6,316 research outputs found
Magnetic properties of 3d-impurities substituted in GaAs
We have calculated the magnetic properties of substituted 3d-impurities
(Cr-Ni) in a GaAs host by means of first principles electronic structure
calculations. We provide a novel model explaining the ferromagnetic long rang
order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism
is shown to be due to delocalized spin-uncompensated As dangling bond
electrons. Besides the quantitative prediction of the magnetic moments, our
model provides an understanding of the halfmetallicity, and the raise of the
critical temperature with the impurity concentration
Ab-initio density functional studies of stepped TaC surfaces
We report on density functional total energy calculations of the step
formation and interaction energies for vicinal TaC(001) surfaces. Our
calculations show that double and triple-height steps are favored over
single-height steps for a given vicinal orientation, which is in agreement with
recent experimental observations. We provide a description of steps in terms of
atomic displacements and charge localization and predict an experimentally
observable rumpled structure of the step-edges, where the Ta atoms undergo
larger displacements compared to the C atoms.Comment: 4 pages, 4 figure
Electronic structure of the (111) and (-1-1-1) surfaces of cubic BN: A local-density-functional ab initio study
We present ab initio local-density-functional electronic structure
calculations for the (111) and (-1-1-1) surfaces of cubic BN. The energetically
stable reconstructions, namely the N adatom, N3 triangle models on the (111),
the (2x1), boron and nitrogen triangle patterns on the (-1-1-1) surface are
investigated. Band structure and properties of the surface states are discussed
in detail.Comment: 8 pages, 12 figure
Kohn Anomalies and Electron-Phonon Interaction in Graphite
We demonstrate that graphite phonon dispersions have two Kohn anomalies at
the Gamma-E_2g and K-A'1 modes. The anomalies are revealed by two sharp kinks.
By an exact analytic derivation, we show that the slope of these kinks is
proportional to the square of the electron-phonon coupling (EPC). Thus, we can
directly measure the EPC from the experimental dispersions. The Gamma-E_2g and
K-A'1 EPCs are particularly large, whilst they are negligible for all the other
modes at Gamma and K.Comment: 4 pages, 2 figure
First-principles study of spontaneous polarization in multiferroic BiFeO
The ground-state structural and electronic properties of ferroelectric
BiFeO are calculated using density functional theory within the local
spin-density approximation and the LSDA+U method. The crystal structure is
computed to be rhombohedral with space group , and the electronic
structure is found to be insulating and antiferromagnetic, both in excellent
agreement with available experiments. A large ferroelectric polarization of
90-100 C/cm is predicted, consistent with the large atomic
displacements in the ferroelectric phase and with recent experimental reports,
but differing by an order of magnitude from early experiments. One possible
explanation is that the latter may have suffered from large leakage currents.
However both past and contemporary measurements are shown to be consistent with
the modern theory of polarization, suggesting that the range of reported
polarizations may instead correspond to distinct switching paths in structural
space. Modern measurements on well-characterized bulk samples are required to
confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables
Broken parity and a chiral ground state in the frustrated magnet CdCr2O4
We present a model describing the lattice distortion and incommensurate
magnetic order in the spinel CdCr2O4, a good realization of the Heisenberg
"pyrochlore" antiferromagnet. The magnetic frustration is relieved through the
spin-Peierls distortion of the lattice involving a phonon doublet with odd
parity. The distortion stablizes a collinear magnetic order with the
propagation wavevector q=2\pi(0,0,1). The lack of inversion symmetry makes the
crystal structure chiral. The handedness is transferred to magnetic order by
the relativistic spin-orbit coupling: the collinear state is twisted into a
long spiral with the spins in the ac plane and q shifted to 2\pi(0,\delta,1).Comment: Incremental changes in response to referee report
The random phase approximation applied to ice
Standard density functionals without van der Waals interactions yield an
unsatisfactory description of ice phases, specifically, high density phases
occurring under pressure are too unstable compared to the common low density
phase I observed at ambient conditions. Although the description is
improved by using functionals that include van der Waals interactions, the
errors in relative volumes remain sizable. Here we assess the random phase
approximation (RPA) for the correlation energy and compare our results to
experimental data as well as diffusion Monte Carlo data for ice. The RPA yields
a very balanced description for all considered phases, approaching the accuracy
of diffusion Monte Carlo in relative energies and volumes. This opens a route
towards a concise description of molecular water phases on surfaces and in
cavities
Structural, electronic and magnetic properties of SrRuO under epitaxial strain
Using density functional theory within the local spin density approximation,
structural, electronic and magnetic properties of SRO are investigated. We
examine the magnitude of the orthorhombic distortion in the ground state and
also the effects of applying epitaxial constraints, whereby the influence of
large (in the range of ) in-plane strain resulting from coherent
epitaxy, for both [001] and [110] oriented films, have been isolated and
investigated. The overall pattern of the structural relaxations reveal coherent
distortions of the oxygen octahedra network, which determine stability of the
magnetic moment on the Ru ion. The structural and magnetic parameters exhibit
substantial changes allowing us to discuss the role of symmetry and
possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page
A model for the formation energies of alanates and boranates
We develop a simple model for the formation energies (FEs) of alkali and
lkaline earth alanates and boranates, based upon ionic bonding between metal
cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained
from first principles calculations and with experimental FEs. The model shows
that details of the crystal structure are relatively unimportant. The small
size of the (BH4)- anion causes a strong bonding in the crystal, which makes
boranates more stable than alanates. Smaller alkali or alkaline earth cations
do not give an increased FE. They involve a larger ionization potential that
compensates for the increased crystal bonding.Comment: 3 pages, 2 figure
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