492 research outputs found
Decoupling method for dynamical mean field theory calculations
In this paper we explore the use of an equation of motion decoupling method
as an impurity solver to be used in conjunction with the dynamical mean field
self-consistency condition for the solution of lattice models. We benchmark the
impurity solver against exact diagonalization, and apply the method to study
the infinite Hubbard model, the periodic Anderson model and the model.
This simple and numerically efficient approach yields the spectra expected for
strongly correlated materials, with a quasiparticle peak and a Hubbard band. It
works in a large range of parameters, and therefore can be used for the
exploration of real materials using LDA+DMFT.Comment: 30 pages, 7 figure
Time-dependent energy absorption changes during ultrafast lattice deformation
The ultrafast time-dependence of the energy absorption of covalent solids
upon excitation with femtosecond laser pulses is theoretically analyzed. We use
a microscopic theory to describe laser induced structural changes and their
influence on the electronic properties. We show that from the time evolution of
the energy absorbed by the system important information on the electronic and
atomic structure during ultrafast phase transitions can be gained. Our results
reflect how structural changes affect the capability of the system to absorb
external energy.Comment: 7 pages RevTeX, 8 ps figures, submitted to Journal of Appl. Physic
Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions
Using density functional theory, we determine parameters of tight-binding
Hamiltonians for a variety of Fabre charge transfer salts, focusing in
particular on the effects of temperature and pressure. Besides relying on
previously published crystal structures, we experimentally determine two new
sets of structures; (TMTTF)SbF at different temperatures and
(TMTTF)PF at various pressures. We find that a few trends in the
electronic behavior can be connected to the complex phase diagram shown by
these materials. Decreasing temperature and increasing pressure cause the
systems to become more two-dimensional. We analyze the importance of
correlations by considering an extended Hubbard model parameterized using
Wannier orbital overlaps and show that while charge order is strongly activated
by the inter-site Coulomb interaction, the magnetic order is only weakly
enhanced. Both orders are suppressed when the effective pressure is increased.Comment: 12 pages, 16 figure
Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br and Cu4Te5O12Cl4
We present a comparative study of the coupled-tetrahedra quantum spin systems
Cu2Te2O5X2, X=Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12Cl4
(Cu-45124(Cl)) based on ab initio Density Functional Theory calculations. The
magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state
at a lower critical temperature T=13.6K than in Cu-2252(Cl) (T=18K) can
be well understood in terms of the modified interaction paths. We identify the
relevant structural changes between the two systems and discuss the
hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio
relaxed structure using Car-Parrinello Molecular Dynamics.Comment: 2 pages, 1 figure; submitted to Proceedings of M2S-HTSC VIII, Dresden
200
Multiferroic FeTeOBr: Alternating spin chains with frustrated interchain interactions
A combination of density functional theory calculations, many-body model
considerations, magnetization and electron spin resonance measurements shows
that the multiferroic FeTeOBr should be described as a system of
alternating antiferromagnetic chains with strong Fe-O-Te-O-Fe bridges
weakly coupled by two-dimensional frustrated interactions, rather than the
previously reported tetramer models. The peculiar temperature dependence of the
incommensurate magnetic vector can be explained in terms of interchain exchange
striction being responsible for the emergent net electric polarization.Comment: 7 pages, 6 figure
Na2IrO3 as a molecular orbital crystal
Contrary to previous studies that classify Na2IrO3 as a realization of the
Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this
system represents a highly unusual case in which the electronic structure is
dominated by the formation of quasi-molecular orbitals (QMOs), with substantial
quenching of the orbital moments. The QMOs consist of six atomic orbitals on an
Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of
such QMOs in solids invokes very different physics compared to the models
considered previously. Employing density functional theory calculations and
model considerations we find that both the insulating behavior and the
experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow
from the QMO model.Comment: Final version, accepted by PR
Local moments and symmetry breaking in metallic PrMnSbO
We report a combined experimental and theoretical investigation of the
layered antimonide PrMnSbO which is isostructural to the parent phase of the
iron pnictide superconductors. We find linear resistivity near room temperature
and Fermi liquid-like T^{2} behaviour below 150 K. Neutron powder diffraction
shows that unfrustrated C-type Mn magnetic order develops below \sim 230 K,
followed by a spin-flop coupled to induced Pr order. At T \sim 35 K, we find a
tetragonal to orthorhombic (T-O) transition. First principles calculations show
that the large magnetic moments observed in this metallic compound are of local
origin. Our results are thus inconsistent with either the itinerant or
frustrated models proposed for symmetry breaking in the iron pnictides. We show
that PrMnSbO is instead a rare example of a metal where structural distortions
are driven by f-electron degrees of freedom
The monoclinic crystal structure of -RuCl and the zigzag antiferromagnetic ground state
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to
realize a Kitaev spin model with strongly frustrated, bond-dependent,
anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic
moments. Here we report a detailed study of the three-dimensional crystal
structure using x-ray diffraction on untwinned crystals combined with
structural relaxation calculations. We consider several models for the stacking
of honeycomb layers and find evidence for a crystal structure with a monoclinic
unit cell corresponding to a stacking of layers with a unidirectional in-plane
offset, with occasional in-plane sliding stacking faults, in contrast with the
currently-assumed trigonal 3-layer stacking periodicity. We report electronic
band structure calculations for the monoclinic structure, which find support
for the applicability of the jeff=1/2 picture once spin orbit coupling and
electron correlations are included. We propose that differences in the
magnitude of anisotropic exchange along symmetry inequivalent bonds in the
monoclinic cell could provide a natural mechanism to explain the spin gap
observed in powder inelastic neutron scattering, in contrast to spin models
based on the three-fold symmetric trigonal structure, which predict a gapless
spectrum within linear spin wave theory. Our susceptibility measurements on
both powders and stacked crystals, as well as neutron powder diffraction show a
single magnetic transition at TN ~ 13K. The analysis of the neutron data
provides evidence for zigzag magnetic order in the honeycomb layers with an
antiferromagnetic stacking between layers. Magnetization measurements on
stacked single crystals in pulsed field up to 60T show a single transition
around 8T for in-plane fields followed by a gradual, asymptotic approach to
magnetization saturation, as characteristic of strongly anisotropic exchange
interactions.Comment: 13 pages, 9 figures, published in Physical Review
Theory for the ultrafast ablation of graphite films
The physical mechanisms for damage formation in graphite films induced by
femtosecond laser pulses are analyzed using a microscopic electronic theory. We
describe the nonequilibrium dynamics of electrons and lattice by performing
molecular dynamics simulations on time-dependent potential energy surfaces. We
show that graphite has the unique property of exhibiting two distinct laser
induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we
find nonequilibrium melting followed by fast evaporation. For low intensities
above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of
intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re
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