144 research outputs found
Stopping dynamics of ions passing through correlated honeycomb clusters
A combined nonequilibrium Green functions-Ehrenfest dynamics approach is
developed that allows for a time-dependent study of the energy loss of a
charged particle penetrating a strongly correlated system at zero and finite
temperature. Numerical results are presented for finite inhomogeneous
two-dimensional Fermi-Hubbard models, where the many-electron dynamics in the
target are treated fully quantum mechanically and the motion of the projectile
is treated classically. The simulations are based on the solution of the
two-time Dyson (Keldysh-Kadanoff-Baym) equations using the second-order Born,
third-order and T-matrix approximations of the self-energy. As application, we
consider protons and helium nuclei with a kinetic energy between 1 and 500
keV/u passing through planar fragments of the two-dimensional honeycomb lattice
and, in particular, examine the influence of electron-electron correlations on
the energy exchange between projectile and electron system. We investigate the
time dependence of the projectile's kinetic energy (stopping power), the
electron density, the double occupancy and the photoemission spectrum. Finally,
we show that, for a suitable choice of the Hubbard model parameters, the
results for the stopping power are in fair agreement with ab-initio simulations
for particle irradiation of single-layer graphene.Comment: 13 pages, 12 figure
Configuration Path Integral Monte Carlo Approach to the Static Density Response of the Warm Dense Electron Gas
Precise knowledge of the static density response function (SDRF) of the
uniform electron gas (UEG) serves as key input for numerous applications, most
importantly for density functional theory beyond generalized gradient
approximations. Here we extend the configuration path integral Monte Carlo
(CPIMC) formalism that was previously applied to the spatially uniform electron
gas to the case of an inhomogeneous electron gas by adding a spatially periodic
external potential. This procedure has recently been successfully used in
permutation blocking path integral Monte Carlo simulations (PB-PIMC) of the
warm dense electron gas [Dornheim \textit{et al.}, Phys. Rev. E in press,
arXiv:1706.00315], but this method is restricted to low and moderate densities.
Implementing this procedure into CPIMC allows us to obtain exact finite
temperature results for the SDRF of the electron gas at \textit{high to
moderate densities} closing the gap left open by the PB-PIMC data. In this
paper we demonstrate how the CPIMC formalism can be efficiently extended to the
spatially inhomogeneous electron gas and present the first data points.
Finally, we discuss finite size errors involved in the quantum Monte Carlo
results for the SDRF in detail and present a solution how to remove them that
is based on a generalization of ground state techniques
Heat Transport in Confined Strongly Coupled 2D Dust Clusters
Dusty plasmas are a model system for studying strong correlation. The dust
grains' size of a few micro-meters and their characteristic oscillation
frequency of a few hertz allows for an investigation of many particle effects
on an atomic level. In this article, we model the heat transport through an
axially confined 2D dust cluster from the center to the outside. The system
behaves particularly interesting since heat is not only conducted within the
dust component but also transfered to the neutral gas. Fitting the analytical
solution to the obtained radial temperature profiles allows to determine the
heat conductivity \kheat. The heat conductivity is found to be constant over
a wide range of coupling strengths even including the phase transition from
solid to liquid here, as it was also found in extended systems by V. Nosenko et
al. in 2008 \cite{PhysRevLett.100.025003
Hubbard nanoclusters far from equilibrium
The Hubbard model is a prototype for strongly correlated many-particle
systems, including electrons in condensed matter and molecules, as well as for
fermions or bosons in optical lattices. While the equilibrium properties of
these systems have been studied in detail, the nonequilibrium dynamics
following a strong non-perturbative excitation only recently came into the
focus of experiments and theory. It is of particular interest how the dynamics
depend on the coupling strength and on the particle number and whether there
exist universal features in the time evolution. Here, we present results for
the dynamics of finite Hubbard clusters based on a selfconsistent
nonequilibrium Green functions (NEGF) approach invoking the generalized
Kadanoff--Baym ansatz (GKBA). We discuss the conserving properties of the GKBA
with Hartree--Fock propagators in detail and present a generalized form of the
energy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we
demonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF
simulations. Besides, this approach substantially speeds up the numerical
calculations and thus presents the capability to study comparatively large
systems and to extend the analysis to long times allowing for an accurate
computation of the excitation spectrum via time propagation. Our data obtained
within the second Born approximation compares favorably with exact
diagonalization results (available for up to 13 particles) and are expected to
have predictive capability for substantially larger systems in the weak
coupling limit
Ion-Streaming Induced Order Transition in 3D Dust Clusters
Dust Dynamics Simulations utilizing a dynamical screening approach are
performed to study the effect of ion-streaming on the self-organized structures
in a three-dimensional spherically confined complex (dusty) plasma. Varying the
Mach number M - the ratio of ion drift velocity to the sound velocity, the
simulations reproduce the experimentally observed cluster configurations in the
two limiting cases: at M=0 strongly correlated crystalline structures
consisting of nested spherical shells (Yukawa balls) and, for M\geq1,
flow-aligned dust chains, respectively. In addition, our simulations reveal a
discontinuous transition between these two limits. It is found that already a
moderate ion drift velocity (M\approx0.1) destabilizes the highly ordered
Yukawa balls and initiates an abrupt melting transition. The critical value of
M is found to be independent of the cluster size
Permutation Blocking Path Integral Monte Carlo approach to the Static Density Response of the Warm Dense Electron Gas
The static density response of the uniform electron gas is of fundamental
importance for numerous applications. Here, we employ the recently developed
\textit{ab initio} permutation blocking path integral Monte Carlo (PB-PIMC)
technique [T.~Dornheim \textit{et al.}, \textit{New J.~Phys.}~\textbf{17},
073017 (2015)] to carry out extensive simulations of the harmonically perturbed
electron gas at warm dense matter conditions. In particular, we investigate in
detail the validity of linear response theory and demonstrate that PB-PIMC
allows to obtain highly accurate results for the static density response
function and, thus, the static local field correction. A comparison with
dielectric approximations to our new \textit{ab initio} data reveals the need
for an exact treatment of correlations. Finally, we consider a superposition of
multiple perturbations and discuss the implications for the calculation of the
static response function
Permutation blocking path integral Monte Carlo: A highly efficient approach to the simulation of strongly degenerate non-ideal fermions
Correlated fermions are of high interest in condensed matter (Fermi liquids,
Wigner molecules), cold atomic gases and dense plasmas. Here we propose a novel
approach to path integral Monte Carlo (PIMC) simulations of strongly degenerate
non-ideal fermions at finite temperature by combining a fourth-order
factorization of the density matrix with antisymmetric propagators, i.e.,
determinants, between all imaginary time slices. To efficiently run through the
modified configuration space, we introduce a modification of the widely used
continuous space worm algorithm, which allows for an efficient sampling at
arbitrary system parameters. We demonstrate how the application of determinants
achieves an effective blocking of permutations with opposite signs, leading to
a significant relieve of the fermion sign problem. To benchmark the capability
of our method regarding the simulation of degenerate fermions, we consider
multiple electrons in a quantum dot and compare our results with other ab
initio techniques, where they are available. The present permutation blocking
path integral Monte Carlo approach allows us to obtain accurate results even
for electrons at low temperature and arbitrary coupling, where no other
ab initio results have been reported, so far
The ion potential in warm dense matter: wake effects due to streaming degenerate electrons
The effective dynamically screened potential of a classical ion in a
stationary flowing quantum plasma at finite temperature is investigated. This
is a key quantity for thermodynamics and transport of dense plasmas in the warm
dense matter regime. This potential has been studied before within hydrodynamic
approaches or based on the zero temperature Lindhard dielectric function. Here
we extend the kinetic analysis by including the effects of finite temperature
and of collisions based on the Mermin dielectric function. The resulting ion
potential exhibits an oscillatory structure with attractive minima (wakes) and,
thus, strongly deviates from the static Yukawa potential of equilibrium
plasmas. This potential is analyzed in detail for high-density plasmas with
values of the Brueckner parameter in the range , for a broad
range of plasma temperature and electron streaming velocity. It is shown that
wake effects become weaker with increasing temperature of the electrons.
Finally, we obtain the minimal electron streaming velocity for which attraction
between ions occurs. This velocity turns out to be less than the electron Fermi
velocity. Our results allow, for the first time, for reliable predictions of
the strength of wake effects in nonequilibrium quantum plasmas with fast
streaming electrons showing that these effects are crucial for transport under
warm dense matter conditions, in particular for laser-matter interaction,
electron-ion temperature equilibration and for stopping power.Comment: Substantially extended version. New figures adde
Correlation effects in strong-field ionization of heteronuclear diatomic molecules
We develop a time-dependent theory to investigate electron dynamics and
photoionization processes of diatomic molecules interacting with strong laser
fields including electron-electron correlation effects. We combine the recently
formulated time-dependent generalized-active-space configuration interaction
theory [D. Hochstuhl and M. Bonitz, Phys. Rev. A 86, 053424 (2012); S. Bauch,
et al., Phys. Rev. A 90, 062508 (2014)] with a prolate spheroidal basis set
including localized orbitals and continuum states to describe the bound
electrons and the outgoing photoelectron. As an example, we study the
strong-field ionization of the two-center four-electron lithium hydride
molecule in different intensity regimes. By using single-cycle pulses, two
orientations of the asymmetric heteronuclear molecule are investigated: Li-H,
with the electrical field pointing from H to Li, and the opposite case of H-Li.
The preferred orientation for ionization is determined and we find a transition
from H-Li, for low intensity, to Li-H, for high intensity. The influence of
electron correlations is studied at different levels of approximation, and we
find a significant change in the preferred orientation. For certain intensity
regimes, even an interchange of the preferred configuration is observed,
relative to the uncorrelated simulations. Further insight is provided by
detailed comparisons of photoelectron angular distributions with and without
correlation effects taken into account
Permutation blocking path integral Monte Carlo approach to the uniform electron gas at finite temperature
The uniform electron gas (UEG) at finite temperature is of high current
interest due to its key relevance for many applications including dense plasmas
and laser excited solids. In particular, density functional theory heavily
relies on accurate thermodynamic data for the UEG. Until recently, the only
existing first-principle results had been obtained for electrons with
restricted path integral Monte Carlo (RPIMC), for low to moderate density,
. This data has been complemented by
Configuration path integral Monte Carlo (CPIMC) simulations for
that substantially deviate from RPIMC towards smaller and low
temperature. In this work, we present results from an independent third
method---the recently developed permutation blocking path integral Monte Carlo
(PB-PIMC) approach [T. Dornheim \textit{et al.}, NJP \textbf{17}, 073017
(2015)] which we extend to the UEG. Interestingly, PB-PIMC allows us to perform
simulations over the entire density range down to half the Fermi temperature
() and, therefore, to compare our results to both
aforementioned methods. While we find excellent agreement with CPIMC, where
results are available, we observe deviations from RPIMC that are beyond the
statistical errors and increase with density
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