160 research outputs found
Plasmon signatures in high harmonic generation
High harmonic generation in polarizable multi-electron systems is
investigated in the framework of multi-configuration time-dependent
Hartree-Fock. The harmonic spectra exhibit two cut offs. The first cut off is
in agreement with the well established, single active electron cut off law. The
second cut off presents a signature of multi-electron dynamics. The strong
laser field excites non-linear plasmon oscillations. Electrons that are ionized
from one of the multi-plasmon states and recombine to the ground state gain
additional energy, thereby creating the second plateau.Comment: Major revision, 12 pages, 5 figures, submitted to J. Phys. B (2005),
accepte
Phase Space Approach to Laser-driven Electronic Wavepacket Propagation
We propose a phase space method to propagate a quantum wavepacket driven by a
strong external field. The method employs the so-called biorthogonal von
Neumann basis recently introduced for the calculation of the energy eigenstates
of time-independent quantum systems [A. Shimshovitz and D.J. Tannor,
arXiv:1201.2299v1]. While the individual elements in this basis set are
time-independent, a small subset is chosen in a time-dependent manner to adapt
to the evolution of the wavepacket in phase space. We demonstrate the accuracy
and efficiency of the present propagation method by calculating the electronic
wavepacket in a one-dimensional soft-core atom interacting with a superposition
of an intense, few-cycle, near-infrared laser pulse and an attosecond
extreme-ultraviolet laser pulse.Comment: 4 pages, 4 figures. The following article has been submitted to the
Journal of Chemical Physics. After it is published, it will be found at
http://jcp.aip.org
Two Component Heat Diffusion Observed in CMR Manganites
We investigate the low-temperature electron, lattice, and spin dynamics of
LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance
spectroscopy. Probing the high-spin d-d transition as a function of time delay
and probe energy, we compare the responses of the Mott insulator and the
double-exchange metal to the photoexcitation. Attempts have previously been
made to describe the sub-picosecond dynamics of CMR manganites in terms of a
phenomenological three temperature model describing the energy transfer between
the electron, lattice and spin subsystems followed by a comparatively slow
exponential decay back to the ground state. However, conflicting results have
been reported. Here we first show clear evidence of an additional component in
the long term relaxation due to film-to-substrate heat diffusion and then
develop a modified three temperature model that gives a consistent account for
this feature. We confirm our interpretation by using it to deduce the bandgap
in LMO. In addition we also model the non-thermal sub-picosecond dynamics,
giving a full account of all observed transient features both in the insulating
LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434
v2: Abstract correcte
Frank's constant in the hexatic phase
Using video-microscopy data of a two-dimensional colloidal system the
bond-order correlation function G6 is calculated and used to determine the
temperature-dependence of both the orientational correlation length xi6 in the
isotropic liquid phase and the Frank constant F_A in the hexatic phase. F_A
takes the value 72/pi at the hexatic to isotropic liquid phase transition and
diverges at the hexatic to crystal transition as predicted by the KTHNY-theory.
This is a quantitative test of the mechanism of breaking the orientational
symmetry by disclination unbinding
Two-photon ionization of Helium studied with the multiconfigurational time-dependent Hartree-Fock method
The multiconfigurational time-dependent Hartree-Fock method (MCTDHF) is
applied for simulations of the two-photon ionization of Helium. We present
results for the single- and double ionization from the groundstate for photon
energies in the non-sequential regime, and compare them to direct solutions of
the Schr\"odinger equation using the time-dependent (full) Configuration
Interaction method (TDCI). We find that the single-ionization is accurately
reproduced by MCTDHF, whereas the double ionization results correctly capture
the main trends of TDCI
Many-body theory for systems with particle conversion: Extending the multiconfigurational time-dependent Hartree method
We derive a multiconfigurational time-dependent Hartree theory for systems
with particle conversion. In such systems particles of one kind can convert to
another kind and the total number of particles varies in time. The theory thus
extends the scope of the available and successful multiconfigurational
time-dependent Hartree methods -- which were solely formulated for and applied
to systems with a fixed number of particles -- to new physical systems and
problems. As a guiding example we treat explicitly a system where bosonic atoms
can combine to form bosonic molecules and vise versa. In the theory for
particle conversion, the time-dependent many-particle wavefunction is written
as a sum of configurations made of a different number of particles, and
assembled from sets of atomic and molecular orbitals. Both the expansion
coefficients and the orbitals forming the configurations are time-dependent
quantities that are fully determined according to the Dirac-Frenkel
time-dependent variational principle. Particular attention is paid to the
reduced density matrices of the many-particle wavefunction that appear in the
theory and enter the equations of motion. There are two kinds of reduced
density matrices: particle-conserving reduced density matrices which directly
only couple configurations with the same number of atoms and molecules, and
particle non-conserving reduced density matrices which couple configurations
with a different number of atoms and molecules. Closed-form and compact
equations of motion are derived for contact as well as general two-body
interactions, and their properties are analyzed and discussed.Comment: 46 page
Time-dependent quantum Monte Carlo: preparation of the ground state
We study one-dimensional (1D) and two-dimensional (2D) Helium atoms using a
new time-dependent quantum Monte Carlo (TDQMC) method. The TDQMC method employs
random walkers, with a separate guiding wave attached to each walker. The
ground state is calculated by a self-consistent solution of complex-time
Schroedinger equations for the guiding waves and of equations for the velocity
fields of the walkers. Our results show that the many-body wavefunction and the
ground state energy of the model atoms are very close to those predicted by the
standard diffusion quantum Monte Carlo method. The obtained ground state can
further be used to examine correlated time-dependent processes which include,
for example, interaction of atoms and molecules with external electromagnetic
fields.Comment: 9 pages, 5 figure
The multi-configurational time-dependent Hartree method for bosons: Many-body dynamics of bosonic systems
The evolution of Bose-Einstein condensates is amply described by the
time-dependent Gross-Pitaevskii mean-field theory which assumes all bosons to
reside in a single time-dependent one-particle state throughout the propagation
process. In this work, we go beyond mean-field and develop an essentially-exact
many-body theory for the propagation of the time-dependent Schr\"odinger
equation of interacting identical bosons. In our theory, the time-dependent
many-boson wavefunction is written as a sum of permanents assembled from
orthogonal one-particle functions, or orbitals, where {\it both} the expansion
coefficients {\it and} the permanents (orbitals) themselves are {\it
time-dependent} and fully determined according to a standard time-dependent
variational principle. By employing either the usual Lagrangian formulation or
the Dirac-Frenkel variational principle we arrive at two sets of coupled
equations-of-motion, one for the orbitals and one for the expansion
coefficients. The first set comprises of first-order differential equations in
time and non-linear integro-differential equations in position space, whereas
the second set consists of first-order differential equations with
time-dependent coefficients. We call our theory multi-configurational
time-dependent Hartree for bosons, or MCTDHB(), where specifies the
number of time-dependent orbitals used to construct the permanents. Numerical
implementation of the theory is reported and illustrative numerical examples of
many-body dynamics of trapped Bose-Einstein condensates are provided and
discussed.Comment: 30 pages, 2 figure
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