arXiv subjects

Henning Schomerus

Publications and source records attributed to Henning Schomerus.

107 records · Page 6Linked to original sources

arXiv2006

Transport in chaotic quantum dots: effects of spatial symmetries which interchange the leads

We investigate the effect of spatial symmetries on phase coherent electronic transport through chaotic quantum dots. For systems which have a spatial symmetry that interchanges the source and drain leads, we find in the framework of random matrix theory that the density of the transmission eigenvalues is indepedent of the number of channels N in the leads. As a consequence, the weak localization correction to the conductance vanishes in these systems, and the shot noise suppression factor F is independent of N. We confirm this prediction by means of numerical calculations for stadium billiards with various lead geometries. These calculations also uncover transport signatures of partially preserved symmetries.

cond-mat.mes-hall↗

arXiv2005

Non-Markovian master equations from entanglement with stationary unobserved degrees of freedom

We deduce a class of non-Markovian completely positive master equations which describe a system in a composite bipartite environment, consisting of a Markovian reservoir and additional stationary unobserved degrees of freedom that modulate the dissipative coupling. The entanglement-induced memory effects can persist for arbitrary long times and affect the relaxation to equilibrium, as well as induce corrections to the quantum-regression theorem. By considering the extra degrees of freedom as a discrete manifold of energy levels, strong non-exponential behavior can arise, as for example power law and stretched exponential decays.

quant-ph↗

arXiv2005

Quantum-to-classical correspondence in open chaotic systems

We review properties of open chaotic mesoscopic systems with a finite Ehrenfest time tau_E. The Ehrenfest time separates a short-time regime of the quantum dynamics, where wave packets closely follow the deterministic classical motion, from a long-time regime of fully-developed wave chaos. For a vanishing Ehrenfest time the quantum systems display a degree of universality which is well described by random-matrix theory. In the semiclassical limit, tau_E becomes parametrically larger than the scattering time off the boundaries and the dwell time in the system. This results in the emergence of an increasing number of deterministic transport and escape modes, which induce strong deviations from random-matrix universality. We discuss these deviations for a variety of physical phenomena, including shot noise, conductance fluctuations, decay of quasibound states, and the mesoscopic proximity effect in Andreev billiards.

cond-mat.mes-hall↗

arXiv2004

Quantum-to-classical crossover of quasi-bound states in open quantum systems

In the semiclassical limit of open ballistic quantum systems, we demonstrate the emergence of instantaneous decay modes guided by classical escape faster than the Ehrenfest time. The decay time of the associated quasi-bound states is smaller than the classical time of flight. The remaining long-lived quasi-bound states obey random-matrix statistics, renormalized in compliance with the recently proposed fractal Weyl law for open systems [W. T. Lu, S. Sridhar, and M. Zworski, Phys. Rev. Lett. 91, 154101 (2003)]. We validate our theory numerically for a model system, the open kicked rotator.

cond-mat.mes-hall↗

arXiv2004

Opto-mechanical probes of resonances in amplifying microresonators

We investigate whether the force and torque exerted by light pressure on an irregularly shaped dielectric resonator allow to detect resonant frequencies, delivering information complemental to the scattering cross section by mechanical means. The peak-to-valley ratio in the torque signal can be many times larger than in the scattering cross section, and, furthermore, depends on the structure of the resonance wave pattern. The far-field emission pattern of the associated quasi-bound states can be tested by the angular dependence of the mechanical probes at finite amplification rate. We relate the force and torque to the scattering matrix and present numerical results for an annularly shaped dielectric resonator.

physics.optics↗

arXiv2004

Aharonov-Bohm Physics with Spin I: Geometric Phases in One-dimensional Ballistic Rings

We analytically calculate the spin-dependent electronic conductance through a one-dimensional ballistic ring in the presence of an inhomogeneous magnetic field and identify signatures of geometric and Berry phases in the general non-adiabatic situation. For an in-plane magnetic field, we rigorously prove the spin-flip effect presented in Frustaglia et al., Phys. Rev. Lett. 87, 256602 (2001), which allows to control and switch the polarization of outgoing electrons by means of an Aharonov-Bohm flux, and derive analytical expressions for the energy-averaged magneto-conductance. Our results support numerical calculations for two-dimensional ballistic rings presented in the second paper (Frustaglia et al., submitted to Phys. Rev. B) of this series.

cond-mat.mes-hall↗

arXiv2002

Quasibound states at thresholds in multichannel impurity scattering

We investigate the threshold behavior of transmission resonances and quasibound states in the multichannel scattering problems of a one dimensional (1D) time-dependent impurity potential, and the related problem of a single impurity in a quasi 1D wire. It was claimed before in the literature that a quasibound state disappears when a transmission zero collides with the subband boundary. However, the transmission line shape, the Friedel sum rule, and the delay time show that the quasibound states still survive and affect the physical quantities. We discuss the relation between threshold behavior of transmission resonances, and quasibound states and their boundary conditions in the general context of multichannel scatterings.

cond-mat.mes-hall↗

arXiv2002

Husimi functions at dielectric interfaces: Inside-outside duality for optical systems and beyond

We introduce generalized Husimi functions at the interfaces of dielectric systems. Four different functions can be defined, corresponding to the incident and departing wave on both sides of the interface. These functions allow to identify mechanisms of wave confinement and escape directions in optical microresonators, and give insight into the structure of resonance wave functions. Off resonance, where systematic interference can be neglected, the Husimi functions are related by Snell's law and Fresnel's coefficients.

physics.optics↗

arXiv1999

Semiclassical singularities from bifurcating orbits

We study how the singular behaviour of classical systems at bifurcations is reflected by their quantum counterpart. The semiclassical contributions of individual periodic orbits to trace formulae of Gutzwiller type are known to diverge when orbits bifurcate, a situation characteristic for systems with a mixed phase space. This singular behaviour is reflected by the quantum system in the semiclassical limit, in which the individual contributions remain valid closer to a bifurcation, while the true collective amplitude at the bifurcation increases with some inverse power of Planck's constant (with an exponent depending on the type of bifurcation). We illustrate the interplay between the two competing limits (closeness to a bifurcation and smallness of $\hbar$) numerically for a generic dynamical system, the kicked top.

chao-dyn↗

arXiv1999

Conductance enhancement in quantum point contact-semiconductor-superconductor devices

We present numerical calculations of the conductance of an interface between a phase-coherent two-dimensional electron gas and a superconductor with a quantum point contact in the normal region. Using a scattering matrix approach we reconsider the geometry of De Raedt, Michielsen, and Klapwijk [Phys. Rev. B, 50, 631 (1994)] which was studied within the time-dependent Bogoliubov-de Gennes formalism. We find that the factor-of-two enhancement of the conductance G_NS compared to the normal state conductance G_N for ideal interfaces may be suppressed for interfaces with a quantum point contact with only a few propagating modes. The suppression is found to depend strongly on the position of the Fermi level. We also study the suppression due to a barrier at the interface and find an anomalous behavior caused by quasiparticle interference. Finally, we consider the limit of sequential tunneling and find a suppression of the factor-of-two enhancement which may explain the absence of conductance enhancement in experiments on metal-superconductor structures.

cond-mat.supr-con↗

arXiv1999

Twofold-broken rational tori and the uniform semiclassical approximation

We investigate broken rational tori consisting of a chain of four (rather than two) periodic orbits. The normal form that describes this configuration is identified and used to construct a uniform semiclassical approximation, which can be utilized to improve trace formulae. An accuracy gain can be achieved even for the situation when two of the four orbits are ghosts. This is illustrated for a model system, the kicked top.

chao-dyn↗

arXiv1997

Periodic orbits near bifurcations of codimension two: Classical mechanics, semiclassics, and Stokes transitions

We investigate classical and semiclassical aspects of codimension--two bifurcations of periodic orbits in Hamiltonian systems. A classification of these bifurcations in autonomous systems with two degrees of freedom or time-periodic systems with one degree of freedom is presented. We derive uniform approximations to be used in semiclassical trace formulas and determine also certain global bifurcations in conjunction with Stokes transitions that become important in the ensuing diffraction catastrophe integrals.

chao-dyn↗

arXiv1997

Uniform Approximation for Period-Quadrupling Bifurcations

We derive a uniform approximation for semiclassical contributions of periodic orbits to the spectral density which is valid for generic period-quadrupling bifurcations in systems with a mixed phase space. These bifurcations involve three periodic orbits which coalesce at the bifurcation. In the vicinity of the bifurcation the three orbits give a collective contribution to the spectral density while the individual contributions of Gutzwiller's type would diverge at the bifurcation. The uniform approximation is obtained by mapping the action function onto the normal form corresponding to the bifurcation. The present article is a continuation of previous work in which uniform approximations for generic period-$m$-tupling bifurcations with $m \neq 4$ were derived.

chao-dyn↗

arXiv1997

Semiclassical spectra from periodic-orbit clusters in a mixed phase space

We determine semiclassical quasienergy spectra from periodic orbits for a system with a mixed phase space, the kicked top. Throughout the transition from integrability to well developed chaos the standard error incurred for the quasienergies is a small percentage of their mean spacing. Such fine accuracy does not even require the angular momentum quantum number $j\propto 1/\hbar$ to be large; it already prevails for $j=1,2,3$. To open the way towards a reliable spectrum the conventional trace formula à la Gutzwiller has to be extended by including (i) ghosts, i.e., complex predecessors of bifurcating orbits and the possibility of Stokes transitions undergone by such complex orbits and (ii) collective contributions of clusters of periodic orbits near bifurcations. Even bifurcations of codimension higher than one must be reckoned with by accounting for the clusters involved through the appropriate diffraction integrals.

chao-dyn↗

arXiv1997

Semiclassical interference of bifurcations

In semiclassical studies of systems with mixed phase space, the neighbourhood of bifurcations of co-dimension two is felt strongly even though such bifurcations are ungeneric in classical mechanics. We discuss a scenario which reveals this fact and derive the correct semiclassical contribution of the bifurcating orbits to the trace of the unitary time evolution operator. That contribution has a certain collective character rather than being additive in the individual periodic orbits involved. The relevance of our observation is demonstrated by a numerical study of the kicked top; the collective contribution derived is found to considerably improve the semiclassical approximation of the trace.

chao-dyn↗

arXiv1997

Bifurcations of Periodic Orbits and Uniform Approximations

We derive uniform approximations for contributions to Gutzwiller's periodic-orbit sum for the spectral density which are valid close to bifurcations of periodic orbits in systems with mixed phase space. There, orbits lie close together and give collective contributions, while the individual contributions of Gutzwiller's type would diverge at the bifurcation. New results for the tangent, the period doubling and the period tripling bifurcation are given. They are obtained by going beyond the local approximation and including higher order terms in the normal form of the action. The uniform approximations obtained are tested on the kicked top and are found to be in excellent agreement with exact quantum results.

chao-dyn↗

arXiv1996

Secular determinants of random unitary matrices

We consider the characteristic polynomials of random unitary matrices $U$ drawn from various circular ensembles. In particular, the statistics of the coefficients of these polynomials are studied. The variances of these ``secular coefficients'' are given explicitly for arbitrary dimension and continued analytically to arbitrary values of the level repulsion exponent $β$. The latter secular coefficients are related to the traces of powers of $U$ by Newton's well-known formulae. While the traces tend to have Gaussian distributions and to be statistically independent among one another in the limit as the matrix dimension grows large, the secular coefficients exhibit strong mutual correlations due to Newton's mixing of traces to coefficients. These results might become relevant for current efforts at combining semiclassics and random-matrix theory in quantum treatments of classically chaotic dynamics.

chao-dyn↗