137 research outputs found
Nonthermal fragmentation of C60
A theoretical study of the subpicosecond fragmentation of C60 clusters in
response to ultrafast laser pulses is presented. We simulate the laser
excitation and the consequent nonequilibrium relaxation dynamics of the
electronic and nuclear degrees of freedom. The first stages of the
nonequilibrium dynamics are dominated by a coherent breathing mode followed by
the cold ejection of single C atoms, in contrast to the dimer emission which
characterizes the thermal relaxation. We also determine the nonequilibrium
damage thresholds as a function of the pulse duration.Comment: 5 pages, 4 figures, submitted to Chem. Phys. Let
C in intense femtosecond laser pulses: nonlinear dipole response and ionization
We study the interaction of strong femtosecond laser pulses with the C
molecule employing time-dependent density functional theory with the ionic
background treated in a jellium approximation. The laser intensities considered
are below the threshold of strong fragmentation but too high for perturbative
treatments such as linear response. The nonlinear response of the model to
excitations by short pulses of frequencies up to 45eV is presented and analyzed
with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond
laser pulses of 800nm wavelength ionization is found to occur multiphoton-like
rather than via excitation of a ``giant'' resonance.Comment: 14 pages, including 1 table, 5 figure
Calculations of the A_1 phonon frequency in photoexcited Tellurium
Calculations of the A_1 phonon frequency in photoexcited tellurium are
presented. The phonon frequency as a function of photoexcited carrier density
and phonon amplitude is determined. Recent pump probe experiments are
interpreted in the light of these calculatons. It is proposed that, in
conjunction with measurements of the phonon period in ultra-fast pump-probe
reflectivity experiments, the calculated frequency shifts can be used to infer
the evolution of the density of photoexcited carriers on a sub-picosecond
time-scale.Comment: 15 pages Latex, 3 postscript figure
Ultrafast changes in lattice symmetry probed by coherent phonons
The electronic and structural properties of a material are strongly
determined by its symmetry. Changing the symmetry via a photoinduced phase
transition offers new ways to manipulate material properties on ultrafast
timescales. However, in order to identify when and how fast these phase
transitions occur, methods that can probe the symmetry change in the time
domain are required. We show that a time-dependent change in the coherent
phonon spectrum can probe a change in symmetry of the lattice potential, thus
providing an all-optical probe of structural transitions. We examine the
photoinduced structural phase transition in VO2 and show that, above the phase
transition threshold, photoexcitation completely changes the lattice potential
on an ultrafast timescale. The loss of the equilibrium-phase phonon modes
occurs promptly, indicating a non-thermal pathway for the photoinduced phase
transition, where a strong perturbation to the lattice potential changes its
symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure
Characterizing temporary hydrological regimes at a European scale
Monthly duration curves have been constructed from climate data across Europe to help address the relative frequency of ecologically critical low flow stages in temporary rivers, when flow persists only in disconnected pools in the river bed. The hydrological model is 5 based on a partitioning of precipitation to estimate water available for evapotranspiration and plant growth and for residual runoff. The duration curve for monthly flows has then been analysed to give an estimate of bankfull flow based on recurrence interval. The corresponding frequency for pools is then based on the ratio of bank full discharge to pool flow, arguing from observed ratios of cross-sectional areas at flood 10 and low flows to estimate pool flow as 0.1% of bankfull flow, and so estimate the frequency of the pool conditions that constrain survival of river-dwelling arthropods and fish. The methodology has been applied across Europe at 15 km resolution, and can equally be applied under future climatic scenarios
Color Superconducting Phases of Cold Dense Quark Matter
We investigate color superconducting phases of cold quark matter at densities
relevant for the interiors of compact stars. At these densities, electrically
neutral and weak-equilibrated quark matter can have unequal numbers of up,
down, and strange quarks. The QCD interaction favors Cooper pairs that are
antisymmetric in color and in flavor, and a crystalline color superconducting
phase can occur which accommodates pairing between flavors with unequal number
densities. In the crystalline color superconductor, quarks of different flavor
form Cooper pairs with nonzero total momentum, yielding a condensate that
varies in space like a sum of plane waves. Rotational and translational
symmetry are spontaneously broken. We use a Ginzburg-Landau method to evaluate
candidate crystal structures and predict that the favored structure is
face-centered-cubic. We predict a robust crystalline phase with gaps comparable
in magnitude to those of the color-flavor-locked phase that occurs when the
flavor number densities are equal. Crystalline color superconductivity will be
a generic feature of the QCD phase diagram, occurring wherever quark matter
that is not color-flavor locked is to be found. If a very large flavor
asymmetry forbids even the crystalline state, single-flavor pairing will occur;
we investigate this and other spin-one color superconductors in a survey of
generic color, flavor, and spin pairing channels. Our predictions for the
crystalline phase may be tested in an ultracold gas of fermionic atoms, where a
similar crystalline superfluid state can occur. If a layer of crystalline quark
matter occurs inside of a compact star, it could pin rotational vortices,
leading to observable pulsar glitches.Comment: Ph.D. thesis, submitted to the MIT Department of Physics, May 2003.
Five chapters and two appendices (180 pages, 30 figures). Chapters 1 and 5
are new: chapter 1 is a detailed review of previous work, and chapter 5
discusses applications of the crystalline phase for the physics of pulsar
spin glitches and cold trapped atom
Enhanced Lifetime Of Excitons In Nonepitaxial Au/cds Core/shell Nanocrystals
The ability of metal nanoparticles to capture light through plasmon excitations offers an opportunity for enhancing the optical absorption of plasmon-coupled semiconductor materials via energy transfer. This process, however, requires that the semiconductor component is electrically insulated to prevent a backward charge flow into metal and interfacial states, which causes a premature dissociation of excitons. Here we demonstrate that such an energy exchange can be achieved on the nanoscale by using nonepitaxial Au/CdS core/shell nanocomposites. These materials are fabricated via a multistep cation exchange reaction, which decouples metal and semiconductor phases leading to fewer interfacial defects. Ultrafast transient absorption measurements confirm that the lifetime of excitons in the CdS shell (tau approximate to 300 ps) is much longer than lifetimes of excitons in conventional, reduction-grown Au/CdS heteronanostructures. As a result, the energy of metal nanoparticles can be efficiently utilized by the semiconductor component without undergoing significant nonradiative energy losses, an important property for catalytic or photovoltaic applications. The reduced rate of exciton dissociation in the CdS domain of Au/CdS nanocomposites was attributed to the nonepitaxial nature of Au/CdS interfaces associated with low defect density and a high potential barrier of the interstitial phase
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