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H Schomerus

Publications and source records attributed to H Schomerus.

At least 19 recordsLinked to original sources
PubMed2005

Nonuniversality of anderson localization in short-range correlated disorder.

We provide an analytic theory of Anderson localization on a lattice with a weak short-range correlated disordered potential. Contrary to the general belief, we demonstrate that already next-neighbor statistical correlations in the potential can give rise to strong anomalies in the localization length and the density of states, and to the complete violation of single-parameter scaling. Such anomalies originate in additional symmetries of the lattice model in the limit of weak disorder. The results of numerical simulations are in full agreement with our theory, with no adjustable parameters.

Journal Article↗
PubMed2004

Exponential sensitivity to dephasing of electrical conduction through a quantum dot.

According to random-matrix theory, interference effects in the conductance of a ballistic chaotic quantum dot should vanish proportional to (tau(phi)/tau(D))(p) when the dephasing time tau(phi) becomes small compared to the mean dwell time tau(D). Aleiner and Larkin have predicted that the power law crosses over to an exponential suppression proportional to exp((-tau(E)/tau(phi)) when tau(phi) drops below the Ehrenfest time tau(E). We report the first observation of this crossover in a computer simulation of universal conductance fluctuations. Their theory also predicts an exponential suppression proportional to exp((-tau(E)/tau(D)) in the absence of dephasing--which is not observed. We show that the effective random-matrix theory proposed previously for quantum dots without dephasing explains both observations.

Journal Article↗
PubMed2004

Antibunched photons emitted by a quantum point contact out of equilibrium.

Motivated by the experimental search for "GHz nonclassical light," we identify the conditions under which current fluctuations in a narrow constriction generate sub-Poissonian radiation. Antibunched electrons generically produce bunched photons, because the same photon mode can be populated by electrons decaying independently from a range of initial energies. Photon antibunching becomes possible at frequencies close to the applied voltage V x e/variant Planck's over 2pi, when the initial energy range of a decaying electron is restricted. The condition for photon antibunching in a narrow frequency interval below eV/variant Planck's over 2pi reads [SigmanTn(1-Tn)]2<2Sigman[Tn(1-Tn)]2, with Tn an eigenvalue of the transmission matrix. This condition is satisfied in a quantum point contact, where only a single Tn differs from 0 or 1. The photon statistics is then a superposition of binomial distributions.

Journal Article↗
PubMed2003

Anomalous wave function statistics on a one-dimensional lattice with power-law disorder.

Within a general framework, we discuss the wave function statistics in the Lloyd model of Anderson localization on a one-dimensional lattice with a Cauchy distribution for random on-site potential. We demonstrate that already in leading order in the disorder strength, there exists a hierarchy of anomalies in the probability distributions of the wave function, the conductance, and the local density of states, for every energy which corresponds to a rational ratio of wavelength to lattice constant. Power-law rather than log-normal tails dominate the short-distance wave-function statistics.

Journal Article↗
PubMed2003

Quantum Andreev map: a paradigm of quantum chaos in superconductivity.

We introduce quantum maps with particle-hole conversion (Andreev reflection) and particle-hole symmetry, which exhibit the same excitation gap as quantum dots in the proximity to a superconductor. Computationally, the Andreev maps are much more efficient than billiard models of quantum dots. This makes it possible to test analytical predictions of random-matrix theory and semiclassical chaos that were previously out of reach of computer simulations. We have observed the universal distribution of the excitation gap for a large Lyapunov exponent and the logarithmic reduction of the gap when the Ehrenfest time becomes comparable to the quasiparticle dwell time.

Journal Article↗
PubMed2002

Statistics of finite-time Lyapunov exponents in a random time-dependent potential.

The sensitivity of trajectories over finite-time intervals t to perturbations of the initial conditions can be associated with a finite-time Lyapunov exponent lambda, obtained from the elements M(ij) of the stability matrix M. For globally chaotic dynamics, lambda tends to a unique value (the usual Lyapunov exponent lambda(infinity)) for almost all trajectories as t is sent to infinity, but for finite t it depends on the initial conditions of the trajectory and can be considered as a statistical quantity. We compute for a particle moving in a randomly time-dependent, one-dimensional potential how the distribution function P(lambda;t) approaches the limiting distribution P(lambda; infinity)=delta(lambda-lambda(infinity)). Our method also applies to the tail of the distribution, which determines the growth rates of moments of M(ij). The results are also applicable to the problem of wave-function localization in a disordered one-dimensional potential.

Journal Article↗
PubMed2002

Shot noise in ballistic quantum dots with a mixed classical phase space.

We investigate shot noise for quantum dots whose classical phase space consists of both regular and chaotic regions. The noise is systematically suppressed below the universal value of fully chaotic systems, by an amount which varies with the positions of the leads. We analyze the dynamical origin of this effect by a novel way to incorporate diffractive impurity scattering. The dependence of the shot noise on the scattering rate shows that the suppression arises due to the deterministic nature of transport through regular regions and along short chaotic trajectories. Shot noise can be used to probe phase-space structures of quantum dots with generic classical dynamics.

Journal Article↗
PubMed2001

Transmission delay times of localized waves.

We investigate the effects of wave localization on the delay time tau (frequency sensitivity of the scattering phase shift) of a wave transmitted through a disordered waveguide. Localization results in a separation tau=chi+chi(') of the delay time into two independent but equivalent contributions, associated to the left and right end of the waveguide. For N=1 propagating modes, chi and chi(') are identical to half the reflection delay time of each end of the waveguide. In this case the distribution function P(tau) in an ensemble of random disorder can be obtained analytically. For N>1 propagating modes the distribution function can be approximated by a simple heuristic modification of the single-channel problem. We find a strong correlation between channels with long reflection delay times and the dominant transmission channel.

Journal Article↗
PubMed2001

Limits to error correction in quantum chaos.

We study the correction of errors that have accumulated in an entangled state of spins as a result of unknown local variations in the Zeeman energy ( B) and spin-spin interaction energy ( J). A nondegenerate code with error rate kappa can recover the original state with high fidelity within a time t(R) approximately Planck's over 2pikappa(1/2)/max(B,J)-independent of the number of encoded qubits. Whether the Hamiltonian is chaotic or not does not affect this time scale, but it does affect the complexity of the error-correcting code.

Journal Article↗
PubMed2001

Semiclassical singularities from bifurcating orbits.

We investigated numerically, for a generic quantum system (a kicked top), how the singular behavior of classical systems at bifurcations is reflected by their quantum counterpart. Good agreement is found with semiclassical predictions.

Journal Article↗
PubMed2001

Localization-induced coherent backscattering effect in wave dynamics.

We investigate the statistics of single-mode delay times of waves reflected from a disordered waveguide in the presence of wave localization. The distribution of delay times is qualitatively different from the distribution in the diffusive regime, and sensitive to coherent backscattering: The probability of finding small delay times is enhanced by a factor close to sqrt[2] for reflection angles near the angle of incidence. This dynamic effect of coherent backscattering disappears in the diffusive regime.

Journal Article↗
PubMed2001

Counting statistics of photons produced by electronic shot noise.

A theory is presented for the photodetection statistics of radiation produced by current fluctuations in a phase-coherent conductor. Deviations are found from the Poisson statistics that would result from a classical current. For detection in a narrow frequency interval delta omega, the photocount distribution has the negative-binomial form of blackbody radiation if e delta omega is less than the mean current I in the conductor. When electronic localization sets in, I drops below e delta omega and a different type of super-Poissonian photon statistics results.

Journal Article↗
PubMed2001

Quality of life in cirrhotics with minimal hepatic encephalopathy.

Minimal hepatic encephalopathy (HE) is a major cause of premature retirement in cirrhotics. The decision on the earning capability of a patient is usually based on clinical judgement, considering the patient's complaints and clinical findings such as nervousness and depression. In a comprehensive psychometric study we were able to show that cirrhotic patients, who are considered to be unable to earn their living, differ significantly from those who are working, in tests evaluating psychomotor function and in personality and subjective well-being scores representing nervousness, aggressiveness, depression. The latter scores are considered to represent the individual discrepancies between professional demands and cerebral performance. Since minimal HE affects psychomotor function but not verbal abilities this discrepancy exists predominantly in "blue collar workers." In accordance with this 60% of "blue collar" (in contrast to 20% of "white collar") workers of our patient group were considered unfit for work. Working ability is an essential element of quality of life in Western societies. Thus, an impairment of working capability is of major impact on quality of life in cirrhotics.

Employment↗
PubMed2000

Search for two-scale localization in disordered wires in a magnetic field

A recent paper [A. V. Kolesnikov and K. B. Efetov, Phys. Rev. Lett. 83, 3689 (1999)] predicts a two-scale behavior of wave function decay in disordered wires in the crossover regime from preserved to broken time-reversal symmetry. We have tested this prediction by a transmission approach, relying on the Borland conjecture that relates the decay length of the transmittance to the decay length of the wave functions. Our numerical simulations show no indication of two-scale behavior.

Journal Article↗
PubMed2000

Hepatic encephalopathy in liver cirrhosis: pathogenesis, diagnosis and management.

The pathogenesis of hepatic encephalopathy (HE) is unknown. Many theories have been proposed. Most established therapies are based on such theories but since no theory has have ever been proved, therapies have to be considered empiric. The spectrum of HE ranges from minimal cerebral functional deficits, which can only be found by sensitive psychometric tests, to coma with signs of decerebration. HE has arbitrarily been divided into stages. A number of precipitating factors are known and the first line of therapy should always be the elimination of these factors. The differential diagnosis includes all states of impaired consciousness and deficits in cerebral function in patients with chronic liver disease, and clinical and biochemical tests to differentiate are indicated. The therapeutic options for HE include: protein restriction only for a limited time in comatous patients; nonabsorbable antibiotics (aminoglycosides), which because of adverse effects are also limited to higher grades of HE: intestinal cleansing which is applicable in all degrees of HE; lactulose, branched chain aminoacids and ornithin aspartate which have been proven to be effective and can be applied long term in patients with lower grades of HE.

Anti-Bacterial Agents↗
PubMed1998

Neuropsychological aspects of portal-systemic encephalopathy.

An extensive psychometric test program was performed in 96 patients with proven liver cirrhosis and clinical signs of portal hypertension as well as in 20 patients with alcoholic pancreatitis, in 19 patients without cirrhosis but with alcoholic cerebral atrophy and in 163 normal controls. The study population comprised six groups of subjects as follows: Group 1. 27 patients with non-alcoholic cirrhosis and normal EEG pattern. Group 2. 48 patients with alcoholic cirrhosis and normal EEG pattern. Group 3. 21 patients with cirrhosis and minimal EEG changes. Group 4. 20 patients with alcoholic pancreatitis. Group 5. 19 patients without cirrhosis but with alcoholic cerebral atrophy. Group 6. 163 normal controls. A one way analysis of variances comparing asymptomatic patients (group 1, 2 and 4) with controls (group 6) revealed no significant differences between patients with alcoholic and non-alcoholic cirrhosis, both cirrhotic groups scoring significantly lower than patients with alcoholic pancreatitis and normal controls, who did not differ significantly. Comparing symptomatic patients (group 3 and 5) with normal controls both patient groups scored significantly lower than controls, the cirrhotic group (group 3) scoring significantly lower than patients with alcoholic cerebral atrophy. A two way analysis of variances revealed that in clinically asymptomatic patients cerebral functional defects revealed by psychometry are only due to cirrhosis and that in patients with clinical evidence of cerebral impairment the factors alcohol and cirrhosis are additive - not synergistic. A multiple group stepwise discriminant analysis revealed that tests evaluating psychomotor functions contributed most to the discrimination. Especially "line tracing " proved to be most sensitive and most specific followed by dexterity, steadiness, aiming, digit symbols in sensitivity and by reaction time, steadiness and dexterity in specificity. A test program for clinical use is proposed.

Alcohol Drinking↗