4,268 research outputs found
Rational Hadamard products via Quantum Diagonal Operators
We use the remark that, through Bargmann-Fock representation, diagonal
operators of the Heisenberg-Weyl algebra are scalars for the Hadamard product
to give some properties (like the stability of periodic fonctions) of the
Hadamard product by a rational fraction. In particular, we provide through this
way explicit formulas for the multiplication table of the Hadamard product in
the algebra of rational functions in \C[[z]]
Electron Confinement, Orbital Ordering, and Orbital Moments in - Oxide Heterostructures
The (SrTiO)/(SrVO) multilayer system is studied
with first principles methods through the observed insulator-to-metal
transition with increasing thickness of the SrVO layer. When correlation
effects with reasonable magnitude are included, crystal field splittings from
the structural relaxations together with spin-orbit coupling (SOC) determines
the behavior of the electronic and magnetic structures. These confined slabs of
SrVO prefer =() orbital ordering of and
() orbitals within the plane, accompanied by
=(0,0) spin order (ferromagnetic alignment). The result is a
SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75
strongly compensates the spin moment on the sublattice. The
insulator-metal transition for (occurring between =4 and
=5) is reproduced. Unlike in the isoelectronic TiO/VO
(rutile structure) system and in spite of some similarities in orbital
ordering, no semi-Dirac point [{\it Phys. Rev. Lett.} {\bf 102}, 166803 (2009)]
is encountered, but the insulator-to-metal transition occurs through a
different type of unusual phase. For n=5 this system is very near (or at) a
unique semimetallic state in which the Fermi energy is topologically determined
and the Fermi surface consists of identical electron and hole Fermi circles
centered at =0. The dispersion consists of what can be regarded as a
continuum of radially-directed Dirac points, forming a "Dirac circle".Comment: 9 pages, 8 figure
First principles investigation of the electronic structure of La2MnNiO6: A room-temperature insulating ferromagnet
Using first principles calculations within DFT based on the full potential
APW+lo method, we calculated the electronic and magnetic structures for the
ferromagnetic and antiferromagnetic states of La2MnNiO6 and analyzed the site
projected density of states and electronic band structures. Our calculations
show that the ground state of La2MnNiO6 is ferromagnetic insulating with the
magnetization in agreement with Hund's first rule and experimental findings.Comment: 10 pages, 3 figure
Statistics on Graphs, Exponential Formula and Combinatorial Physics
The concern of this paper is a famous combinatorial formula known under the
name "exponential formula". It occurs quite naturally in many contexts
(physics, mathematics, computer science). Roughly speaking, it expresses that
the exponential generating function of a whole structure is equal to the
exponential of those of connected substructures. Keeping this descriptive
statement as a guideline, we develop a general framework to handle many
different situations in which the exponential formula can be applied
Reversible strain effect on the magnetization of LaCoO3 films
The magnetization of ferromagnetic LaCoO3 films grown epitaxially on
piezoelectric substrates has been found to systematically decrease with the
reduction of tensile strain. The magnetization change induced by the reversible
strain variation reveals an increase of the Co magnetic moment with tensile
strain. The biaxial strain dependence of the Curie temperature is estimated to
be below 4K/% in the as-grown tensile strain state of our films. This is in
agreement with results from statically strained films on various substrates
t-J model of coupled CuO ladders in SrCaCuO
Starting from the proper charge transfer model for CuO coupled
ladders in SrCaCuO we derive the low energy
Hamiltonian for this system. It occurs that the widely used ladder t-J model is
not sufficient and has to be supplemented by the Coulomb repulsion term between
holes in the neighboring ladders. Furthermore, we show how a simple mean-field
solution of the derived t-J model may explain the onset of the charge density
wave with the odd period in SrCaCuO.Comment: 8 pages, 4 figures, 2 table
Exchange parameters from approximate self-interaction correction scheme
The approximate atomic self-interaction corrections (ASIC) method to density
functional theory is put to the test by calculating the exchange interaction
for a number of prototypical materials, critical to local exchange and
correlation functionals. ASIC total energy calculations are mapped onto an
Heisenberg pair-wise interaction and the exchange constants J are compared to
those obtained with other methods. In general the ASIC scheme drastically
improves the bandstructure, which for almost all the cases investigated
resemble closely available photo-emission data. In contrast the results for the
exchange parameters are less satisfactory. Although ASIC performs reasonably
well for systems where the magnetism originates from half-filled bands, it
suffers from similar problems than those of LDA for other situations. In
particular the exchange constants are still overestimated. This reflects a
subtle interplay between exchange and correlation energy, not captured by the
ASIC.Comment: 10 page
First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite
We present a first-principles density functional study of the structural,
electronic and magnetic properties of the ferroelectric domain walls in
multiferroic BiFeO3. We find that domain walls in which the rotations of the
oxygen octahedra do not change their phase when the polarization reorients are
the most favorable, and of these the 109 degree domain wall centered around the
BiO plane has the lowest energy. The 109 degree and 180 degree walls have a
significant change in the component of their polarization perpendicular to the
wall; the corresponding step in the electrostatic potential is consistent with
a recent report of electrical conductivity at the domain walls. Finally, we
show that changes in the Fe-O-Fe bond angles at the domain walls cause changes
in the canting of the Fe magnetic moments which can enhance the local
magnetization at the domain walls.Comment: 9 pages, 20 figure
High Energy Positrons From Annihilating Dark Matter
Recent preliminary results from the PAMELA experiment indicate the presence
of an excess of cosmic ray positrons above 10 GeV. In this letter, we consider
possibility that this signal is the result of dark matter annihilations taking
place in the halo of the Milky Way. Rather than focusing on a specific particle
physics model, we take a phenomenological approach and consider a variety of
masses and two-body annihilation modes, including W+W-, ZZ, b bbar, tau+ tau-,
mu+ mu-, and e+e. We also consider a range of diffusion parameters consistent
with current cosmic ray data. We find that a significant upturn in the positron
fraction above 10 GeV is compatible with a wide range of dark matter
annihilation modes, although very large annihilation cross sections and/or
boost factors arising from inhomogeneities in the local dark matter
distribution are required to produce the observed intensity of the signal. We
comment on constraints from gamma rays, synchrotron emission, and cosmic ray
antiproton measurements.Comment: 4 pages, 1 figur
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