2,508 research outputs found
Symplectic Cobordism in Small Dimensions and a Series of Elements of Order Four
We present the structure of symplectic cobordism ring in dimensions
up to 51 and give a construction of an infinite series of elements ,
, of order four in this ring, where . The key element of the series is in dimension 103.Comment: 117 pages, 18 table
Correlation strength, Lifshitz transition and the emergence of a two- to three-dimensional crossover in FeSe under pressure
We report a detailed theoretical study of the electronic structure, spectral
properties, and lattice parameters of bulk FeSe under pressure using a fully
charge self-consistent implementation of the density functional theory plus
dynamical mean-field theory method (DFT+DMFT). In particular, we perform a
structural optimization and compute the evolution of the lattice parameters
(volume, ratio, and the internal position of Se) and the electronic
structure of the tetragonal (space group ) paramagnetic FeSe. Our
results for the lattice parameters are in good quantitative agreement with
experiment. The ratio is slightly overestimated by about ~\%,
presumably due to the absence of the van der Waals interactions between the
FeSe layers in our calculations. The lattice parameters determined within DFT
are off the experimental values by a remarkable -~\%, implying a
crucial importance of electron correlations. Upon compression to ~GPa, the
ratio and the lattice volume show a decrease by and ~\%,
respectively, while the Se coordinate weakly increases by ~\%.
Most importantly, our results reveal a topological change of the Fermi surface
(Lifshitz transition) which is accompanied by a two- to three-dimensional
crossover. Our results indicate a small reduction of the quasiparticle mass
renormalization by about ~\% for the and less than ~\% for
the states, as compared to ambient pressure. The behavior of the
momentum-resolved magnetic susceptibility shows no topological
changes of magnetic correlations under pressure, but demonstrates a reduction
of the degree of the in-plane stripe-type nesting. Our results for
the electronic structure and lattice parameters of FeSe are in good qualitative
agreement with recent experiments on its isoelectronic counterpart
FeSeS.Comment: 10 pages, 6 figure
Calculations of the thermodynamic and kinetic properties of LiV3O8
The phase behavior and kinetic pathways of Li1+xV3O8 are investigated by
means of density functional theory (DFT) and a cluster expansion (CE)
methodology that approximates the system Hamiltonian in order to identify the
lowest energy configurations. Although DFT calculations predict the correct
ground state for a given composition, both GGA and LDA fail to obtain phase
stability consistent with experiment due to strongly localized vanadium 3d
electrons. A DFT+U methodology recovers the correct phase stability for an
optimized U value of 3.0eV. GGA+U calculations with this value of U predict
electronic structures that qualitatively agree with experiment. The resulting
calculations indicate solid solution behavior from LiV3O8 to Li2.5V3O8 and
two-phase coexistence between Li2.5V3O8 and Li4V3O8. Analysis of the lithiation
sequence from LiV3O8 to Li2.5V3O8 reveals the mechanism by which lithium
intercalation proceeds in this material. Calculations of lithium migration
energies for different lithium concentrations and configurations provides
insight into the relevant diffusion pathways and their relationship to
structural properties
The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3
We present first principles LDA+U calculations of electronic structure and
magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state
transition was found in our calculations using experimental crystallographic
data for both materials with a much higher transition temperature for HoCoO3,
which agrees well with the experimental estimations. Low Spin state t6e0
(non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+)
ions happens due to the competition between crystal field t_2g-e_g splitting
and effective exchange interaction between 3 spin-orbitals. We show that the
difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the
smaller ionic radius of Ho ion comparing with La ion results in stronger
crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3)
and hence tip the balance between the Low Spin and Intermediate Spin states to
the non-magnetic solution in HoCoO3.Comment: 13 pages, 6 figure
Exact Kohn-Sham potential of strongly correlated finite systems
The dissociation of molecules, even the most simple hydrogen molecule, cannot
be described accurately within density functional theory because none of the
currently available functionals accounts for strong on-site correlation. This
problem has led to a discussion of properties that the local Kohn-Sham
potential has to satisfy in order to correctly describe strongly correlated
systems. We derive an analytic expression for this potential at the
dissociation limit and show that the numerical calculations for a
one-dimensional two electron model system indeed approach and reach this limit.
It is shown that the functional form of the potential is universal, i.e.
independent of the details of the system.Comment: 17 pages, 3 figures, submitted to JC
Wannier functions and exchange integrals: The example of LiCuO
Starting from a single band Hubbard model in the Wannier function basis, we
revisit the problem of the ligand contribution to exchange and derive explicit
formulae for the exchange integrals in metal oxide compounds in terms of atomic
parameters that can be calculated with constrained LDA and LDA+U. The analysis
is applied to the investigation of the isotropic exchange interactions of
LiCuO, a compound where the Cu-O-Cu angle of the dominant exchange
path is close to 90. Our results show that the magnetic moments are
localized in Wannier orbitals which have strong contribution from oxygen atomic
orbitals, leading to exchange integrals that considerably differ from the
estimates based on kinetic exchange only. Using LSDA+U approach, we also
perform a direct {\it ab-initio} determination of the exchange integrals
LiCuO. The results agree well with those obtained from the Wannier
function approach, a clear indication that this modelization captures the
essential physics of exchange. A comparison with experimental results is also
included, with the conclusion that a very precise determination of the Wannier
function is crucial to reach quantitative estimates.Comment: 8 pages, 8 figure
Orbital-spin order and the origin of structural distortion in MgTiO
We analyze electronic, magnetic, and structural properties of the spinel
compound MgTiO using the local density approximation+U method. We show
how MgTiO undergoes to a canted orbital-spin ordered state, where
charge, spin and orbital degrees of freedom are frozen in a geometrically
frustrated network by electron interactions. In our picture orbital order
stabilize the magnetic ground state and controls the degree of structural
distortions. The latter is dynamically derived from the cubic structure in the
correlated LDA+U potential. Our ground-state theory provides a consistent
picture for the dimerized phase of MgTiO, and might be applicable to
frustrated materials in general.Comment: 6 pages, 6 figure
Optical investigation of the metal-insulator transition in
We present a comprehensive optical study of the narrow gap
semiconductor. From the optical reflectivity, measured from the far infrared up
to the ultraviolet spectral range, we extract the complete absorption spectrum,
represented by the real part of the complex optical
conductivity. With decreasing temperature below 80 K, we find a progressive
depletion of below cm, the
semiconducting optical gap. The suppressed (Drude) spectral weight within the
gap is transferred at energies and also partially piles up over a
continuum of excitations extending in the spectral range between zero and
. Moreover, the interaction of one phonon mode with this continuum leads
to an asymmetric phonon shape. Even though several analogies between
and were claimed and a Kondo-insulator scenario was also invoked for
both systems, our data on differ in several aspects from those of
. The relevance of our findings with respect to the Kondo insulator
description will be addressed.Comment: 17 pages, 5 figure
Hydrodynamic Waves in Regions with Smooth Loss of Convexity of Isentropes. General Phenomenological Theory
General phenomenological theory of hydrodynamic waves in regions with smooth
loss of convexity of isentropes is developed based on the fact that for most
media these regions in p-V plane are anomalously small. Accordingly the waves
are usually weak and can be described in the manner analogous to that for weak
shock waves of compression. The corresponding generalized Burgers equation is
derived and analyzed. The exact solution of the equation for steady shock waves
of rarefaction is obtained and discusses.Comment: RevTeX, 4 two-column pages, no figure
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