110 research outputs found
Exact-exchange kernel of time-dependent density functional theory: Frequency dependence and photoabsorption spectra of atoms
In this work we have calculated excitation energies and photoionization cross
sections of Be and Ne in the exact-exchange (EXX) approximation of
time-dependent density functional theory (TDDFT). The main focus has been on
the frequency dependence of the EXX kernel and on how it affects the spectrum
as compared to the corresponding adiabatic approximation. We show that for some
discrete excitation energies the frequency dependence is essential to reproduce
the results of time-dependent Hartree-Fock theory. Unfortunately, we have found
that the EXX approximation breaks down completely at higher energies, producing
a response function with the wrong analytic structure and making inner-shell
excitations disappear from the calculated spectra. We have traced this failure
to the existence of vanishing eigenvalues of the Kohn-Sham non-interacting
response function. Based on the adiabatic TDDFT formalism we propose a new way
of deriving the Fano parameters of autoionizing resonances.Comment: 14 pages, 7 figure
Diffuse versus square-well confining potentials in modelling @C atoms
Attention: this version- of the manuscript differs from its previously
uploaded version- (arXiv:1112.6158v1) and subsequently published in 2012 J.
Phys. B \textbf{45} 105102 only by a removed typo in Eq.(2) of version-;
there was the erroneous factor "2" in both terms in the right-hand-side of the
Eq.(2) of version-. Now that the typo is removed, Eq.(2) is correct.
A perceived advantage for the replacement of a discontinuous square-well
pseudo-potential, which is often used by various researchers as an
approximation to the actual C cage potential in calculations of
endohedral atoms @C, by a more realistic diffuse potential is
explored. The photoionization of endohedral H@C and Xe@C is
chosen as the case study. The diffuse potential is modelled by a combination of
two Woods-Saxon potentials. It is demonstrated that photoionization spectra of
@C atoms are largely insensitive to the degree of diffuseness
of the potential borders, in a reasonably broad range of 's.
Alternatively, these spectra are found to be insensitive to discontinuity of
the square-well potential either. Both potentials result in practically
identical calculated spectra. New numerical values for the set of square-well
parameters, which lead to a better agreement between experimental and
theoretical data for @C spectra, are recommended for future studies.Comment: 11 pages, 4 figure
Ionization of clusters in strong X-ray laser pulses
The effect of intense X-ray laser interaction on argon clusters is studied
theoretically with a mixed quantum/classical approach. In comparison to a
single atom we find that ionization of the cluster is suppressed, which is in
striking contrast to the observed behavior of rare-gas clusters in intense
optical laser pulses. We have identified two effects responsible for this
phenomenon: A high space charge of the cluster in combination with a small
quiver amplitude and delocalization of electrons in the cluster. We elucidate
their impact for different field strengths and cluster sizes.Comment: 4 pages, 4 figure
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
Calculation of Stark-induced absorption on the 6s6p ^3P_1 - 6s^2 ^1S_0 transition in Hg
We carry out relativistic many-body calculations of the Stark-induced
absorption coefficient on the 254-nm 6s6p ^3P_1 (F=1/2) - 6s^2 ^1S_0 line of
Hg atom, the effect considered before by Lamoreaux and Fortson using a
simple central field estimate [Phys. Rev. A {\bf 46}, 7053 (1992)]. The
Stark-induced admixing of states of opposite parity opens additional M1 and E2
transition channels. We find that the resulting M1-E1 absorption dominates over
E2-E1 absorption. The value of the E2-E1 absorption coefficient depends
strongly on the details of treatment of the correlation problem. As a result,
our numerical values differ substantially from those of the earlier central
field calculation. Reliable calculation of this effect can enable a useful
experimental check on the optical technique being used to search for a
permanent electric dipole moment of the Hg atom.Comment: 5 page
Non-physical consequences of the Muffin-tin-type intra-molecular potential
We demonstrate using a simple model that in the frame of muffin-tin - like
potential non-physical peculiarities appear in molecular photoionization
cross-sections that are a consequence of jumps in the potential and its first
derivative at some radius. The magnitude of non-physical effects is of the same
order as the physical oscillations in the cross-section of a two-atomic
molecule. The role of the size of these jumps is illustrated by choosing three
values of it. The results obtained are connected to the studied previously
effect of non-analytical behavior as a function of r the potential V(r)acting
upon a particle on its photoionization cross-section. In reality, such
potential has to be analytic in magnitude and first derivative function in
distance. Introduction of non-analytic features in model potential leads to
non-physical features in the corresponding cross-section - oscillations,
additional maxima etc.Comment: 11 pages, 5 figure
Oscillator strengths with pseudopotentials
The time-dependent local-density approximation (TDLDA) is shown to remain
accurate in describing the atomic response of IB elements under the additional
approximation of using pseudopotentials to treat the effects of core electrons.
This extends the work of Zangwill and Soven who showed the utility of the
all-electron TDLDA in the atomic response problem.Comment: 13 pages including 3 Postscript figure
Quasiparticles of strongly correlated Fermi liquids at high temperatures and in high magnetic fields
Strongly correlated Fermi systems are among the most intriguing, best
experimentally studied and fundamental systems in physics. There is, however,
lack of theoretical understanding in this field of physics. The ideas based on
the concepts like Kondo lattice and involving quantum and thermal fluctuations
at a quantum critical point have been used to explain the unusual physics.
Alas, being suggested to describe one property, these approaches fail to
explain the others. This means a real crisis in theory suggesting that there is
a hidden fundamental law of nature. It turns out that the hidden fundamental
law is well forgotten old one directly related to the Landau---Migdal
quasiparticles, while the basic properties and the scaling behavior of the
strongly correlated systems can be described within the framework of the
fermion condensation quantum phase transition (FCQPT). The phase transition
comprises the extended quasiparticle paradigm that allows us to explain the
non-Fermi liquid (NFL) behavior observed in these systems. In contrast to the
Landau paradigm stating that the quasiparticle effective mass is a constant,
the effective mass of new quasiparticles strongly depends on temperature,
magnetic field, pressure, and other parameters. Our observations are in good
agreement with experimental facts and show that FCQPT is responsible for the
observed NFL behavior and quasiparticles survive both high temperatures and
high magnetic fields.Comment: 17 pages, 17 figures. Dedicated to 100th anniversary of A.B.Migdal
birthda
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