Persistent Currents in the Double Aharonov-Bohm Ring Connected to Electron Reservoirs

We study persistent currents in the double Aharonov-Bohm ring connected to two electron reservoirs by quantum waveguide theory. It is found that the persistent currents in the double Aharonov-Bohm ring depend on the direction of the current flow from one reservoir to another. When the direction of the current flow reverses, the persistent current in each ring of the double Aharonov-Bohm ring changes. If the two rings are of the same size, the persistent currents in the left and the right rings exchange at the reversal of the current flow direction.     

Neutral and charged excitons in a quantum tube

The optical transition energies of neutral and charged excitons in a quantum tube are calculated as a function of the Aharonov-Bohm magnetic flux Φ. The oscillation amplitude of the ground state energy of the electron-hole relative motion is shown to be larger in a quantum tube than a quantum ring with strong confinement in the axis direction. We find a double maxima structure in the optical transition energy for a quantum tube with radius R = 0.5 in units of the effective Bohr radius because of the difference in the Φ dependencies between the single electron energy and the relative-motion energy of a charged exciton state.     

Influence of topology in a quantum ring

In this Letter we study the quantum rings in the presence of a topological defect. We use geometric theory of defects to describe one and two-dimensional quantum rings in the presence of a single screw dislocation. In addition we consider some potential in a two dimensional ring and calculate their energy spectrum. It is shown that the energy spectrum depend on the parabolic way on the burgers vectors of the screw dislocation. We also show that the presence of a topological defect introduces a new contribution for the Aharonov-Bohm effect in the quantum ring.     

Eccentricity effects on the quantum confinement in double quantum rings

This work presents a theoretical study of the energy spectrum of GaAs/AlGaAs concentric double quantum rings, under an applied magnetic field directed perpendicular to the ring plane. The Schrödinger equation for this system is solved in a realistic model consisting of rings with finite barrier potentials. Numerical results show that increasing the magnetic field intensity leads to oscillations in the ground state energy which, in contrast to the usual Aharonov-Bohm oscillations, do not have a well defined period, due to the coupling between inner and outer ring states. However, when one considers an elliptical geometry for the rings, the energy spectra of the inner and outer ring states are decoupled and the periodicity of the oscillations is recovered.     

Transport properties of a single-quantum dot Aharonov-Bohm interferometer

We consider a two-terminal Aharonov-Bohm (AB) interferometer with a quantum dot inserted in one path of the AB ring. We investigate the transport properties of this system in and out of the Kondo regime. We utilize perturbation theory to calculate the electron self-energy of the quantum dot with respect to the intradot Coulomb interaction. We show the expression of the Kondo temperature as a function of the AB phase together with its dependence on other characteristics such as the linewidth of the ring and the finite Coulomb interaction and the energy levels of the quantum dot. The current oscillates periodically as a function of the AB phase. The amplitude of the current oscillation decreases with increasing Coulomb interaction. For a given temperature, the electron transport through the AB interferometer can be selected to be in or out of the Kondo regime by changing the magnetic flux threading perpendicular to the AB ring of the system.     

Effect of interdiffusion and magnetic field on two-electron states in Gaussian-shaped double quantum rings

The effects of interdiffusion and electrons' Coulomb interaction on the energy spectrum in Gaussian-shaped single and double quantum rings in the presence of magnetic field has been considered in the framework of exact diagonalization method. The one-electron energies as functions of magnetic field for different values of diffusion parameter have been obtained. The two-electron energies and electron probability density distributions are obtained as well. It is shown that the energy oscillations which are more pronounced for a single quantum ring, smooth out due to the interdiffusion. The Coulomb interaction transforms the crossings of the two-electron levels to anticrossings and can lead to the appearance of an additional level between the anticrossing levels.     

Oscillatory persistent currents in self-assembled quantum rings

We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs/GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry.     

Quantum Rings with Two Deeply Bound Electrons under a Magnetic Field

A model is proposed to study the quantum rings with two deeply bound electrons under a variable magnetic field. The emphasis is placed to clarify the effect of the size （diameter） and the width of the ring on the fractional Aharonov-Bohm oscillation. It was found that the reduction of size will lead to a very strong oscillation in the ground state energy and in the persistent current. The electronic correlation has also been demonstrated by showing the nodal structures of wave functions.     

Ring interferometer on the basis of 2D electron gas in a double quantum well

Magnetotransport properties of submicron rings fabricated on the basis of 2D electron gas in a GaAs double quantum well are studied. It is shown that, in such interferometers, the Aharonov-Bohm effect is caused by coherent processes in two weakly coupled rings, which have different widths of electron channels. In these interferometers, a phase inversion of h/e oscillations is observed under the action of the parallel component of a tilted magnetic field. This phenomenon is qualitatively explained by a redistribution of charge carriers in the two rings.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

Magnetic flux and strain effects on electron transport in a linear array of nanoscopic rings

Electron transport through a linear array of nanoscopic rings with six quantum dot sites per ring is investigated in the presence of an external magnetic flux producing an Aharonov-Bohm phase shift effect. A tight-binding model is employed to analytically calculate the transmission as a function of electron energy, external flux, and inter-site coupling parameters. Current vs. voltage relationships of the ring system are computed using a standard scattering theory of transport and shown to modulate between semiconductor and ohmic characteristics. System parameters are adjusted in order to study the effects of a longitudinal strain on the transmission properties of the linear multiple-ring array. Longitudinal strain is modeled with a Slater-Koster type theory and is demonstrated to affect the transmission properties primarily by narrowing the transmission bands and opening up additional bandgaps in the band structure. In addition, a universal resonant transmission condition as a function of flux is extended to show that the application of strain causes the resonant transmission peaks to converge towards one-half of a flux quantum.     

Cyclic hydrocarbons: nanoscopic (π)-SQUIDs?

A nonperturbative method for calculating persistent currents in molecules and nanoscopic quantum rings is presented. Starting from the extended Hubbard model on a ring threaded by an Aharonov-Bohm flux, a feedback term through which the current can generate magnetic flux is added. Another extension of the Hamiltonian describes the energy stored in the internally generated field. This model is evaluated using exact diagonalization and an iterative scheme to find the minima of the free energy with respect to the current. The magnetic properties due to electron delocalization of conjugated hydrocarbons like benzene [magnetic anisotropy, magnetic susceptibility exaltation, nucleus-independent chemical shift (NICS)] — that have become important criteria for aromaticity — can be examined using this model. A possible novel mechanism for a permanent orbital magnetic moment in quantum rings analogous to the one in π-SQUIDs is found in the framework of the proposed model. The quantum rings must satisfy two conditions to exhibit this kind of permanent orbital magnetic moment: a negative Drude weight and an inductivity above the critical level.     

Electronic States of Elliptical Quantum Rings Subjected to a Magnetic Field

We calculate the energy states and Aharonov--Bohm oscillations of an electron in elliptical quantum rings in the presence of a uniform magnetic field by using an exact numerical diagonalization. The calculated results show that the elliptical quantum rings are flatter, larger amplitudes and periods of the Aharonov--Bohm oscillations are observed. In addition, in the limits of a circular quantum ring, the results of our approach are in good agreement with those of earlier theories.     

Multisubband transport and magnetic deflection of Fermi electron trajectories in three terminal junctions and rings

We study the electron transport in three terminal junctions and quantum rings looking for the classical deflection of electron trajectories in the presence of intersubband scattering. We indicate that although the Aharonov-Bohm oscillations and the Lorentz force effects co-exist in the low subband transport, for higher Fermi energies a simultaneous observation of both effects is difficult and calls for carefully formed structures. In particular, in quantum rings with channels wider than the input lead the Lorentz force is well resolved but the Aharonov-Bohm periodicity is lost in chaotic scattering events. In quantum rings with equal lengths of the channels and T-shaped junctions the Aharonov-Bohm oscillations are distinctly periodic but the Lorentz force effects are not well pronounced. We find that systems with wedge-shaped junctions allow for observation of both the periodic Aharonov-Bohm oscillations and the magnetic deflection.     

Conductance through strongly interacting rings in a magnetic field

We study the conductance through finite Aharonov-Bohm rings of interacting electrons weakly coupled to non-interacting leads at two arbitrary sites. This model can describe an array of quantum dots with a large charging energy compared to the interdot overlap. As a consequence of the spin-charge separation, which occurs in these highly correlated systems, the transmittance is shown to present pronounced dips for particular values of the magnetic flux piercing the ring. We analyze this effect by numerical and analytical means and show that the zero-temperature equilibrium conductance in fact presents these striking features which could be observed experimentally.     

Optical detection of the Aharonov-Bohm effect on a charged particle in a nanoscale quantum ring

We study spectroscopically the current produced by a charged particle moving in a nanosize semiconductor quantum ring subject to a perpendicular magnetic field. Several Aharonov-Bohm oscillations are observed in the emission of a charged exciton confined in a single ring structure. The magnetic field period of the oscillations correlates well with the size of the rings.     

Electronic structures of GaAs/AlGaAs concentric double rings in applied electric and magnetic field

The electronic structures of GaAs/Al_0.35Ga_0.65As concentric double rings are calculated based on the effective mass envelope function theory, with and without the applied electric and magnetic field along the growth direction. The Hamiltonian matrix elements are determined through the Fourier transform method. As the heterostructure evolves from a single ring to the concentric double rings, our simulation is performed on the bound state energies of the electron and the hole. The results show that the energy levels undulate with the evolution of the ring. The applied magnetic field increases the ground state energies both of the electron and of the hole, as well as the transition energy between the first conduction subband and valence subband. However, the electric field decreases the electronic energies linearly.     

Influence of lateral electric field on intraband optical absorption in concentric double quantum rings

The influence of lateral electric field on one-electron states and intraband absorption in two-dimensional concentric double quantum rings is investigated. The confining potential of the rings is modeled as a double harmonic central potential. Using the exact diagonalization technique, we calculate the dependence of the electron energy spectrum as a function of the electric field strength as well as the inner ring radius. Also, different values of confinement strength are considered. Selection rule is obtained for intraband transitions, caused by the direction of incident light polarization. The intraband absorption coefficient is calculated for different values of electric field strength, inner ring radius, confinement strength and incident light polarization direction. The combined influence of electric field strength and change of confining strength show that while the increment of the first one leads only to blueshift of absorption spectrum, the augment of the second one makes the redshift. In addition, both blueshift and redshift of the spectrum have been obtained with the enlargement of inner ring radius. Finally, we show that the absorption spectrum undergoes redshift by changing the polarization of incident light from x- to y-axis.     

Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect

We present a simple model of transmission across a metallic mesoscopic ring. In one of its arm an electron interacts with a single magnetic impurity via an exchange coupling. We show that entanglement between electron and spin impurity states leads to reduction of Aharonov-Bohm oscillations in the transmission coefficient. The spin-conductance is asymmetric in the flux reversal as opposed to the two-probe electrical conductance which is symmetric. In the same model, in contradiction to the naive expectation of a current magnification effect, we observe enhancement as well as suppression of this effect depending on the system parameters. The limitations of this model to the general notion of dephasing or decoherence in quantum systems are pointed out.     

Flux-dependent anti-crossing of resonances in parallel non-coupled double quantum dots

We present novel resonant phenomena through parallel non-coupled double quantum dots (QDs) embedded in each arm of an Aharonov-Bohm (AB) ring with magnetic flux passing through its center. The electron transmission through this AB ring with each QD formed by two short-range potential barriers is calculated using a scattering matrix at each junction and a transfer matrix in each arm. We show that as the magnetic flux modulates, a distortion of the grid-like square transmission occurs and an anti-crossing of the resonances appears. Hence, the modulation of magnetic flux in this system can have an equivalent effect to the control of inter-dot coupling between the two QDs.