Aharonov–Bohm effect in Kane-type semiconductor quantum wire

In the present study, the Aharonov–Bohm effect for bound states in Kane-type quantum wire is analysed. It is demonstrated that the wave function and the energy spectra of carriers depend on the magnetic flux. The energy and the g factor of electrons are of a periodic function of the magnetic flux. The statistical property of the orbital magnetism of electrons at low temperatures and strong magnetic fields is presented. It is shown that the magnetisation of electron oscillations depends on the magnetic flux.     

Phase slips in Aharonov–Bohm oscillations

We give a possible mechanism for periodically occurring phaseslips in Aharonov–Bohm oscillations of mesoscopic rings.     

Investigation of the mesoscopic Aharonov–Bohm effect in low magnetic fields

We have investigated the Aharonov–Bohm effect in mesoscopic semiconductor GaAs/GaA1As rings in low magnetic fields. The oscillatory magnetoconductance of these systems is systematically studied as a function of electron density. We observe phase shifts of π in the magnetoconductance oscillations, and halving of the fundamental h/e period, as the density is varied. Theoretically, we find agreement with the experiment, by introducing an asymmetry between the two arms of the ring.     

Quantum coherence in quantum dot—Aharonov–Bohm ring hybrid systems

We investigate coherent transport through hybrid systems of quantum dots and Aharonov–Bohm (AB) rings. Strong coherence over the entire system leads to the Fano effect, which originates from the interference and the phase shift caused by the discrete states in the dots. The high controllability of the system parameters reveals that the Fano effect in mesoscopic transport can be a powerful tool for detecting the phase shift of electrons. We apply it to detect electrostatic phase modulation and the phase shift in a quantum wire with a side-coupled dot. Finally, we provide an experimental answer to the problem of “neighboring in-phase Coulomb peaks”.     

Control of Aharonov–Bohm oscillations in a AlGaAs/GaAs ring by asymmetric and symmetric gate biasing

The control of the Aharonov–Bohm effect on a AlGaAs/GaAs ring structure is studied by employing two in-plane-gates. By applying a gate voltage to one of the gates, a change of the oscillation pattern due to the additional potential induced in one branch of the ring is observed. The change of the oscillation frequency as well as the phase is attributed to the multi-channel transport. In case of a symmetric biasing, where both gates are biased simultaneously, a larger voltage is required to change the oscillation pattern than for the case when only one gate is used. This effect is explained by a partial compensation of the phase difference between both branches of the ring.     

Precession and Interference in the Aharonov–Casher and Scalar Aharonov–Bohm Effects

The ideal scalar Aharonov–Bohm (SAB) and Aharonov–Casher (AC) effect involve a magnetic dipole pointing in a certain fixed direction: along a purely time dependent magnetic field in the SAB case and perpendicular to a planar static electric field in the AC case. We extend these effects to arbitrary direction of the magnetic dipole. The precise conditions for having nondispersive precession and interference effects in these generalized set ups are delineated both classically and quantally. Under these conditions the dipole is affected by a nonvanishing torque that causes pure precession around the directions defined by the ideal set ups. It is shown that the precession angles are in the quantal case linearly related to the ideal phase differences, and that the nonideal phase differences are nonlinearly related to the ideal phase differences. It is argued that the latter nonlinearity is due to the appearance of a geometric phase associated with the nontrivial spin path. It is further demonstrated that the spatial force vanishes in all cases except in the classical treatment of the nonideal AC set up, where the occurring force has to be compensated by the experimental arrangement. Finally, for a closed space-time loop the local precession effects can be inferred from the interference pattern characterized by the nonideal phase differences and the visibilities. It is argued that this makes it natural to regard SAB and AC as essentially local and nontopological effects.     

Aharonov–Bohm oscillations observed in nanoscale open-dot structures fabricated in an InAs surface inversion layer

We fabricated nanoscale open-dot structures in an InAs surface inversion layer using an atomic-force-microscope oxidation process. Due to its superior nanofabrication capability, small open-dot structures with the feature size ranging between 100 and 300 nm were successfully fabricated. The magnetoresistance signal measured at 4.2 K showed reproducible fluctuations and a periodic oscillation component that varies in both amplitude and periodicity depending on the dot size. We show that the period of the oscillations corresponds to that of the Aharonov–Bohm effect and propose that the possible mechanism for the oscillations is due to the formation of a one-dimensional electron channel enclosing the open-dot structure as a result of the electron transfer from the InAs oxide to InAs.     

Period doubling effects in the oscillations of persistent currents in discretized Aharonov–Bohm rings

One shows that the oscillations with respect to the magnetic flux in the one-dimensional discretized Aharonov–Bohm rings are sensitive with respect to the parity of the number of electrons N_e. Resorting to basic flux intervals of unit length indicates that the period of such oscillations is given by the flux quantum Φ₀=hc/e and by 2Φ₀ if N_e is odd and even, respectively. Accordingly, one deals with a period doubling effect when passing from odd N_e-values to even ones, which may be useful in the design of further nanodevices. Interaction free limits of the Drude weight are also discussed.     

Gate voltage-dependent Aharanov–Bohm experiment in the presence of Rashba spin–orbit interaction

We measured gate voltage-dependent Aharonov–Bohm oscillations in an InGaAs-based two-dimensional electron gas ring with a gate on top of one of the branches. After ensemble averaging, the h/e oscillation spectrum showed smooth oscillatory behavior as a function of the gate voltage. This could be a manifestation of the spin–orbit interaction induced interference.     

Time‐resolved reconstruction of defect creation sequences in diode lasers

The propagation of defect networks in failed 980 nm emitting high‐power diode lasers is analyzed. This is accomplished ex post facto by electron‐beam based techniques applied without device preparation and in situ by thermographic microscopy with 1 µs time resolution. Moreover, an iterative model is established, which allows for describing both the shape of the observed defect networks as well as the kinetics of their spread. This concerted approach allows the clear assignment of starting points of extended defect systems as well as analysis of their evolution kinetics. Eventually this knowledge may help in making devices more resistive against defect creation and extension.     

First‐principles identification of defect levels in Er‐doped GaN

Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first‐principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er‐related defect complexes in GaN. We discover for the first time that Er_Ga–C_N–V_N, a defect complex of Er, a C impurity, and an N vacancy, and Er_Ga–O_N–V_N, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 eV and 0.46 eV below the conduction band, respectively. Together with Er_Ga–V_N, a complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er‐related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed.     

The stability of the fractional quantum Hall effect in topological insulators

With the recent observation of graphene-like Landau levels at the surface of topological insulators, the possibility of fractional quantum Hall effect, which is a fundamental signature of strong correlations, has become of interest. Some experiments have reported intra-Landau level structure that is suggestive of fractional quantum Hall effect. This paper discusses the feasibility of fractional quantum Hall effect from a theoretical perspective, and argues that while this effect should occur, ideally, in the n=0 and |n|=1 Landau levels, it is ruled out in higher |n| Landau levels. Unlike graphene, the fractional quantum Hall effect in topological insulators is predicted to show an interesting asymmetry between n=1 and n=−1 Landau levels due to spin-orbit coupling.     

Transmission of an X‐ray beam through a two‐dimensional photonic crystal and the Talbot effect

Results of computer simulations of the transmission of an X‐ray beam through a two‐dimensional photonic crystal as well as the propagation of an X‐ray beam in free space behind the photonic crystal are reported. The photonic crystal consists of a square lattice of silicon cylinders of diameter 0.5 µm. The amount of matter in the path of the X‐ray beam rapidly decreases at the sides of the cylinder projections. Therefore the transmission is localized near the boundaries, and appears like a channeling effect. The iterative method of computer simulations is applied. This method is similar to the multi‐slice method that is widely used in electron microscopy. It allows a solution to be obtained with acceptable accuracy. A peculiarity in the intensity distribution inside the Talbot period z_T in free space was found when the intensity is approximately equal to the initial value at a distance 0.46z_T, and it is shifted by half a period at distance 0.5z_T. The reason for this effect is the existence of a periodic phase of the wavefunction of radiation inside the intensity peaks. Simulations with zero phase do not show this effect. Symmetry rules for the Talbot effect are discussed.     

Misinterpretation yields supervelocities during transmission of wave packets through a barrier

This paper is concerned with the transmission time of an incident Gaussian wave packet through a symmetric rectangular barrier. Following Hartman (J. Appl. Phys. 33, 3427 (1962)), the transmission time is usually taken as the difference between the time at which the peak of the transmitted packet leaves the barrier of thickness and the time at which the peak of the incident Gaussian wave packet arrives at the barrier. This yields a corresponding transmission velocity which appears under certain conditions as a supervelocity, i.e. becomes larger than the corresponding propagation velocity in free space which is the group velocity for electrons or the velocity of light for photons, respectively. By analysing the propagation of a broadband wave packet (which leads in free space to an extremely concentrated wave packet at a certain time) we obtain the pulse response function of the barrier and show that the insertion of the barrier is physically unable to produce a supervelocity. Therefore, the peak of an incident Gaussian wave packet and the peak of the transmitted wave packet are in no causal relationship. The shape of the transmitted wave packet is produced from the incident wave by convolution with the pulse response of the barrier. This yields a distortion of the shape of the wave packet which includes also the observed negative time shift of the peak. We demonstrate further that the phenomenon of Hartman's supervelocities is not restricted to barriers with their exponentially decaying fields but occurs for instance also in transmission lines with an inserted LCR circuit. Received 7 January 1999 and Received in final form 22 April 1999     

X‐ray and neutron scattering on disordered nanosize clusters: a case study of lead‐zirconate‐titanate solid solutions

Defects and frequently used defect models of solids are reviewed. Signatures for identifying the disorder from x‐ray and neutron scattering data are given. To give illustrative examples how technologically important defects contribute to x‐ray and neutron scattering numerical method able to treat non‐periodical solids possessing several simultaneous defect types is given for simulating scattering in nanosize disordered clusters. The approach takes particle size, shape, and defects into account and isolates element specific signals. As a case study a statistical approximation model for lead‐zirconate titanate [Pb(Zr_xTi)O₃, PZT] is introduced. PZT is a material possessing several defect types, including substitutional, displacement and surface defects. Spatial composition variation is taken into account by introducing a model in which the edge lengths of each cell depend on the distribution of Zr and Ti ions in the cluster. Spatially varying edge lengths and angles is referred to as microstrain. The model is applied to compute the scattering from ellipsoid shaped PZT clusters and to simulate the structural changes as a function of average composition. Two‐phase co‐existence range, the so called morphotropic phase boundary composition is given correctly. The composition at which the rhombohedral and tetragonal cells are equally abundant was . Selected x‐ray and neutron Bragg reflection intensities and line shapes were simulated. Examples of the effect of size and shape of the scattering clusters on diffraction patterns are given and the particle dimensions, computed through Scherrer equation, are compared with the exact cluster dimensions. Scattering from two types of 180° domains in spherical particles, one type assigned to Ti‐rich PZT and the second to the MPB and Zr‐rich PZT, is computed. We show how the method can be used for modelling polarization reversal.

     

Quantum regime of a plasma‐wave‐pumped free‐electron laser in the presence of an axial magnetic field

The quantum regime of a plasma‐whistler‐wave‐pumped free‐electron laser (FEL) in the presence of an axial‐guide magnetic field is presented. By quantizing both the plasma whistler field and axial magnetic field, an N‐particle three‐dimensional Hamiltonian of quantum‐FEL (QFEL) has been derived. Employing Heisenberg evolution equations and introducing a new collective operator which controls the vertical motion of electrons, a quantum dispersion relation of the plasma whistler wiggler has been obtained analytically. Numerical results indicate that, by increasing the intrinsic quantum momentum spread and/or increasing the axial magnetic field strength, the bunching and the radiation fields grow exponentially. In addition, a spiking behavior of the spectrum was observed with increasing cyclotron frequency which provides an enormous improvement in the coherence of QFEL radiation even in a limit close‐to‐classical regime, where an overlapping of these spikes is observed. Also, an upper limit of the intrinsic quantum momentum spread which depends on the value of the cyclotron frequency was found.     

Luminescence of defect centers in yttrium–aluminum garnet crystals

Spectral–kinetic properties of the 2.38 eV emission in neutron-irradiated and as-received yttrium aluminum garnet (Y₃Al₅O₁₂) crystals are investigated. This luminescence band as well as its lifetime and the temperature dependence are analyzed. It is suggested that the 2.38 eV luminescence band is due to the strongly allowed transition of the F⁺-center in garnet crystals. Possible reasons that may explain the observed temperature dependence of this emission intensity are discussed.     

Scattering effect of atoms through a Bose-Einstein condensate in an optical Lattice with a single defect

We study the transport of atoms across a localized Bose-Einstein condensate in an onedimensional optical lattice with a single defect. Our analytical and numerical results show that the defect as well as the nonlinear parameter can control the transmission of the atoms beam and the position of total reflection caused by Fano resonance. These interesting features may be a very useful basis for devising tunable atom filters or a button.