Due to the lack of simulation tools that take into account the actual geometry of complicated quantum Hall samples there are lots of experiments that are not yet fully understood. Already some years ago R. G. Mani recorded a shift of the Hall resistance transitions to lower magnetic fields in samples of a Hall bar with embedded anti-Hall bar by using partial gating. We use a Nonequilibrium Network Model (NNM) to simulate this geometry and find qualitative agreement. Fitting the simulated resistance curves to the experimental results we can not only determine the carrier concentration but also obtain an estimate of the screened gating potential and especially the amplitude and lengthscale of potential fluctuations from charge inhomogenities which are not easily accessible by experiment.

We present a first approach to magneto transport of a Hall bar in non-uniform magnetic fields, which is based on a novel non-equilibrium network model. Such non-uniform fields generate magnetic barriers for current flow, which can be observed by a change of the longitudinal resistance R-xx that becomes a function of orientation and field amplitude of the superimposed magnetic field. For the case of a hybrid electric and magnetic confinement we find a significant suppression of equilibration between edge and bulk, resulting in a suppression of the longitudinal resistance.

We developed a non-equilibrium network model for magneto transport in 2D electronic systems in the quantum Hall effect regime, which is able to capture the real sample geometry, including contacts, leads and in-homogeneities like introduced by gate electrodes. In this paper we investigate non-local effects in magneto transport for a number of contact configurations of a quantum Hall system, including the effect of having unused metallic contacts between the active region of classical current flow and the remote region where the non-local signals are obtained.

A computational investigation of HCN → HNC isomerization in the electronic ground state by one- and few-cycle infrared pulses is presented. Starting from a vibrationally pre-excited reagent state, isomerization yields of more than 50% are obtained using single one- to five-cycle pulses. The principal mechanism includes two steps of population transfer by dipole-resonance (DR), and hence, the success of the method is closely linked to the polarity of the system and, in particular, the stepwise change of the dipole moment from reactant to transition state and on to products. The yield drops massively if the diagonal dipole matrix elements are artificially set to zero. In detail, the mechanism includes DR-induced preparation of a delocalized vibrational wavepacket, which traverses the barrier region and is finally trapped in the product well by DR-dominated de-excitation. The excitation and de-excitation steps are triggered by pulse lobes of opposite field direction. As the number of optical cycles is increased, the leading field lobes prepare a vibrational superposition state by off-resonant ladder climbing, which is then subjected to the three steps of the principal isomerization mechanism. DR excitation is more efficient from a preformed vibrational wavepacket than from a molecular eigenstate. The entire process can be loosely described as Tannor-Kosloff-Rice type transfer mechanism on a single potential surface effected by a single pulse, individual field lobes assuming the roles of pump- and dump-pulses. Pre-excitation to a transient wavepacket can be enhanced by applying a separate, comparatively weak few-cycle prepulse, in which the prepulse prepares a vibrational wavepacket. The two-pulse setup corresponds to a double Tannor-Kosloff-Rice control scheme on a single potential surface.

We investigate theoretically population dynamics in an N-level system for pulses of arbitrary shape. In order to use Floquet theory the pulse only has to have finite support in time, which is well fulfilled in experiments. Furthermore the dynamics must be approximately time-local, which restricts to negligible memory effects. Despite the few requirements we are able to derive criteria for resonances, which means that all population rests in a preselected state after the pulse has been applied. We show that the necessary criterion on the Floquet quasienergies is a generalization of the pi-pulse criterion, derived within the Rotating Wave Approximation and adiabatic aproximation for the envelope by Holthaus et. al., to pulses of arbitrary shape. Furthermore we find that only the eigenvectors contain enough information to give a sufficient criterion for a resonance. We constuct the resonant N-level propagator and show that it is block diagonal, leading to a description by an effective 2-level-system composed of the initial and final state.

The plateau transitions of the integer quantum Hall effect (QHE) are accompanied by dissipative bulk current flow, which causes the corresponding peaks in the longitudinal magneto-resistance Rxx. On the basis of a non-equilibrium network model for magneto-transport, we propose an experimental setup for suppressing the dissipation in the plateau transition regime. This is achieved by an appropriate biasing of a narrow stripe shaped gate electrode, which is aligned to the longitudinal direction of the sample and needs to cover only a vanishing small part (few percent) of the bulk region. Our calculations demonstrate, that the reduction of the dissipation is accompanied by a drastic narrowing of the plateau transitions. Besides the practical aspect in context with metrology, such an investigation provides additional insight into the role of quantum phase transitions and phase coherence for the appearance of the QHE.

On the basis of a non-equilibrium network model for magneto transport we propose an approach for creating a quite narrow incompressible stripe far from the sample edges without creating edge channels (EC) at it's boundaries. This is achieved by an appropriate biasing of a narrow stripe like gate electrode, which is aligned to the longitudinal direction of the sample and covers only a vanishing small part (few percent) of the bulk region. By adjusting the magnetic field in order to tune the bulk region to the vicinity of a plateau transition, the bulk gets resistive (dissipative). In this way we create 3 distinguishable possibilities for current flow: (i) the original ECs at the real edge of the sample; (ii) the dissipative bulk region which covers nearly all of the sample area and (iii) the narrow incompressible stripe in the middle of the bulk region. Our calculations clearly demonstrate, that in this regime all the sample current appears as a dissipation-less current in the narrow incompressible stripe, while the edge channels as well as the bulk region loose their importance.

In the present paper we investigate the influence of the contact region on the distribution of the chemical potential in integer quantum Hall samples as well as the longitudinal and Hall resistance as a function of the magnetic field. First we use a standard quantum Hall sample geometry and analyze the influence of the length of the leads where current enters/leaves the sample and the ratio of the contact width to the width of these leads. Furthermore we investigate potential barriers in the current-injecting leads and the measurement arms in order to simulate nonideal contacts. Second we simulate nonlocal quantum Hall samples with applied gating voltage at the metallic contacts. For such samples it has been found experimentally that both the longitudinal and Hall resistance as a function of the magnetic field can change significantly. Using the nonequilibrium network model we are able to reproduce most qualitative features of the experiments.

We investigate population dynamics in N-level systems driven beyond the linear regime by a strong external field, which couples to the system through an operator with nonzero diagonal elements. As concrete example we consider the case of dipolar molecular systems. We identify limiting cases of the Hamiltonian leading to wave functions that can be written in terms of ordinary exponentials, and focus on the limits of slowly and rapidly varying fields of arbitrary strength. For rapidly varying fields we prove for arbitrary N that the population dynamics is independent of the sign of the projection of the field onto the dipole coupling. In the opposite limit of slowly varying fields the population of the target level is optimized by a dipole resonance condition. As a result population transfer is maximized for one sign of the field and suppressed for the other one, so that a switch based on flopping the field polarization can be devised. For significant sign dependence the resonance linewidth with respect to the field strength is small. In the intermediate regime of moderate field variation, the integral of lowest order in the coupling can be rewritten as a sum of terms resembling the two limiting cases, plus correction terms for N > 2, so that a less pronounced sign-dependence still exists.

The authors introduce and describe pulse train control (PTC) of population branching in strongly coupled processes as a novel control tool for the separation of competing multiphoton processes. Control strategies are presented based on the different responses of processes with different photonicities and/or different frequency detunings to the pulse-to-pulse time delay and the pulse-to-pulse phase shift in pulse trains. The control efficiency is further enhanced by the property of pulse trains that complete population transfer can be obtained over an extended frequency range that replaces the resonance frequency of simple pulses. The possibility to freely tune the frequency assists the separation of the competing processes and reduces the number of subpulses required for full control. As a sample application, PTC of leaking multiphoton resonances is demonstrated by numerical simulations. In model systems exhibiting sizable background (intruder) state population if excited with single pulses, PTC leading to complete accumulation of population in the target state and elimination of background population is readily achieved. The analysis of the results reveals different mechanisms of control and provides clues on the mechanisms of the leaking process itself. In an alternative setup, pulse trains can be used as a phase-sensitive tool for level switching. By changing only the pulse-to-pulse phase shift of a train with otherwise unchanged parameters, population can be transferred to any of two different target states in a near-quantitative manner.

The properties of pulse-train induced multiphoton excitation in anharmonic progressions and the accumulation of population in a specific rung state are investigated by means of numerical simulations. It is shown how and under which conditions resonant π-pulses and multiple-π pulses can be split into trains of fractional π-pulses driving the same transition. Standardized train forms are considered with sub-pulses of equal (gaussian) shapes and equal, but tunable pulse-to-pulse delays and pulse-to-pulse phase shifts. The increased number of tuning parameters together with the handle on the number of sub-pulses gives rise to a remarkable variability in the control of state-specific population transfer, where simple zero-order estimates assist the determination of the parameters. Each π- or multiple-π pulse is replaced by a resonance locus in parameter space representing an infinite set of π-trains. The loci span extended frequency ranges that increase with increasing sub-pulse number. Their projection onto the frequency-field strength plane gives rise to elliptically shaped closed curves, termed resonance ridges, which replace the singular points mapped out by simple π- and multiple-π pulses. In the subspace of pulse-to-pulse delays and pulse-to-pulse phase shifts the resonance loci are characterized by phase recurrence relations, whose number and complexity increases with increasing numbers of sub-pulses. Our results indicate that pulse trains may be a powerful tool for the control of parallel or branching multiphoton transitions and for the elimination of background and intruder state population.

Single pulse dipole mediated tunnelling at frequencies tuned approximately to the vicinity of the direct 1-photon and multiphoton tunnel transitions is explored as a mechanism of population transfer across asymmetric dipolar 3-level lambda- (Λ-) or N-level extended Λ-systems. These systems may be purely sequentially coupled, with zero dipole matrix coupling elements for non-neighbouring states along the legs of the Λ. In simulations for model systems with typical molecular parameters, dipole mediated tunnelling is found to be a highly efficient strong-field mechanism of population transfer with remarkable robustness properties, in particular so for higher-order multiphoton processes.

The counterdiabatic principle [M. Demirplak and S. A. Rice, J. Phys. Chem. A 107, 9937 (2003)] is used in a pragmatic way to formulate a practical control strategy for perturbed population transfer. Interpreting the appearance of population in undesirable intruder or background states as phenomenological consequences of diabatic perturbations, such branching is suppressed as soon as it arises. By invoking a penalty term that is sensitive to any transitional population in undesirable levels, a correction field is created which effectively prevents diabatic behavior. This strategy is applied to the control of background state population in multiphoton excitations. For a model five-level system we show that leaking of a resonant three-photon transition to a background state can readily be suppressed by simple correction fields obtained from our intermediate-branching driven implementation of counterdiabatic control.

Using 550 previously calculated vibrational energy levels and dipole moments we performed simulations of the HCN-->HNC isomerization dynamics induced by sub-one-cycle and few-cycle IR pulses, which we represent as Gaussian pulses with 0.25-2 optical cycles in the pulse width. Starting from vibrationally pre-excited states, isomerization probabilities of up to 50% are obtained for optimized pulses. With decreasing number of optical cycles a strong dependence on the carrier-envelope phase (CEP) emerges. Although the optimized pulse parameters change significantly with the number of optical cycles, the distortion by the Gaussian envelope produces nearly equal fields, with a positive lobe followed by a negative one. The positions and areas of the lobes are also almost unchanged, irrespective of the number of cycles in the half-width. Isomerization proceeds via a pump-dumplike mechanism induced by the sequential lobes. The first lobe prepares a wave packet incorporating many delocalized states above the barrier. It is the motion of this wave packet across the barrier, which determines the timing of the pump and dump lobes. The role of the pulse parameters, and in particular of the CEP, is to produce the correct lobe sequence, size and timing within a continuous pulse.

We investigate population transfer across the barrier in a double-well potential, induced by a pair of time-delayed single-lobe half-cycle pulses. We apply this setup both to a one-dimensional (1D) quartic model potential and to a three-dimensional potential representing HCN-->HNC isomerization. Overall the results for the two systems are similar, although in the 3D system some additional features appear not seen in the 1D case. The generic mechanism of population transfer is the preparation by the pump pulse of a wave packet involving delocalized states above the barrier, followed by the essentially 1D motion of the delocalized part of wave packet across the barrier, and the eventual de-excitation by the dump pulse to localized states in the other well. The correct timing is given by the well-to-well passage time of the wave packet and its recurrence properties, and by the signs of the field lobes which determine the direction and acceleration or deceleration of the wave packet motion. In the 3D system an additional pump-pump-dump mechanism linked to wave packet motion in the reagent well can mediate isomerization. Since the transfer time and the pulse durations are of the same order of magnitude, there is also a marked dependence of the dynamics and the transfer yield on the pulse duration. Our analysis also sheds light on the pronounced carrier envelope phase dependence previously observed for isomerization and molecular dissociation with one-cycle and sub-one-cycle pulses.

The magnetic anisotropy of epitaxial Co-N/Cu(111),1less than or equal toNless than or equal to7, films is investigated in terms of the relativistic spin-polarized screened Korringa-Kohn-Rostoker method by taking into account uniform relaxations of the Co interlayer distance between -4% and +3% with respect to the Cu parent lattice. While the spin-orbit coupling induced (band energy) part of the magnetic anisotropy is found to favor a perpendicular magnetization for Ngreater than or equal to2, because of the dominating contribution of the magnetic dipole-dipole interaction to the magnetic anisotropy energy, an in-plane magnetization is energetically preferred for essentially all relaxations and layer thicknesses. Only for N=2, 3 the anisotropy between an in-plane and a perpendicular orientation of the magnetization is not significantly different. The theoretical results are in good agreement with recent experiments based on pulsed layer deposition.

We present a nonequilibrium band-structure calculation of a magnetic tunnel junction using the ab initio screened Korringa–Kohn–Rostoker multiple scattering formalism together with the local spin density approximation. We find that the current is determined by the surface density of states (DOS) and the bias dependence of the barrier in our Fe(100)/vacuum(30 Å)/Fe(100) model junction. The existence of quasilocalized states at the interfaces leads to the effect of negative differential resistance at low bias. For high bias the surface DOS yields an enhanced spin polarization of the current. Therefore, the magnetoresistance ratio becomes strongly quenched since the current is dominated by the minority channel for both the parallel and the antiparallel configuration of the magnetic electrodes.

When an electron tunnels from a metal into the barrier in a magnetic tunnel junction it has to cross the interface. Deep in the metal the eigenstates for the electron can be labelled by the point symmetry group of the bulk but around the interface this symmetry is reduced and one has to use linear combinations of the bulk states to form the eigenstates labelled by the irreducible representations of the point symmetry group of the interface. In this way there can be states localized at the interface which control tunneling. The conclusions as to which are the dominant tunneling states are different from that conventionally found.