1,771 research outputs found
Stability of Branched Flow from a Quantum Point Contact
In classically chaotic systems, small differences in initial conditions are
exponentially magnified over time. However, it was observed experimentally that
the (necessarily quantum) "branched flow" pattern of electron flux from a
quantum point contact (QPC) traveling over a random background potential in
two-dimensional electron gases(2DEGs) remains substantially invariant to large
changes in initial conditions. Since such a potential is classically chaotic
and unstable to changes in initial conditions, it was conjectured that the
origin of the observed stability is purely quantum mechanical, with no
classical analog. In this paper, we show that the observed stability is a
result of the physics of the QPC and the nature of the experiment. We show that
the same stability can indeed be reproduced classically, or quantum
mechanically. In addition, we explore the stability of the branched flow with
regards to changes in the eigenmodes of quantum point contact.Comment: 4 figures, To appear in PR
Quasiresonance
The concept of quasiresonance was introduced in connection with inelastic
collisions between one atom and a vibro-rotationally excited diatomic molecule.
In its original form, the collisions induce {\sl quasiresonant} transfer of
energy between the internal degrees of freedom of the diatom: there is a
surprisingly accurate low order rational value for the ratio of the changes in
the vibrational and rotational classical actions, provided the vibrational and
rotational frequencies of the diatom are approximately related by low order
rational values, and the collision was longer that the rotational period of the
molecule. In this paper we show that quasiresonance can be extended to many
other processes and systems, and that it may be understood in terms of the
adiabatic invariance theory and the method of averaging
Unification of perturbation theory, RMT and semiclassical considerations in the study of parametrically-dependent eigenstates
We consider a classically chaotic system that is described by an Hamiltonian
where x is a constant parameter. Our main interest is in the case of
a gas-particle inside a cavity, where controls a deformation of the
boundary or the position of a `piston'. The quantum-eigenstates of the system
are . We describe how the parametric kernel
evolves as a function of . We explore both the perturbative
and the non-perturbative regimes, and discuss the capabilities and the
limitations of semiclassical as well as of random-waves and
random-matrix-theory (RMT) considerations.Comment: 4 pages, 1 figure. Improved presentation. To be published in Phys.
Rev. Let
Multiple Scattering and Plasmon Resonance in the Intermediate Regime
The collective excitation of the conduction electrons in subwavelength
structures gives rise to the Localized Surface Plasmon(LSP). The system
consisting of two such LSPs, known as the dimer system,is of fundamental
interest and is being actively investigated in the literature. Three regimes
have been previously identified and they are the photonic regime, the strong
coupling regime and the quantum tunneling regime. In this Letter, we propose a
new regime for this intriguing systems, the intermediate regime. In this new
regime, the quasistatic approximation, which is widely used to study such LSP
systems, fails to capture the main physics: the multiple scattering of the
electromagnetic waves between the two LSPs, which significantly modifies the
properties of the resonant modes in the system. This intermediate regime
provides a new route to explore in plasmonics, where controlling both the
excited plasmon modes and the damping rates are of paramount significance
Shaping Electromagnetic Fields
The ability to control electromagnetic fields on the subwavelength scale
could open exciting new venues in many fields of science. Transformation optics
provides one way to attain such control through the local variation of the
permittivity and permeability of a material. Here, we demonstrate another way
to shape electromagnetic fields, taking advantage of the enormous size of the
configuration space in combinatorial problems and the resonant scattering
properties of metallic nanoparticles. Our design does not require the
engineering of a material's electromagnetic properties and has relevance to the
design of more flexible platforms for probing light-matter interaction and many
body physics
Quantum Multiple Scattering: Eigenmode Expansion and Its Applications to Proximity Resonance
We show that for a general system of N s-wave point scatterers, there are
always N eigenmodes. These eigenmodes or eigenchannels play the same role as
spherical harmonics for a spherically symmetric target--they give a phase shift
only. In other words, the T matrix of the system is of rank N and the
eigenmodes are eigenvectors corresponding to non-0 eigenvalues of the T matrix.
The eigenmode expansion approach can give insight to the total scattering cross
section; the position, width, and superradiant or subradiant nature of
resonance peaks; the unsymmetric Fano lineshape of sharp proximity resonance
peaks based on the high energy tail of a broad band; and other properties.
Off-resonant eigenmodes for identical proximate scatterers are approximately
angular momentum eigenstates.Comment: Accepted by PRA. Research is underway currently to apply this method
to superradianc
Classical approach to collision complexes in ultracold chemical reactions
Inspired by Wannier's threshold law, we recognize that collision complex
decay meets the requirements of quantum-classical correspondence in
sufficiently exothermic ultracold reactions. We make use of this correspondence
to elucidate the classical foundations of ultracold reactions and to help bring
calculations currently beyond the capabilities of quantum mechanics within
reach. A classical method with a simplified model of many-body interactions is
provided for determination of the collision complex lifetime and demonstrated
for a reduced-dimensional system, as preliminary to the calculation of
collision complex lifetimes in the full-dimensional system.Comment: 27 pages, 8 figure
Fidelity Decay for Phase Space Displacements
In this letter we analyse the behavior of fidelity decay under a very
specific kind of perturbation: phase space displacements. Under these
perturbations, systems will decay following the Lyapunov regime only. Others
universal regimes discussed in the literature are not presented in this case;
instead, for small values of the perturbation we observe quantum freeze of the
fidelity. We also show that it is possible to connect this result with the
incoherent neutron scattering problemComment: 4 pages, 2 figure
The No-sticking Effect in Ultra-cold Collisions
We provide the theoretical basis for understanding the phenomenon in which an
ultra cold atom incident on a possibly warm target will not stick, even in the
large limit where is the number of internal degrees of freedom of the
target. Our treatment is non-perturbative in which the full many-body problem
is viewed as a scattering event purely within the context of scattering theory.
The question of sticking is then simply and naturally identified with the
formation of a long lived resonance. One crucial physical insight that emerges
is that the many internal degrees of freedom serve to decohere the incident one
body wavefunction, thus upsetting the delicate interference process necessary
to form a resonance in the first place. This is the physical reason for not
sticking.Comment: 19 pages single column, 2 figures. Submitted to Physical Review
The Polyacetylene Raman Spectrum, Decoded
More than 30 years ago, polyacetylene was very much in the limelight, an
early example of a conducting polymer and source of many unusual spectroscopic
features spawning disparate ideas as to their origin. Several versions of the
polyacetylene spectrum story emerged, with contradictory conclusions. In this
paper both ordinary and peculiar polyacetylene spectral features are explained
in terms of standard (if disused) spectroscopic concepts, including the
dependence of electronic transition moments on phonon coordinates,
Born-Oppenheimer energy surface properties, and (much more familiarly) electron
and phonon band structure. Raman sideband dispersion and line shapes are very
well matched by theory in a fundamental way. Most importantly, clear
ramifications emerge for the Raman spectroscopy of a wide range of extended
systems, including graphene and beyond, suggesting changes to some common
practice in condensed matter spectroscopy.Comment: Seven figure
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