3,212 research outputs found
Interface relaxation and electrostatic charge depletion in the oxide heterostructure LaAlO3/SrTiO3
Performing an analysis within density functional theory, we develop insight
into the structural and electronic properties of the oxide heterostructure
LaAlO3/SrTiO3. Electrostatic surface effects are decomposed from the internal
lattice distortion in order to clarify their interplay. We first study the
interface relaxation by a multi-layer system without surface, and the surface
effects, separately, by a substrate-film system. While elongation of the TiO6
octahedra at the interface enhances the metallicity, reduction of the film
thickness has the opposite effect due to a growing charge depletion. The
interplay of these two effects, as reflected by the full lattice relaxation in
the substrate-film system, however, strongly depends on the film thickness. An
inversion of the TiO6 distortion pattern for films thinner than four LaAlO3
layers results in an insulating state.Comment: 10 pages, 7 figures, accepted by Europhysics Letter
Self-assembled Pt nanowires on Ge(001): Relaxation effects
Absorption of Pt on the Ge(001) surface results in stable self-organized Pt
nanowires, extending over some hundred nanometers. Based on band structure
calculations within density functional theory and the generalized gradient
approximation, the structural relaxation of the Ge--Pt surface is investigated.
The surface reconstruction pattern obtained agrees well with findings from
scanning tunneling microscopy. In particular, strong Pt--Pt dimerization is
characteristical for the nanowires. The surface electronic structure is
significantly perturbed due to Ge--Pt interaction, which induces remarkable
shifts of Ge states towards the Fermi energy. As a consequence, the topmost Ge
layers are subject to a metal-insulator transition.Comment: 4 pages, 2 figures, final version accepted by Europhys. Lett., minor
modifications of the tex
Magnetic ground state of coupled edge-sharing CuO_2 spin-chains
By means of density functional theory, we investigate the magnetic ground
state of edge-sharing CuO_2 spin-chains, as found in the
(La,Ca,Sr)_14Cu_24O_41system, for instance. Our data rely on spin-polarized
electronic structure calculations including onsite interaction (LDA+U) and an
effective model for the interchain coupling. Strong doping dependence of the
magnetic order is characteristic for edge-sharing CuO_2 spin-chains. We
determine the ground state magnetic structure as function of the spin-chain
filling and quantify the competing exchange interactions.Comment: 5 pages, 2 figures, 3 tables, accepted by Phys. Rev. Let
One-dimensional Hubbard model at quarter filling on periodic potentials
Using the Hubbard chain at quarter filling as a model system, we study the
ground state properties of highly doped antiferromagnets. In particular, the
Hubbard chain at quarter filling is unstable against 2k_F- and 4k_F-periodic
potentials, leading to a large variety of charge and spin ordered ground
states. Employing the density matrix renormalization group method, we compare
the energy gain of the ground state induced by different periodic potentials.
For interacting systems the lowest energy is found for a 2k_F-periodic magnetic
field, resulting in a band insulator with spin gap. For strong interaction, the
4k_F-periodic potential leads to a half-filled Heisenberg chain and thus to a
Mott insulating state without spin gap. This ground state is more stable than
the band insulating state caused by any non-magnetic 2k_F-periodic potential.
Adding more electrons, a cluster-like ordering is preferred.Comment: 8 pages, 5 figures, accepted by Phys. Rev.
Terrestrial and Solar Limits on Long-Lived Particles in a Dark Sector
Dark matter charged under a new gauge sector, as motivated by recent data,
suggests a rich GeV-scale "dark sector" weakly coupled to the Standard Model by
gauge kinetic mixing. The new gauge bosons can decay to Standard Model leptons,
but this mode is suppressed if decays into lighter dark sector particles are
kinematically allowed. These particles in turn typically have macroscopic decay
lifetimes that are constrained by two classes of experiments, which we discuss.
Lifetimes of 10 cm < c tau < 10^8 cm are constrained by existing terrestrial
beam-dump experiments. If, in addition, dark matter captured in the Sun (or
Earth) annihilates into these particles, lifetimes up to 10^15 cm are
constrained by solar observations. These bounds span fourteen orders of
magnitude in lifetime, but they are not exhaustive. Accordingly, we identify
promising new directions for experiments including searches for displaced
di-muons in B-factories, studies at high-energy and -intensity proton beam
dumps, precision gamma-ray and electronic measurements of the Sun, and
milli-charge searches re-analyzed in this new context.Comment: 9 pages, 9 figure
Magnetic ordering in the striped nickelate La5/3Sr1/3NiO4: A band structure point of view
We report on a comprehensive study of the electronic and magnetic structure
of the striped nickelate La5/3Sr1/3NiO4. The investigation is carried out using
band structure calculations based on density functional theory. A magnetic
structure compatible with experiment is obtained from spin-polarized
calculations within the generalized gradient approximation (GGA), whereas
inclusion of a local Coulomb interaction in the LDA+U framework results in a
different ground state. The influence of the various interaction parameters is
discussed in detail.Comment: 5 pages, 4 figures, 2 tables, accepted by Europhys. Let
On the Origin of Light Dark Matter Species
TeV-mass dark matter charged under a new GeV-scale gauge force can explain
electronic cosmic-ray anomalies. We propose that the CoGeNT and DAMA direct
detection experiments are observing scattering of light stable states --
"GeV-Matter" -- that are charged under this force and constitute a small
fraction of the dark matter halo. Dark higgsinos in a supersymmetric dark
sector are natural candidates for GeV-Matter that scatter off protons with a
universal cross-section of 5 x 10^{-38} cm^2 and can naturally be split by
10-30 keV so that their dominant interaction with protons is down-scattering.
As an example, down-scattering of an O(5) GeV dark higgsino can simultaneously
explain the spectra observed by both CoGeNT and DAMA. The event rates in these
experiments correspond to a GeV-Matter abundance of 0.2-1% of the halo mass
density. This abundance can arise directly from thermal freeze-out at weak
coupling, or from the late decay of an unstable TeV-scale WIMP. Our proposal
can be tested by searches for exotics in the BaBar and Belle datasets.Comment: 31 text pages, 4 figures, revision includes corrected Germanium
quenching factor and clarified text in Sec.
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