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 $H(Q,P;x)$ where x is a constant parameter. Our main interest is in the case of a gas-particle inside a cavity, where $x$ controls a deformation of the boundary or the position of a `piston'. The quantum-eigenstates of the system are $|n(x)>$. We describe how the parametric kernel $P(n|m)=||^2$ evolves as a function of $\delta x = (x-x_0)$. 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 $n$ limit where $n$ 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