390 research outputs found
Extreme ultraviolet-excited time-resolved luminescence spectroscopy using an ultrafast table-top high-harmonic generation source
We present a table-top extreme ultraviolet (XUV) beamline for measuring time-
and frequency-resolved XUV excited optical luminescence (XEOL) with additional
femtosecond-resolution XUV transient absorption spectroscopy functionality. XUV
pulses are generated via high-harmonic generation using a near-infrared pulse
in a noble gas medium, and focused to excite luminescence from a solid sample.
The luminescence is collimated and guided into a streak camera, where its
spectral components are temporally resolved with picosecond temporal
resolution. We time-resolve XUV excited luminescence and compare the results to
luminescence decays excited at longer wavelengths for three different materials
: (i) sodium salicylate, an often used XUV scintillator, (ii) fluorescent
labeling molecule 4-carbazole benzoic acid (CB), and (iii) a zirconium metal
oxo-cluster labeled with CB, which is a photoresist candidate for
extreme-ultraviolet lithography. Our results establish time-resolved XEOL as a
new technique to measure transient XUV-driven phenomena in solid-state samples,
and identify decay mechanisms of molecules following XUV and soft-X-ray
excitation
Following the Nonthermal Phase Transition in Niobium Dioxide by Time-Resolved Harmonic Spectroscopy
Photoinduced phase transitions in correlated materials promise diverse applications from ultrafast switches to optoelectronics. Resolving those transitions and possible metastable phases temporally are key enablers for these applications, but challenge existing experimental approaches. Extreme nonlinear optics can help probe phase changes, as higher-order nonlinearities have higher sensitivity and temporal resolution to band structure and lattice deformations. Here the ultrafast transition from the semiconducting to the metallic phases in polycrystalline thin-film NbO2 is investigated by time-resolved harmonic spectroscopy. The emission strength of all harmonic orders shows a steplike suppression when the excitation fluence exceeds a threshold (∼11-12 mJ/cm2), below the fluence required for the thermal transition - a signature of the nonthermal emergence of a metallic phase within 100 ± 20 fs. This observation is backed by full ab initio simulations as well as a 1D chain model of high-harmonic generation from both phases. Our results demonstrate femtosecond harmonic probing of phase transitions and nonthermal dynamics in solids.</p
One-loop effective action for Einstein gravity in special background gauge
The one-loop effective action for Einstein gravity in a special one-parameter
background gauge is calculated up to first order in a gauge parameter. It is
shown that the effective action does not depend upon the gauge parameter on
shell.Comment: 17 pages, Latex, no figure
Five-Brane Configurations without a Strong Coupling Regime
Five-brane distributions with no strong coupling problems and high symmetry
are studied. The simplest configuration corresponds to a spherical shell of
branes with S^3 geometry and symmetry. The equations of motions with
delta-function sources are carefully solved in such backgrounds. Various other
brane distributions with sixteen unbroken supercharges are described. They are
associated to exact world-sheet superconformal field theories with domain-walls
in space-time. We study the equations of gravitational fluctuations, find
normalizable modes of bulk 6-d gravitons and confirm the existence of a mass
gap. We also study the moduli of the configurations and derive their
(normalizable) wave-functions. We use our results to calculate in a
controllable fashion using holography, the two-point function of the stress
tensor of little string theory in these vacua.Comment: LateX, 32 pages, 4 figures; (v2) A reference adde
Multiagent cooperation for solving global optimization problems: an extendible framework with example cooperation strategies
This paper proposes the use of multiagent cooperation for solving global optimization problems through the introduction of a new multiagent environment, MANGO. The strength of the environment lays in itsflexible structure based on communicating software agents that attempt to solve a problem cooperatively. This structure allows the execution of a wide range of global optimization algorithms described as a set of interacting operations. At one extreme, MANGO welcomes an individual non-cooperating agent, which is basically the traditional way of solving a global optimization problem. At the other extreme, autonomous agents existing in the environment cooperate as they see fit during run time. We explain the development and communication tools provided in the environment as well as examples of agent realizations and cooperation scenarios. We also show how the multiagent structure is more effective than having a single nonlinear optimization algorithm with randomly selected initial points
Disappearing Dark Matter in Brane World Cosmology: New Limits on Noncompact Extra Dimensions
We explore cosmological implications of dark matter as massive particles
trapped on a brane embedded in a Randall-Sundrum noncompact higher dimension
space. It is an unavoidable consequence of this cosmology that massive
particles are metastable and can disappear into the bulk dimension. Here, we
show that a massive dark matter particle (e.g. the lightest supersymmetric
particle) is likely to have the shortest lifetime for disappearing into the
bulk. We examine cosmological constraints on this new paradigm and show that
disappearing dark matter is consistent (at the 95% confidence level) with all
cosmological constraints, i.e. present observations of Type Ia supernovae at
the highest redshift, trends in the mass-to-light ratios of galaxy clusters
with redshift, the fraction of X-ray emitting gas in rich clusters, and the
spectrum of power fluctuations in the cosmic microwave background. A best concordance region is identified corresponding to a mean lifetime for
dark matter disappearance of Gyr. The implication
of these results for brane-world physics is discussed.Comment: 7 pages, 7 figures, new cosmological constraints added, accepted for
publication in PR
Quantum Iterated Function Systems
Iterated functions system (IFS) is defined by specifying a set of functions
in a classical phase space, which act randomly on an initial point. In an
analogous way, we define a quantum iterated functions system (QIFS), where
functions act randomly with prescribed probabilities in the Hilbert space. In a
more general setting a QIFS consists of completely positive maps acting in the
space of density operators. We present exemplary classical IFSs, the invariant
measure of which exhibits fractal structure, and study properties of the
corresponding QIFSs and their invariant states.Comment: 12 pages, 1 figure include
The dynamics of quark-gluon plasma and AdS/CFT
In these pedagogical lectures, we present the techniques of the AdS/CFT
correspondence which can be applied to the study of real time dynamics of a
strongly coupled plasma system. These methods are based on solving
gravitational Einstein's equations on the string/gravity side of the AdS/CFT
correspondence. We illustrate these techniques with applications to the
boost-invariant expansion of a plasma system. We emphasize the common
underlying AdS/CFT description both in the large proper time regime where
hydrodynamic dynamics dominates, and in the small proper time regime where the
dynamics is far from equilibrium. These AdS/CFT methods provide a fascinating
arena interrelating General Relativity phenomenae with strongly coupled gauge
theory physics.Comment: 35 pages, 3 figures. Lectures at the 5th Aegean summer school, `From
gravity to thermal gauge theories: the AdS/CFT correspondence'. To appear in
the proceedings in `Lecture Notes in Physics
Holographic Cosmic Quintessence on Dilatonic Brane World
Recently quintessence is proposed to explain the observation data of
supernova indicating a time-varying cosmological constant and accelerating
universe. Inspired by this and its mysterious origin, we look for the
possibility of quintessence as the holographic dark matters dominated in the
late time in the brane world scenarios. We consider both the cases of static
and moving brane in a dilaton gravity background. For the static brane we use
the Hamilton-Jacobi method motivated by holographic renormalization group to
study the intrinsic FRW cosmology on the brane and find out the constraint on
the bulk potential for the quintessence. This constraint requires a negative
slowly varying bulk potential which implies an anti-de Sitter-like bulk
geometry and could be possibly realized from the higher dimensional
supergravities or string theory. We find the similar constraint for the moving
brane cases and that the quintessence on it has the effect as a mildly
time-varying Newton constant.Comment: 16pages, no figure, Latex; revised version, references added, typos
corrected, abstract and comments improved; final version, will appear in PR
- …