8,774 research outputs found
First-principles perturbative computation of dielectric and Born charge tensors in finite electric fields
We present a perturbative treatment of the response properties of insulating
crystals under a dc bias field, and use this to study the effects of such bias
fields on the Born effective charge tensor and dielectric tensor of insulators.
We start out by expanding a variational field-dependent total-energy functional
with respect to the electric field within the framework of density-functional
perturbation theory. The second-order term in the expansion of the total energy
is then minimized with respect to the first-order wave functions, from which
the Born effective charge tensor and dielectric tensor are easily computed. We
demonstrate an implementation of the method and perform illustrative
calculations for the III-V semiconductors AlAs and GaAs under finite bias
field
Image-charge induced localization of molecular orbitals at metal-molecule interfaces: Self-consistent GW calculations
Quasiparticle (QP) wave functions, also known as Dyson orbitals, extend the
concept of single-particle states to interacting electron systems. Here we
employ many-body perturbation theory in the GW approximation to calculate the
QP wave functions for a semi-empirical model describing a -conjugated
molecular wire in contact with a metal surface. We find that image charge
effects pull the frontier molecular orbitals toward the metal surface while
orbitals with higher or lower energy are pushed away. This affects both the
size of the energetic image charge shifts and the coupling of the individual
orbitals to the metal substrate. Full diagonalization of the QP equation and,
to some extent, self-consistency in the GW self-energy, is important to
describe the effect which is not captured by standard density functional theory
or Hartree-Fock. These results should be important for the understanding and
theoretical modeling of electron transport across metal-molecule interfaces.Comment: 7 pages, 6 figure
Sodium: a charge-transfer insulator at high pressures
By means of first-principles methods we analyze the optical response of
transparent dense sodium as a function of applied pressure. We discover an
unusual kind of charge-transfer exciton that proceeds from the interstitial
distribution of valence electrons repelled away from the ionic cores by the
Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum
shows a strong anisotropy with light polarization that just at pressures above
the metal-insulator transition manifests as sodium being optically transparent
in one direction but reflective in the other. This result provides a key
information about the crystal structure of transparent sodium, a new
unconventional inorganic electride.Comment: revtex4, 5+8 page
Local Density of States and Angle-Resolved Photoemission Spectral Function of an Inhomogeneous D-wave Superconductor
Nanoscale inhomogeneity seems to be a central feature of the d-wave
superconductivity in the cuprates. Such a feature can strongly affect the local
density of states (LDOS) and the spectral weight functions. Within the
Bogoliubov-de Gennes formalism we examine various inhomogeneous configurations
of the superconducting order parameter to see which ones better agree with the
experimental data. Nanoscale large amplitude oscillations in the order
parameter seem to fit the LDOS data for the underdoped cuprates. The
one-particle spectral function for a general inhomogeneous configuration
exhibits a coherent peak in the nodal direction. In contrast, the spectral
function in the antinodal region is easily rendered incoherent by the
inhomogeneity. This throws new light on the dichotomy between the nodal and
antinodal quasiparticles in the underdoped cuprates.Comment: 5 pages, 9 pictures. Phys. Rev. B (in press
Friedel sum rule for an interacting multiorbital quantum dot
A generalized Friedel sum rule is derived for a quantum dot with internal
orbital and spin degrees of freedom. The result is valid when all many-body
correlations are taken into account and it links the phase shift of the
scattered electron to the displacement of its SPECTRAL density into the dot.Comment: RevTeX 4.0, 5 page
Efficient quantum transport simulation for bulk graphene heterojunctions
The quantum transport formalism based on tight-binding models is known to be
powerful in dealing with a wide range of open physical systems subject to
external driving forces but is, at the same time, limited by the memory
requirement's increasing with the number of atomic sites in the scattering
region. Here we demonstrate how to achieve an accurate simulation of quantum
transport feasible for experimentally sized bulk graphene heterojunctions at a
strongly reduced computational cost. Without free tuning parameters, we show
excellent agreement with a recent experiment on Klein backscattering [A. F.
Young and P. Kim, Nature Phys. 5, 222 (2009)].Comment: 5 pages, 3 figure
Exotic and native earthworms in various land use systems of Central, southern and Eastern Uruaguay.
From 1995 to 2001 we conducted several surveys of earthworm populations in 7 land use systems, of varying intensity of disturbance, in Central, Southern and Eastern Uruguay. We evaluated the presence and density of various earthworm genera and species in selected land use systems. Most species found were exotic, belonging to the Lumbricidae (6 species) and Megascolecidae (1 species) families. We found only two native species, one ocnerodrilid and one acanthodrilid. Lumbricids were generally in disturbed agroecosystems, although native species were also found in some disturbed sites. Eukerria sp. had a mean density of 196 ind./m2 in a rice field. The apple orchard, Eucalyptus sp. plantation and natural prairie showed maximum mean densities of 102 ind./m2, 733 ind./m2 and 317 ind./m2, respectively. Our results show the great adaptability of lumbricids to various levels of disturbance and suggest the possibility of a large incidence of exotic worm species in Uruguayan soils
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