24,112 research outputs found
Approximation of corner polyhedra with families of intersection cuts
We study the problem of approximating the corner polyhedron using
intersection cuts derived from families of lattice-free sets in .
In particular, we look at the problem of characterizing families that
approximate the corner polyhedron up to a constant factor, which depends only
on and not the data or dimension of the corner polyhedron. The literature
already contains several results in this direction. In this paper, we use the
maximum number of facets of lattice-free sets in a family as a measure of its
complexity and precisely characterize the level of complexity of a family
required for constant factor approximations. As one of the main results, we
show that, for each natural number , a corner polyhedron with basic
integer variables and an arbitrary number of continuous non-basic variables is
approximated up to a constant factor by intersection cuts from lattice-free
sets with at most facets if and that no such approximation is
possible if . When the approximation factor is allowed to
depend on the denominator of the fractional vertex of the linear relaxation of
the corner polyhedron, we show that the threshold is versus .
The tools introduced for proving such results are of independent interest for
studying intersection cuts
Three-dimensional MgB-type superconductivity in hole-doped diamond
We substantiate by calculations that the recently discovered
superconductivity below 4 K in 3% boron-doped diamond is caused by
electron-phonon coupling of the same type as in MgB, albeit in 3
dimensions. Holes at the top of the zone-centered, degenerate -bonding
valence band couple strongly to the optical bond-stretching modes. The increase
from 2 to 3 dimensions reduces the mode-softening crucial for reaching
40 K in MgB Even if diamond had the same \emph{bare} coupling constant
as MgB which could be achieved with 10% doping, would only be 25
K. Superconductivity above 1 K in Si (Ge) requires hole-doping beyond 5% (10%).Comment: revised version, accepted by PR
Interplay between nanometer-scale strain variations and externally applied strain in graphene
We present a molecular modeling study analyzing nanometer-scale strain
variations in graphene as a function of externally applied tensile strain. We
consider two different mechanisms that could underlie nanometer-scale strain
variations: static perturbations from lattice imperfections of an underlying
substrate and thermal fluctuations. For both cases we observe a decrease in the
out-of-plane atomic displacements with increasing strain, which is accompanied
by an increase in the in-plane displacements. Reflecting the non-linear elastic
properties of graphene, both trends together yield a non-monotonic variation of
the total displacements with increasing tensile strain. This variation allows
to test the role of nanometer-scale strain variations in limiting the carrier
mobility of high-quality graphene samples
First-principles scattering matrices for spin-transport
Details are presented of an efficient formalism for calculating transmission
and reflection matrices from first principles in layered materials. Within the
framework of spin density functional theory and using tight-binding muffin-tin
orbitals, scattering matrices are determined by matching the wave-functions at
the boundaries between leads which support well-defined scattering states and
the scattering region. The calculation scales linearly with the number of
principal layers N in the scattering region and as the cube of the number of
atoms H in the lateral supercell. For metallic systems for which the required
Brillouin zone sampling decreases as H increases, the final scaling goes as
H^2*N. In practice, the efficient basis set allows scattering regions for which
H^{2}*N ~ 10^6 to be handled. The method is illustrated for Co/Cu multilayers
and single interfaces using large lateral supercells (up to 20x20) to model
interface disorder. Because the scattering states are explicitly found,
``channel decomposition'' of the interface scattering for clean and disordered
interfaces can be performed.Comment: 22 pages, 13 figure
Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices
We suggest a simple experimental method for probing antiferromagnetic spin
correlations of two-component Fermi gases in optical lattices. The method
relies on a spin selective Raman transition to excite atoms of one spin species
to their first excited vibrational mode where the tunneling is large. The
resulting difference in the tunneling dynamics of the two spin species can then
be exploited, to reveal the spin correlations by measuring the number of doubly
occupied lattice sites at a later time. We perform quantum Monte Carlo
simulations of the spin system and solve the optical lattice dynamics
numerically to show how the timed probe can be used to identify
antiferromagnetic spin correlations in optical lattices.Comment: 5 pages, 5 figure
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
High energy cosmic-ray interactions with particles from the Sun
Cosmic-ray protons with energies above eV passing near the Sun may
interact with photons emitted by the Sun and be excited to a
resonance. When the decays, it produces pions which further decay to
muons and photons which may be detected with terrestrial detectors. A flux of
muons, photon pairs (from decay), or individual high-energy photons
coming from near the Sun would be a rather striking signature, and the flux of
these particles is a fairly direct measure of the flux of cosmic-ray nucleons,
independent of the cosmic-ray composition. In a solid angle within
around the Sun the flux of photon pairs is about \SI{1.3e-3}{}
particles/(kmyr), while the flux of muons is about \SI{0.33e-3}{}
particles/(kmyr). This is beyond the reach of current detectors like
the Telescope Array, Auger, KASCADE-Grande or IceCube. However, the muon flux
might be detectable by next-generation air shower arrays or neutrino detectors
such as ARIANNA or ARA. We discuss the experimental prospects in some detail.
Other cosmic-ray interactions occuring close to the Sun are also briefly
discussed.Comment: 8 pages, 11 figure
First principles theoretical studies of half-metallic ferromagnetism in CrTe
Using full-potential linear augmented plane wave method (FP-LAPW) and the
density functional theory, we have carried out a systematic investigation of
the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in
three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type
(NA) hexagonal. Although the ground state is of NA structure, RS and ZB are
interesting in that these fcc-based structures, which can possibly be grown on
many semiconductor substrates, exhibit half-metallic phases above some critical
values of the lattice parameter. We find that the NA structure is not
half-metallic at its equilibrium volume, while both ZB and RS structures are.
The RS structure is more stable than the ZB, with an energy that is lower by
0.25 eV/atom. While confirming previous results on the half-metallic phase in
ZB structure, we provide hitherto unreported results on the half-metallic RS
phase, with a gap in the minority channel and a magnetic moment of 4.0
per formula unit. A comparison of total energies for the
ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM)
configurations shows the lowest energy configuration to be FM for CrTe in all
the three structures. The FP-LAPW calculations are supplemented by linear
muffin-tin orbital (LMTO) calculations using both local density approximation
(LDA) and LDA+U method. The exchange interactions and the Curie temperatures
calculated via the linear response method in ZB and RS CrTe are compared over a
wide range of the lattice parameter. The calculated Curie temperatures for the
RS phase are consistently higher than those for the ZB phase.Comment: 11 pages, 14 figure
Three-loop HTL gluon thermodynamics at intermediate coupling
We calculate the thermodynamic functions of pure-glue QCD to three-loop order
using the hard-thermal-loop perturbation theory (HTLpt) reorganization of
finite temperature quantum field theory. We show that at three-loop order
hard-thermal-loop perturbation theory is compatible with lattice results for
the pressure, energy density, and entropy down to temperatures .
Our results suggest that HTLpt provides a systematic framework that can used to
calculate static and dynamic quantities for temperatures relevant at LHC.Comment: 24 pages, 13 figs. 2nd version: improved discussion and fixing typos.
Published in JHE
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