Spin-polarized Quantum Transport in Mesoscopic Conductors: Computational Concepts and Physical Phenomena

Mesoscopic conductors are electronic systems of sizes in between nano- and micrometers, and often of reduced dimensionality. In the phase-coherent regime at low temperatures, the conductance of these devices is governed by quantum interference effects, such as the Aharonov-Bohm effect and conductance fluctuations as prominent examples. While first measurements of quantum charge transport date back to the 1980s, spin phenomena in mesoscopic transport have moved only recently into the focus of attention, as one branch of the field of spintronics. The interplay between quantum coherence with confinement-, disorder- or interaction-effects gives rise to a variety of unexpected spin phenomena in mesoscopic conductors and allows moreover to control and engineer the spin of the charge carriers: spin interference is often the basis for spin-valves, -filters, -switches or -pumps. Their underlying mechanisms may gain relevance on the way to possible future semiconductor-based spin devices. A quantitative theoretical understanding of spin-dependent mesoscopic transport calls for developing efficient and flexible numerical algorithms, including matrix-reordering techniques within Green function approaches, which we will explain, review and employ.Comment: To appear in the Encyclopedia of Complexity and System Scienc

Optimal block-tridiagonalization of matrices for coherent charge transport

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms requires the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrary complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green's function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.Comment: 28 pages, 14 figures; submitted to Journal of Computational Physic

Spin currents in rough graphene nanoribbons: Universal fluctuations and spin injection

We investigate spin conductance in zigzag graphene nanoribbons and propose a spin injection mechanism based only on graphitic nanostructures. We find that nanoribbons with atomically straight, symmetric edges show zero spin conductance, but nonzero spin Hall conductance. Only nanoribbons with asymmetrically shaped edges give rise to a finite spin conductance and can be used for spin injection into graphene. Furthermore, nanoribbons with rough edges exhibit mesoscopic spin conductance fluctuations with a universal value of $\mathrm{rms} G_\mathrm{s}\approx 0.4 e/4\pi$.Comment: 4 pages, 5 figures, PdfLaTeX, accepted for publication in Physical Review Letter

Weak localization in mesoscopic hole transport: Berry phases and classical correlations

We consider phase-coherent transport through ballistic and diffusive two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show that intrinsic heavy-hole light-hole coupling gives rise to clear-cut signatures of an associated Berry phase in the weak localization which renders the magneto-conductance profile distinctly different from electron transport. Non-universal classical correlations determine the strength of these Berry phase effects and the effective symmetry class, leading even to antilocalization-type features for circular quantum dots and Aharonov-Bohm rings in the absence of additional spin-orbit interaction. Our semiclassical predictions are quantitatively confirmed by numerical transport calculations

Anwendungsmodalitäten der sakralen Neuromodulation im deutschsprachigen Raum im Jahr 2014

Anwendungsmodalitäten der sakralen Neuromodulation im deutschsprachigen Raum im Jahr 2014 Abstrakt Einführung: Die sakrale Neuromodulation ist ein minimalinvasives operatives Verfahren, das als zentrale Zweitlinien-Therapie in der Behandlung der überaktiven Harnblase mit und ohne Urinverlust, der nicht obstruktiven Harnretention sowie der Stuhlinkontinenz von Urologen Chirurgen und (Uro-)Gynäkologen erfolgreich eingesetzt wird. Trotz deutlich häufigerer Anwendung des Verfahrens, ist keine in gleichem Ausmaß verlaufende Zunahme an standardisierter Vorgehensweise zu sehen. Ziel der Arbeit war es, anhand einer Umfrage die SNM-Behandlungsstrategien im deutschsprachigen Raum zu erfassen und Gemeinsamkeiten und Unterschiede der einzelnen Zentren beziehungsweise der anwendenden Fachgebiete aufzuzeigen, unterschiedliche Strategien zu erkennen und daraus neue Ziele und Fragestellungen zu eruieren. Methoden: Sämtliche für das Verfahren relevante Themenbereiche wurden in einem 30 Fragen umfassenden Fragebogen formuliert und mittels eines Online Umfrageportals an Zentren, die die SNM im deutschsprachigen Raum als Therapiemethode regelmäßig anwenden, versandt. Nach Erhalt der Antworten wurden diese statistisch ausgewertet und kritisch mit dem aktuellen Wissensstand verglichen. Ergebnisse: Von 432 angefragten Einrichtungen in Deutschland Österreich und der Schweiz erhielten wir 83 Antworten. Wie vermutet, zeigte sich in der Untersuchung, dass in vielen Teilbereichen des Verfahrens eine signifikante Variabilität in der Anwendung zu beobachten ist. In Verfahrensabschnitten, in denen aufgrund einer ausreichenden Datenlage klare Empfehlungen vorliegen, werden diese auch vom überwiegenden Teil der Befragten vollständig in ihrer Handlungsweise berücksichtigt. (z.B. Hauptindikationen) Bereiche, für die eine geringere oder keine Evidenz vorliegt, führen nicht nur unter den verschiedenen Fachdisziplinen, sondern auch innerhalb einer Fachrichtung zu einer sehr inhomogenen Vorgehensweise. (unter anderem erweiterte Indikationen, uni-versus bilaterale Stimulation, Antibiose, Anwendung von PNE oder Tined Lead Elektroden) Schlußfolgerung: Für eine Optimierung der klinischen Resultate stellt sich neben der Durchführung weiterer Untersuchungen auch die Forderung nach einem zentralen Datenregister

Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations

We study the symmetry classes of graphene quantum dots, both open and closed, through the conductance and energy level statistics. For abrupt termination of the lattice, these properties are well described by the standard orthogonal and unitary ensembles. However, for smooth mass confinement, special time-reversal symmetries associated with the sublattice and valley degrees of freedom are critical: they lead to block diagonal Hamiltonians and scattering matrices with blocks belonging to the unitary symmetry class even at zero magnetic field. While the effect of this structure is clearly seen in the conductance of open dots, it is suppressed in the spectral statistics of closed dots, because the intervalley scattering time is shorter than the time required to resolve a level spacing in the closed systems but longer than the escape time of the open systems.Comment: 4 pages, 4 figures, RevTex, submitted to Phys. Rev. Let

Objective evaluation of intracochlear electrocochleography: repeatability, thresholds, and tonotopic patterns.

INTRODUCTION Intracochlear electrocochleography (ECochG) is increasingly being used to measure residual inner ear function in cochlear implant (CI) recipients. ECochG signals reflect the state of the inner ear and can be measured during implantation and post-operatively. The aim of our study was to apply an objective deep learning (DL)-based algorithm to assess the reproducibility of longitudinally recorded ECochG signals, compare them with audiometric hearing thresholds, and identify signal patterns and tonotopic behavior. METHODS We used a previously published objective DL-based algorithm to evaluate post-operative intracochlear ECochG signals collected from 21 ears. The same measurement protocol was repeated three times over 3 months. Additionally, we measured the pure-tone thresholds and subjective loudness estimates for correlation with the objectively detected ECochG signals. Recordings were made on at least four electrodes at three intensity levels. We extracted the electrode positions from computed tomography (CT) scans and used this information to evaluate the tonotopic characteristics of the ECochG responses. RESULTS The objectively detected ECochG signals exhibited substantial repeatability over a 3-month period (bias-adjusted kappa, 0.68; accuracy 83.8%). Additionally, we observed a moderate-to-strong dependence of the ECochG thresholds on audiometric and subjective hearing levels. Using radiographically determined tonotopic measurement positions, we observed a tendency for tonotopic allocation with a large variance. Furthermore, maximum ECochG amplitudes exhibited a substantial basal shift. Regarding maximal amplitude patterns, most subjects exhibited a flat pattern with amplitudes evenly distributed over the electrode carrier. At higher stimulation frequencies, we observed a shift in the maximum amplitudes toward the basal turn of the cochlea. CONCLUSIONS We successfully implemented an objective DL-based algorithm for evaluating post-operative intracochlear ECochG recordings. We can only evaluate and compare ECochG recordings systematically and independently from experts with an objective analysis. Our results help to identify signal patterns and create a better understanding of the inner ear function with the electrode in place. In the next step, the algorithm can be applied to intra-operative measurements

Dynamics of Orientation Tuning in Cat V1 Neurons Depend on Location Within Layers and Orientation Maps

Analysis of the timecourse of the orientation tuning of responses in primary visual cortex (V1) can provide insight into the circuitry underlying tuning. Several studies have examined the temporal evolution of orientation selectivity in V1 neurons, but there is no consensus regarding the stability of orientation tuning properties over the timecourse of the response. We have used reverse-correlation analysis of the responses to dynamic grating stimuli to re-examine this issue in cat V1 neurons. We find that the preferred orientation and tuning curve shape are stable in the majority of neurons; however, more than forty percent of cells show a significant change in either preferred orientation or tuning width between early and late portions of the response. To examine the influence of the local cortical circuit connectivity, we analyzed the timecourse of responses as a function of receptive field type, laminar position, and orientation map position. Simple cells are more selective, and reach peak selectivity earlier, than complex cells. There are pronounced laminar differences in the timing of responses: middle layer cells respond faster, deep layer cells have prolonged response decay, and superficial cells are intermediate in timing. The average timing of neurons near and far from pinwheel centers is similar, but there is more variability in the timecourse of responses near pinwheel centers. This result was reproduced in an established network model of V1 operating in a regime of balanced excitatory and inhibitory recurrent connections, confirming previous results. Thus, response dynamics of cortical neurons reflect circuitry based on both vertical and horizontal location within cortical networks

The operating regime of local computations in primary visual cortex

[Abstract] In V1, local circuitry depends on the position in the orientation map: close to pinwheel centers, recurrent inputs show variable orientation preferences; within iso-orientation domains, inputs are relatively uniformly tuned. Physiological properties such as cell's membrane potentials, spike outputs, and temporal characteristics change systematically with map location. We investigate in a firing rate and a Hodgkin–Huxley network model what constraints these tuning characteristics of V1 neurons impose on the cortical operating regime. Systematically varying the strength of both recurrent excitation and inhibition, we test a wide range of model classes and find the likely models to account for the experimental observations. We show that recent intracellular and extracellular recordings from cat V1 provide the strongest evidence for a regime where excitatory and inhibitory recurrent inputs are balanced and dominate the feed-forward input. Our results are robust against changes in model assumptions such as spatial extent and strength of lateral inhibition. Intriguingly, the most likely recurrent regime is in a region of parameter space where small changes have large effects on the network dynamics, and it is close to a regime of “runaway excitation,” where the network shows strong self-sustained activity. This could make the cortical response particularly sensitive to modulation