32,317 research outputs found
Two-color stabilization of atomic hydrogen in circularly polarized laser fields
Dynamic stabilization of atomic hydrogen against ionization in high-frequency
single- and two-color, circularly polarized laser pulses is observed by
numerically solving the three-dimensional, time-dependent Schr\"odinger
equation. The single-color case is revisited and numerically determined
ionization rates are compared with both, exact and approximate high-frequency
Floquet rates. The position of the peaks in the photoelectron spectra can be
explained with the help of dressed initial states. In two-color laser fields of
opposite circular polarization the stabilized probability density may be shaped
in various ways. For laser frequencies and ,
and sufficiently large excursion amplitudes distinct
probability density peaks are observed. This may be viewed as the
generalization of the well-known ``dichotomy'' in linearly polarized laser
fields, i.e, as ``trichotomy,'' ``quatrochotomy,'' ``pentachotomy'' etc. All
those observed structures and their ``hula-hoop''-like dynamics can be
understood with the help of high-frequency Floquet theory and the two-color
Kramers-Henneberger transformation. The shaping of the probability density in
the stabilization regime can be realized without additional loss in the
survival probability, as compared to the corresponding single-color results.Comment: 10 pages, REVTeX4, 11 eps-figures, see also
http://www.physik.tu-darmstadt.de/tqe/dieter/publist.html for a manuscript
with higher-quality figure
A numerical ab initio study of harmonic generation from a ring-shaped model molecule in laser fields
When a laser pulse impinges on a molecule which is invariant under certain
symmetry operations selection rules for harmonic generation (HG) arise. In
other words: symmetry controls which channels are open for the deposition and
emission of laser energy---with the possible application of filtering or
amplification. We review the derivation of HG selection rules and study
numerically the interaction of laser pulses with an effectively one-dimensional
ring-shaped model molecule. The harmonic yields obtained from that model and
their dependence on laser frequency and intensity are discussed. In a real
experiment obvious candidates for such molecules are benzene, other aromatic
compounds, or even nanotubes.Comment: 5 pages, 3 figure
The fundamental group and torsion group of Beauville surfaces
We give a survey on the fundamental group of surfaces isogenous to a higher
product. If the surfaces are regular, e.g. if they are Beauville surfaces, the
first homology group is a finite group. We present a MAGMA script which
calculates the first homology groups of regular surfaces isogenous to a
product.Comment: 14 pages; MAGMA script included; v2: minor corrections, final version
to appear in the Proceedings of the Conference "Beauville Surfaces and
Groups", Newcastle University (UK), 7-9th June 201
Time-dependent Kohn-Sham approach to quantum electrodynamics
We prove a generalization of the van Leeuwen theorem towards quantum
electrodynamics, providing the formal foundations of a time-dependent Kohn-Sham
construction for coupled quantized matter and electromagnetic fields. Thereby
we circumvent the symmetry-causality problems associated with the
action-functional approach to Kohn-Sham systems. We show that the effective
external four-potential and four-current of the Kohn-Sham system are uniquely
defined and that the effective four-current takes a very simple form. Further
we rederive the Runge-Gross theorem for quantum electrodynamics.Comment: 8 page
Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonances regimes
The generation of harmonics by atoms interacting with two circularly
polarized and frequency related laser fields is addressed through ab initio
numerical simulations. A detailed charaterization of a few specific harmonics
is given. In particular, the two different cases where the total energy
absorbed through photons is far-off or close to the energy gap between
different atomic states are investigated. It is found that the conversion
efficiency in the harmonic generation is strongly dependent on the inner atomic
structure and in certain specific cases it can be significantly enhanced within
a small frequency range.Comment: Submitted to Appl. Phys. B, 4 page
Large magnetoresistance in the antiferromagnetic semi-metal NdSb
There has been considerable interest in topological semi-metals that exhibit
extreme magnetoresistance (XMR). These have included materials lacking
inversion symmetry such as TaAs, as well Dirac semi-metals such as Cd3As2.
However, it was reported recently that LaSb and LaBi also exhibit XMR, even
though the rock-salt structure of these materials has inversion symmetry, and
the band-structure calculations do not show a Dirac dispersion in the bulk.
Here, we present magnetoresistance and specific heat measurements on NdSb,
which is isostructural with LaSb. NdSb has an antiferromagnetic groundstate,
and in analogy with the lanthanum monopnictides, is expected to be a
topologically non-trivial semi-metal. We show that NdSb has an XMR of 10^4 %,
even within the AFM state, illustrating that XMR can occur independently of the
absence of time reversal symmetry breaking in zero magnetic field. The
persistence of XMR in a magnetic system offers promise of new functionality
when combining topological matter with electronic correlations. We also find
that in an applied magnetic field below the Neel temperature there is a first
order transition, consistent with evidence from previous neutron scattering
work.Comment: 5 pages, 6 figure
The Tropos Software Development Methodology: Processes, Models and Diagrams
Tropos is a novel agent-oriented software development methodology founded on two key features: (i) the notions of agent, goal, plan and various other knowledge level concepts are fundamental primitives used uniformly throughout the software development process; and (ii) a crucial role is assigned to requirements analysis and specification when the system-to-be is analyzed with respect to its intended environment. This paper provides a (first) detailed account of the Tropos methodology. In particular, we describe the basic concepts on which Tropos is founded and the types of models one builds out of them. We also specify the analysis process through which design flows from external to system actors through a goal analysis and delegation. In addition, we provide an abstract syntax for Tropos diagrams and other linguistic constructs
Perturbations in the relaxation mechanism for a large cosmological constant
Recently, a mechanism for relaxing a large cosmological constant (CC) has
been proposed [arxiv:0902.2215], which permits solutions with low Hubble rates
at late times without fine-tuning. The setup is implemented in the LXCDM
framework, and we found a reasonable cosmological background evolution similar
to the LCDM model with a fine-tuned CC. In this work we analyse analytically
the perturbations in this relaxation model, and we show that their evolution is
also similar to the LCDM model, especially in the matter era. Some tracking
properties of the vacuum energy are discussed, too.Comment: 18 pages, LaTeX; discussion improved, accepted by CQ
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