arXiv subjects

Victor A. Gopar

Publications and source records attributed to Victor A. Gopar.

27 records · Page 2Linked to original sources

arXiv2008

Statistics of orbital entanglement production in quantum-chaotic dots

The production of orbitally entangled electrons in quantum-chaotic dots is investigated from a statistical point of view. The degree of entanglement is quantified through the concurrence and the entanglement of formation. We calculate the complete statistical distributions of the entanglement measures by using random matrix theory. Simple analytical expressions are provided for the concurrence distributions. We identify clear signatures of time-reversal invariance in the production of entanglement at the level of the entanglement-measure distributions, such as the ability of producing maximally entangled (Bell) states, which passed unnoticed in previous works where only the first two moments of the distributions were studied.

cond-mat.mes-hall↗

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↗

arXiv2006

Statistical study of the conductance and shot noise in open quantum-chaotic cavities: Contribution from whispering gallery modes

In the past, a maximum-entropy model was introduced and applied to the study of statistical scattering by chaotic cavities, when short paths may play an important role in the scattering process. In particular, the validity of the model was investigated in relation with the statistical properties of the conductance in open chaotic cavities. In this article we investigate further the validity of the maximum-entropy model, by comparing the theoretical predictions with the results of computer simulations, in which the Schroedinger equation is solved numerically inside the cavity for one and two open channels in the leads; we analyze, in addition to the conductance, the zero-frequency limit of the shot-noise power spectrum. We also obtain theoretical results for the ensemble average of this last quantity, for the orthogonal and unitary cases of the circular ensemble and an arbitrary number of channels. Generally speaking, the agreement between theory and numerics is good. In some of the cavities that we study, short paths consist of whispering gallery modes, which were excluded in previous studies. These cavities turn out to be all the more interesting, as it is in relation with them that we found certain systematic discrepancies in the comparison with theory. We give evidence that it is the lack of stationarity inside the energy interval that is analyzed, and hence the lack of ergodicity that gives rise to the discrepancies. Indeed, the agreement between theory and numerical simulations is improved when the energy interval is reduced to a point and the statistics is then collected over an ensemble. It thus appears that the maximum-entropy model is valid beyond the domain where it was originally derived. An understanding of this situation is still lacking at the present moment.

cond-mat.mes-hall↗

arXiv2005

Effect of temperature and bias voltage on the conductance distribution of disordered 1d quantum wires

The statistical properties of the conductance of one dimensional disordered systems are studied at finite bias voltage V and temperature T, in an independent-electron picture. We calculate the complete distribution of the conductance P(G) in different regimes of V, T within a statistical model of resonant tunneling transmission. We find that P(G) changes from the well-known log-normal distribution at T=0 in the linear response regime to a Gaussian distribution at large V, T. The dependence on T and V of average quantities such as < G >, < ln G > is analyzed as well. Our analytical results are confirmed by numerical simulations. We also discuss the limits of validity of the model and conclude that the effects of finite T, V presented here should be observable.

cond-mat.mes-hall↗

arXiv2003

Electronic transport through domain walls in ferromagnetic nanowires: Co-existence of adiabatic and non-adiabatic spin dynamics

We study the effect of a domain wall on the electronic transport in ferromagnetic quantum wires. Due to the transverse confinement, conduction channels arise. In the presence of a domain wall, spin up and spin down electrons in these channels become coupled. For very short domain walls or at high longitudinal kinetic energy, this coupling is weak, leads to very few spin flips, and a perturbative treatment is possible. For very long domain wall structures, the spin follows adiabatically the local magnetization orientation, suppressing the effect of the domain wall on the total transmission, but reversing the spin of the electrons. In the intermediate regime, we numerically investigate the spin-dependent transport behavior for different shapes of the domain wall. We find that the knowledge of the precise shape of the domain wall is not crucial for determining the qualitative behavior. For parameters appropriate for experiments, electrons with low longitudinal energy are transmitted adiabatically while the electrons at high longitudinal energy are essentially unaffected by the domain wall. Taking this co-existence of different regimes into account is important for the understanding of recent experiments.

cond-mat.mes-hall↗

arXiv2002

Conductance distribution in disordered quantum wires: Crossover between the metallic and insulating regimes

We calculate the distribution of the conductance P(g) for a quasi-one-dimensional system in the metal to insulator crossover regime, based on a recent analytical method valid for all strengths of disorder. We show the evolution of P(g) as a function of the disorder parameter from a insulator to a metal. Our results agree with numerical studies reported on this problem, and with analytical results for the average and variance of g.

cond-mat.mes-hall↗

arXiv2002

Generalization of the DMPK equation beyond quasi one dimension

Electronic transport properties in a disordered quantum wire are very well described by the Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the evolution of the transmission eigenvalues as a function of the length of a multichannel conductor. However, the DMPK equation is restricted to quasi one dimensional systems only. We derive a generalized DMPK equation for higher dimensions, containing dependence on the dimensionality through the properties of the transmission eigenvectors, by making certain statistical assumptions about the transfer matrix. An earlier phenomenological generalization is obtained as a special case.

cond-mat.mes-hall↗

arXiv1997

Mesoscopic Capacitors: A Statistical Analysis

The capacitance of mesoscopic samples depends on their geometry and physical properties, described in terms of characteristic times scales. The resulting ac admittance shows sample to sample fluctuations. Their distribution is studied here -through a random-matrix model- for a chaotic cavity capacitively coupled to a backgate: it is observed from the distribution of scattering time delays for the cavity, which is found analytically for the orthogonal, unitary, and symplectic universality classes, one mode in the lead connecting the cavity to the reservoir and no direct scattering. The results agree with numerical simulations.

cond-mat↗

arXiv1997

The problem of quantum chaotic scattering with direct processes reduced to the one without

We show that the study of the statistical properties of the scattering matrix S for quantum chaotic scattering in the presence of direct processes (charaterized by a nonzero average S matrix ) can be reduced to the simpler case where direct processes are absent ( = 0). Our result is verified with a numerical simulation of the two-energy autocorrelation for two-dimensional S matrices. It is also used to extend Wigner's time delay distribution for one-dimensional S matrices, recently found for = 0, to the case not equal to zero; this extension is verified numerically. As a consequence of our result, future calculations can be restricted to the simpler case of no direct processes.

chao-dyn↗