arXiv subjects

Stefan Rotter

Publications and source records attributed to Stefan Rotter.

133 records · Page 8Linked to original sources

arXiv2008

Decreasing excitation gap in Andreev billiards by disorder scattering

We investigate the distribution of the lowest-lying energy states in a disordered Andreev billiard by solving the Bogoliubov-de Gennes equation numerically. Contrary to conventional predictions we find a decrease rather than an increase of the excitation gap relative to its clean ballistic limit. We relate this finding to the eigenvalue spectrum of the Wigner-Smith time delay matrix between successive Andreev reflections. We show that the longest rather than the mean time delay determines the size of the excitation gap. With increasing disorder strength the values of the longest delay times increase, thereby, in turn, reducing the excitation gap.

cond-mat.mes-hall↗

arXiv2008

Interacting Quantum Dot Coupled to a Kondo Spin: A Universal Hamiltonian Study

We study a mesoscopic interacting quantum dot described by the "universal Hamiltonian" that is coupled to a Kondo spin. The ferromagnetic exchange interaction within the dot leads to a stepwise increase of the ground state spin; this Stoner staircase is modified non-trivially by the Kondo interaction. We find that the spin-transition steps move to lower values of the exchange coupling for weak Kondo interaction, but shift back up for sufficiently strong Kondo coupling. The problem is solved numerically by diagonalizing the system Hamiltonian in a customized good-spin basis and analytically in the weak and strong Kondo coupling limits. The interplay of Kondo and ferromagnetic exchange can be probed with experimentally tunable parameters.

cond-mat.mes-hall↗

arXiv2007

Decomposition of neuronal assembly activity via empirical de-Poissonization

Consider a compound Poisson process with jump measure $ν$ supported by finitely many positive integers. We propose a method for estimating $ν$ from a single, equidistantly sampled trajectory and develop associated statistical procedures. The problem is motivated by the question whether nerve cells in the brain exhibit higher-order interactions in their firing patterns. According to the neuronal assembly hypothesis (Hebb [13]), synchronization of action potentials across neurons of different groups is considered a signature of assembly activity, but it was found notoriously difficult to demonstrate it in recordings of neuronal activity. Our approach based on a compound Poisson model allows to detect the presence of joint spike events of any order using only population spike count samples, thus bypassing both the ``curse of dimensionality'' and the need to isolate single-neuron spike trains in population signals.

math.ST↗

arXiv2007

Statistics of Transmission Eigenvalues in Two-Dimensional Quantum Cavities: Ballistic versus Stochastic Scattering

We investigate the statistical distribution of transmission eigenvalues in phase-coherent transport through quantum dots. In two-dimensional ab-initio simulations for both clean and disordered two-dimensional cavities, we find markedly different quantum-to-classical crossover scenarios for these two cases. In particular, we observe the emergence of ``noiseless scattering states'' in clean cavities, irrespective of sharp-edged entrance and exit lead mouths. We find the onset of these ''classical'' states to be largely independent of the cavity's classical chaoticity, but very sensitive with respect to bulk disorder. Our results suggest that for weakly disordered cavities the transmission eigenvalue distribution is determined both by scattering at the disorder potential and the cavity walls. To properly account for this intermediate parameter regime we introduce a hybrid crossover scheme which combines previous models that are valid in the ballisic and the stochastic limit, respectively.

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↗

arXiv2005

Comment on "Dynamic range of nanotube- and nanowire-based electromechanical systems"

We investigate the role of quantum effects (e.g. zero-point energy fluctuations) in the physics of nanotube- and nanowire-based electromechanical sensors as discussed in a recent article [Postma et al., Appl. Phys. Lett. 86, 223105 (2005)]. Employing the quantum fluctuation-dissipation theorem we find that these effects pose additional limits on the dynamic range of nanomechanical resonators.

cond-mat.mtrl-sci↗

arXiv2005

Simulation of Electron Transport through a Quantum Dot with Soft Walls

We numerically investigate classical and quantum transport through a soft-wall cavity with mixed dynamics. Remarkable differences to hard-wall quantum dots are found which are, in part, related to the influence of the hierarchical structure of classical phase space on features of quantum scattering through the device. We find narrow isolated transmission resonances which display asymmetric Fano line shapes. The dependence of the resonance parameters on the lead mode numbers and on the properties of scattering eigenstates are analyzed. Their interpretation is aided by a remarkably close classical-quantum correspondence. We also searched for fractal conductance fluctuations. For the range of wave numbers $k_F$ accessible by our simulation we can rule out their existence.

cond-mat.mes-hall↗