{ \partial \mu} |$ are small compared with unity for all patches $\alpha$. Here $q_{c}$ is the cutoff of the interaction in wave-vector space, $k_{F}$ is the Fermi wave-vector, $\mu$ is the chemical potential, $F_{0}$ is the usual dimensionless Landau interaction-parameter, and $\nu^{\alpha}$ is the {\it{local}} density of states associated with patch $\alpha$. We also show that the well known cancellation between vertex- and self-energy corrections in one-dimensional systems, which is responsible for the fact that the random-phase approximation for the density-density correlation function is exact in $d=1$, exists also in $d> 1$, provided (1) the interaction cutoff $q_{c}$ is small compared with $k_{F}$, and (2) the energy dispersion is locally linearized at the Fermi the Fermi surface. Finally, we suggest a new systematic method to calculate corrections to the RPA, which is based on the perturbative calculation of the irreducible bosonic self-energy arising from the non-Gaussian terms of the bosonized Hamiltonian.Comment: The abstract has been rewritten. No major changes in the text

Negative Electron-electron Drag Between Narrow Quantum Hall Channels

Momentum transfer due to Coulomb interaction between two parallel, two-dimensional, narrow, and spatially separated layers, when a current I_{drive} is driven through one layer, is studied in the presence of a perpendicular magnetic field B. The current induced in the drag layer, I_{drag}, is evaluated self-consistently with I_{drive} as a parameter. I_{drag} can be positive or negative depending on the value of the filling factor \nu of the highest occupied bulk Landau level (LL). For a fully occupied LL, I_{drag} is negative, i.e., it flows opposite to I_{drive}, whereas it is positive for a half-filled LL. When the circuit is opened in the drag layer, a voltage \Delta V_{drag} develops in it; it is negative for a half-filled LL and positive for a fully occupied LL. This positive \Delta V_{drag}, expressing a negative Coulomb drag, results from energetically favored near-edge inter-LL transitions that occur when the highest occupied bulk LL and the LL just above it become degenerate.Comment: Text file in Latex/Revtex/preprint format, 7 separate PS figures, Physical Review B, in pres

Effective action and density functional theory

The effective action for the charge density and the photon field is proposed as a generalization of the density functional. A simple definition is given for the density functional, as the functional Legendre transform of the generator functional of connected Green functions for the density and the photon field, offering systematic approximation schemes. The leading order of the perturbation expansion reproduces the Hartree-Fock equation. A renormalization group motivated method is introduced to turn on the Coulomb interaction gradually and to find corrections to the Hartree-Fock and the Kohn-Sham schemes.Comment: New references and a numerical algorithm added, to appear in Phys. Rev. B. 30 pages, no figure

Structure Factor and Electronic Structure of Compressed Liquid Rubidium

We have applied the quantal hypernetted-chain equations in combination with the Rosenfeld bridge-functional to calculate the atomic and the electronic structure of compressed liquid-rubidium under high pressure (0.2, 2.5, 3.9, and 6.1 GPa); the calculated structure factors are in good agreement with experimental results measured by Tsuji et al. along the melting curve. We found that the Rb-pseudoatom remains under these high pressures almost unchanged with respect to the pseudoatom at room pressure; thus, the effective ion-ion interaction is practically the same for all pressure-values. We observe that all structure factors calculated for this pressure-variation coincide almost into a single curve if wavenumbers are scaled in units of the Wigner-Seitz radius $a$ although no corresponding scaling feature is observed in the effective ion-ion interaction.This scaling property of the structure factors signifies that the compression in liquid-rubidium is uniform with increasing pressure; in absolute Q-values this means that the first peak-position ($Q_1$) of the structure factor increases proportionally to $V^{-1/3}$ ($V$ being the specific volume per ion), as was experimentally observed by Tsuji et al.Comment: 18 pages, 11 figure