422 research outputs found
Classical Trajectory Perspective on Double Ionization Dynamics of Diatomic Molecules Irradiated by Ultrashort Intense Laser Pulses
In the present paper, we develop a semiclassical quasi-static model
accounting for molecular double ionization in an intense laser pulse. With this
model, we achieve insight into the dynamics of two highly-correlated valence
electrons under the combined influence of a two-center Coulomb potential and an
intense laser field, and reveal the significant influence of molecular
alignment on the ratio of double over single ion yield. Analysis on the
classical trajectories unveils sub-cycle dynamics of the molecular double
ionization. Many interesting features, such as the accumulation of emitted
electrons in the first and third quadrants of parallel momentum plane, the
regular pattern of correlated momentum with respect to the time delay between
closest collision and ionization moment, are revealed and successfully
explained by back analyzing the classical trajectories. Quantitative agreement
with experimental data over a wide range of laser intensities from tunneling to
over-the-barrier regime is presented.Comment: 8 pages, 9 figure
Aharonov-Bohm Oscillation and Chirality Effect in Optical Activity of Single Wall Carbon Nanotubes
We study the Aharonov-Bohm effect in the optical phenomena of single wall
carbon nanotubes (SWCN) and also their chirality dependence. Specially, we
consider the natural optical activity as a proper observable and derive it's
general expression based on a comprehensive symmetry analysis, which reveals
the interplay between the enclosed magnetic flux and the tubule chirality for
arbitrary chiral SWCN. A quantitative result for this optical property is given
by a gauge invariant tight-binding approximation calculation to stimulate
experimental measurements.Comment: Submitted on 15 Jan 04, REVISED on 28 Apr 04, To appear in Phys. Rev.
B(Brief Report
Rosen-Zener Transition in a Nonlinear Two-Level System
We study Rosen-Zener transition (RZT) in a nonlinear two-level system in
which the level energies depend on the occupation of the levels, representing a
mean-field type of interaction between the particles. We find that the
nonlinearity could affect the quantum transition dramatically. At certain
nonlinearity the 100% population transfer between two levels is observed and
found to be robust over a very wide range of external parameters. On the other
hand, the quantum transition could be completely blocked by a strong
nonlinearity. In the sudden and adiabatic limits we have derived analytical
expressions for the transition probability. Numerical explorations are made for
a wide range of parameters of the general case. Possible applications of our
theory to Bose-Einstern Condensates (BECs) are discussed.Comment: 8 pages, 8 figure
Landau-Zener Tunnelling in a Nonlinear Three-level System
We present a comprehensive analysis of the Landau-Zener tunnelling of a
nonlinear three-level system in a linearly sweeping external field. We find the
presence of nonzero tunnelling probability in the adiabatic limit (i.e., very
slowly sweeping field) even for the situation that the nonlinear term is very
small and the energy levels keep the same topological structure as that of
linear case. In particular, the tunnelling is irregular with showing an
unresolved sensitivity on the sweeping rate. For the case of fast-sweeping
fields, we derive an analytic expression for the tunnelling probability with
stationary phase approximation and show that the nonlinearity can dramatically
influence the tunnelling probability when the nonlinear "internal field"
resonate with the external field. We also discuss the asymmetry of the
tunnelling probability induced by the nonlinearity. Physics behind the above
phenomena is revealed and possible application of our model to triple-well
trapped Bose-Einstein condensate is discussed.Comment: 8 pages, 8 figure
The Deformation of an Elastic Substrate by a Three-Phase Contact Line
Young's classic analysis of the equilibrium of a three-phase contact line
ignores the out-of-plane component of the liquid-vapor surface tension. While
it has long been appreciated that this unresolved force must be balanced by
elastic deformation of the solid substrate, a definitive analysis has remained
elusive because conventional idealizations of the substrate imply a divergence
of stress at the contact line. While a number of theories of have been
presented to cut off the divergence, none of them have provided reasonable
agreement with experimental data. We measure surface and bulk deformation of a
thin elastic film near a three-phase contact line using fluorescence confocal
microscopy. The out-of-plane deformation is well fit by a linear elastic theory
incorporating an out-of-plane restoring force due to the surface tension of the
gel. This theory predicts that the deformation profile near the contact line is
scale-free and independent of the substrate elastic modulus.Comment: 4 pages, 3 figure
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Method to determine defect positions below a metal surface by STM
The oscillatory voltage dependence of the conductance of a quantum point
contact in the presence of a single point-like defect has been analyzed
theoretically. Such signals are detectable and may be exploited to obtain
information on defect positions below a metal surface. Both tunnel junctions
and ballistic contacts of adiabatic shape have been considered. The effect of
quantum interference has been taking into account between the principal wave
that is directly transmitted through the contact and the partial wave that is
scattered by the contact and the defect. This effect leads to oscillations of
the conductance as a function of applied voltage. We obtain the dependence of
the period and amplitude of the conductance oscillations on the position of the
defect inside the metal.Comment: 16 pages, 7 figure
The role of particle interactions in a many-body model of Feshbach molecular formation in bosonic systems
In this paper, we investigate the atom-molecule conversion dynamics of a
generalized many-body model that includes the atom-atom, atom-molecule, and
molecule-molecule interactions, emphasizing the efficiency of the Feshbach
molecular formation. We show that the picture of two-body molecular production
depicted by the Landau-Zener model is significantly altered: The energy levels
are dramatically distorted and the conversion efficiency is suppressed by the
particle interactions. According to the rule of constant action and with the
help of phase-space analysis, we derive an analytical expression for the
conversion efficiency in the adiabatic limit. It shows a ceiling for the
conversion efficiency when the interaction strength is larger than a critical
value. We further derive a closed equation for the conversion efficiency with
the stationary phase approximation. In the sudden limit, the conversion
efficiency is twice that predicted by the two-body Landau-Zener formula. Our
analytical formula has been confirmed by numerical calculations.Comment: 7 pages, 5 figure
A Model for the Propagation of Sound in Granular Materials
This paper presents a simple ball-and-spring model for the propagation of
small amplitude vibrations in a granular material. In this model, the
positional disorder in the sample is ignored and the particles are placed on
the vertices of a square lattice. The inter-particle forces are modeled as
linear springs, with the only disorder in the system coming from a random
distribution of spring constants. Despite its apparent simplicity, this model
is able to reproduce the complex frequency response seen in measurements of
sound propagation in a granular system. In order to understand this behavior,
the role of the resonance modes of the system is investigated. Finally, this
simple model is generalized to include relaxation behavior in the force network
-- a behavior which is also seen in real granular materials. This model gives
quantitative agreement with experimental observations of relaxation.Comment: 21 pages, requires Harvard macros (9/91), 12 postscript figures not
included, HLRZ preprint 6/93, (replacement has proper references included
Chaos in generalized Jaynes-Cummings model. Kinetic approach
In this work we study possibility of chaos formation in the dynamics governed
by paradigmatic model of Cavity Quantum Electrodynamics, the so called
James-Cammings model. In particular we consider generalized JC model. It is
shown that even in the case of zero detuning dynamics is chaotic. Kinetic
approach for the problem under study has been applied
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