Intersubband Mixing Research Articles

Interplay between quantum interference and electron interactions in a Rashbasystem

We investigate theoretically a nanoscopic device in which quantum interference of electronwaves takes place in the presence of their mutual Coulomb interaction. The device consistsof interacting quantum dots coupled to spin-polarized leads via quantum point contactswith Rashba interaction. The Rashba spin-flip-assisted inter-subband mixing in quantumpoint contacts induces quantum interference between the tunneling waves, which interactby Coulomb repulsion inside the dot. The spin-dependent Fano resonances, which appearin the conductance through the device, are significantly modified by Coulombinteractions. Their width and shape depend on the quantum dot spin-up and spin-downoccupancies, controlled by electron interactions. On the other hand, correlatorscalculated for the quantum dot spin sub-levels are not influenced by quantuminterference between them and depend rather on the degree of localization of theselevels.

Spin-polarized transport in quantum waveguide systems with attached stubs

We theoretically studied the spin-polarized electron transport properties of quantum waveguide systems with attached stubs in the presence of Rashba spin-orbit coupling （SOC） by the lattice Green function approach. It is shown that the structure-induced Fano resonance and SOC-induced Fano resonance exist in the charge conductance in consequence of the bound states interfering with the conductance states. At the same time, the Fano resonance and antiresonance structures are also found in the spin polarization. In addition, due to the effect of quantum interference and backscattering caused by abrupt change of quantum wire width, a series of resonant peaks and dips appear in the charge conductance and spin conductance. However, all these effects are suppressed and quantization plateau is recovered in the charge conductance and spin conductance when applying a magnetic field to the system. The underlying physics is that the effect of the magnetic field is equivalent to the enhancement of the lateral confining potential, which reduces the intersubband mixing.

Effects of growth-direction electric and magnetic fields on excitons inGaAs−Ga1−xAlxAscoupled double quantum wells

Direct and indirect excitons in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ coupled double quantum wells, under growth-direction applied electric and magnetic fields, have been theoretically investigated within a variational procedure in the effective-mass and parabolic-band approximations. The exciton hydrogenic $1s$-like envelope wave function is obtained through a variational procedure and an appropriate expansion in trigonometric functions of the electron and hole wave functions. The applied electric field produces a polarization of the exciton by pushing the electron and hole away from each other, whereas the magnetic field contracts the exciton by pushing the electron and hole closer to each other. Intersubband mixing produced by the Coulomb interaction of electron-hole pairs is taken into account and a detailed analysis of the properties of direct- and indirect-exciton states in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ coupled double quantum wells is presented, with theoretical results in good agreement with available experimental measurements.

Strongly modulated transmission of a spin-split quantum wire with local Rashba interaction

We investigate the transport properties of ballistic quantum wires in the presence of Zeeman spin splittings and a spatially inhomogeneous Rashba interaction. The Zeeman interaction is extended along the wire and produces gaps in the energy spectrum, which allow electron propagation only for spinors lying along a certain direction. For spins in the opposite direction, the waves are evanescent far away from the Rashba region, which plays the role of the scattering center. The most interesting case occurs when the magnetic field is perpendicular to the Rashba field. Then, the spins of the asymptotic wave functions are not eigenfunctions of the Rashba Hamiltonian, and the resulting coupling between spins in the Rashba region gives rise to sudden changes of the transmission probability when the Fermi energy is swept along the gap. After briefly examining the energy spectrum and eigenfunctions of a wire with extended Rashba coupling, we analyze the transmission through a region of localized Rashba interaction, in which a double interface separates a region of constant Rashba interaction from wire leads free from spin-orbit coupling. For energies slightly above the propagation threshold, we find the ubiquitous occurrence of transmission zeros (antiresonances), which are analyzed by matching methods in the one-dimensional limit. We find that a minimal tight-binding model yields analytical transmission line shapes of Fano antiresonance type. The general angular dependence of the external magnetic field is treated within projected Schr\"odinger equations with the nondiagonal Hamiltonian matrix elements mixing different wave function components. Finally, we consider a realistic quantum wire where the energy subbands are coupled via the Rashba intersubband coupling term and discuss its effect on the transmission zeros. We find that the antiresonances are robust against intersubband mixing, magnetic field changes, and smooth variations of the wire interfaces, which paves the way for possible applications of spin-split Rashba wires as spintronic current modulators.

Semiconductor quantum-wires and nano-wires for optoelectronic applications

Exciton transfer between two parallel GaAs V-groove quantum wires (QWRs) or two planar quantum wells (QWs) separated by AlGaAs barriers ranging from 5.5 nm to 20 nm thickness is studied by photoluminescence (PL) and PL excitation (PLE) spectroscopy. It is found that the transfer is strongly reduced between the widely spaced QWRs as compared with QWs. We have also investigated the optical absorption in single QWRs embedded in an AlGaAs V-shaped channel waveguide. Using a combination of PLE and absorption measurements we construct the full dependence of absorption spectra on the linear polarization. Our studies reveal the importance of inter-subband mixing in determining the energies of the light-hole-like transitions and thus the QWR absorption. Finally we present recent results on the fabrication and structural characterization of GaAs and GaP nanowires (NWs) grown by molecular beam epitaxy (MBE) on GaAs(111)B and Si(111) substrates, using Au-catalyzed vapor–liquid–solid growth technique. It is shown that, apart from optimizing the NW growth parameters, substrate material and the procedure for preparing the substrate before the MBE growth play an important role in controlling the NWs.

Calculation of direct and indirect excitons inGaAs∕Ga1−xAlxAscoupled double quantum wells: The effects of in-plane magnetic fields and growth-direction electric fields

The variational procedure, in the effective-mass and parabolic-band approximations, is used in order to investigate the effects of crossed electric and magnetic fields on the exciton states in $\mathrm{Ga}\mathrm{As}∕{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ coupled double quantum wells. Calculations are performed for double quantum wells under applied magnetic fields parallel to the layers and electric fields in the growth direction. The exciton envelope wave function is obtained through a variational procedure using a hydrogenic $1s$-like wave function and an expansion in a complete set of trigonometric functions for the electron and hole wave functions. We take into account intersubband mixing brought about by the Coulomb interaction of electron-hole pairs in double quantum wells and present a detailed analysis of the properties of direct and indirect exciton states in these systems. The present study clearly reveals anticrossing effects on the dispersion with applied voltage (or growth-direction electric field) of the photoluminescence peaks associated with direct and indirect excitons. Calculated results are found in good agreement with available experimental measurements on the photoluminescence peak position associated with direct and indirect excitons in $\mathrm{Ga}\mathrm{As}\text{\ensuremath{-}}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ double quantum wells under growth-direction applied electric fields or under applied in-plane magnetic fields.

Fano-Rashba effect in a quantum wire

We predict the occurrence of Fano lineshapes in a semiconductor quantum wire with local spin-orbit (Rashba) coupling. We show that the Rashba interaction acts in a strictly one dimensional channel as an attractive impurity, leading to the formation of purely bound states. In a quasi-one dimensional system these bound states couple to the conduction ones through the Rashba intersubband mixing, giving rise to pronounced dips in the linear conductance plateaus. We give exact numerical results and propose an approximate model capturing the main ingredients of the effect.

COMBINED EFFECTS OF RASHBA SPIN-ORBIT INTERACTION STRENGTH AND ITS EXTENDED LENGTH IN QUANTUM WIRES

The combined effects of Rashba spin-orbit interaction (SOI) strength and the length of SOI excited region, which can be controlled by the external gate voltage deposited on the heterostructure, and on the electron spin precession in quasi-one-dimensional quantum wires are investigated by evaluating the relative conductance change, i.e. the ratio of difference of the conductances of the spin-up and spin-down polarized electrons to the total conductance. It is found that for proper wire width, electron spin precession can be smoothly achieved by co-adjusting the SOI strength and the length of SOI excited region. However, for wide quantum wire and strong SOI, the electron spin precession is significantly reduced due to the appearance of inter-subband mixing.

Resonant hydrogenic impurity states and1s−2p0transitions in coupled double quantum wells

Calculations of the $1s$ and resonant ${2p}_{0}$ states of a shallow donor in double-quantum-well structures are performed. The variational method is used to calculate the localized part of the impurity states, taking into account the effect of intersubband mixing. The resonance coupling of the ${2p}_{0}$ state with the first subband is then dealt with using the Green function technique. The results show that for an asymmetric double-quantum-well structure the $1s$ state has a maximum binding energy as the donor is around the center of the wider well while the ${2p}_{0}$ state has in general a maximum binding energy as the donor is in the narrower well. The resonant coupling of the ${2p}_{0}$ state is stronger for the structures with a stronger intersubband mixing, where the ${2p}_{0}$ energy level is closer to the first subband bottom. The resonance-induced broadening of the ${2p}_{0}$ state can be as large as 6 meV, corresponding to a lifetime of $\ensuremath{\sim}0.1$ ps. The resonance in general causes a negligibly small blueshift but can give a redshift of the order of 1 meV when the resonance is strong. A phase transition of the ${2p}_{0}$ state can occur from the resonant nature to the bound nature by modulation of the interwell coupling. The $1s\ensuremath{-}{2p}_{0}$ transition energy is also calculated. The possibility of population inversion between the $1s$ and ${2p}_{0}$ states is discussed.

Orbital effect of an in-plane magnetic field on quantum transport in chaotic lateral dots

We show that an in-plane magnetic field is able to break time-reversal symmetry of the orbital motion of electrons in two-dimensional semiconductor structures, due to the momentum-dependent inter-subband mixing, which results in supression of weak localization effect. Then, we analyze the influence of the in-plane field on weak localization correction and universal fluctuations of conductance in large-area chaotic semiconductor quantum dots.

Energy structure of quantum rings in a magnetic field

Electron and hole energy levels of self-assembled quantum rings [A. Lorke et al., Phys. Rev. Lett. 84, 4537 (2000)] in a magnetic field are calculated. We go beyond the simple two-dimensional ring model with parabolic confining potential and use the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method with rectangular band-offset potentials in three dimensions. The one-band model is employed for the electron states and the two-band Hamiltonian is used for the hole states. Our calculations describe qualitatively well the positions of experimental resonances found in the far-infrared transmission spectra. The calculated hole energy spectrum differs significantly from the electron one. This difference originates from the light-hole and heavy-hole intersubband mixing.

The role of intersubband mixing in single-electron charging of open quantum dot

We compare the electrostatics and electron transmission through the quantum dots that have the inlet/outlet constrictions made by (i) split finger gates and (ii) overlaying finger gates. It has been found numerically that the intermode mixing is large in the former case while almost absent in the latter. We beleive this difference is responsible for single-electron charging of the open quantum dot of new type (ii) observed in C.-T. Liang et al. (Phys. Rev. Lett. 81 (1998) 3507). Our calculations show that the measured periods of conductance oscillations agree well with the change of the total charge of the dot by one elemental charge as the gate voltages change. Conventionally, single-electron charging does not show up in the transport through open quantum dots due to high transition probability from localized states to fully open channels.

Band Mixing Effects in Quantum Well Magnetoexcitons

The influence of intersubband mixing in quantum wells of semiconductors with zinc-blende structure is studied both experimentally and theoretically. A multiband magnetoexciton model is described which takes into account k . p mixing between valence subbands and the effective Coulomb interaction for an arbitrary confinement potential shape. Theoretical results reproduce very well the photoluminescence excitation spectra of GaAs/AlGaAs single quantum wells of various widths. In particular, the characteristic avoided crossing between the lowest light-hole exciton Landau level and excited heavy-hole exciton Landau level occurring at σ— polarization is accurately described by our theory.

Inter-layer subb and mixing in MBE-grown HgTe/CdTe superlattices

We report transport results for molecular beam epitaxially grown HgTe/CdTe superlattices (SLs) with CdTe barrier thickness of Lb systematically varied from 100A down to 20A, and demonstrate the increasing intersubband mixing between adjacent layers with decreasing Lb. Magnetotransport data measured at 1.5K and for B up to 10T show monotonously varying features with Lb, implying a dimensional change from 2D to quasi-3D. As Lb decreases, the quantum Hall plateaus become weaker and the number of effective 2D channels are decreased, which indicates increasing growth-direction energy dispersion. This enhanced interlayer subband coupling with decreasing Lb is further confirmed by the angle-dependence of SdH oscillations. For Lb = 20A, all of 2D-related transport behavior are washed out. A quantitative analysis with calculated subband energy dispersion relations demonstrates the close interplay between superlattice barrier thickness and the strength of the intersubband mixing. These transport results are directly related to successful control of the growth-axis carrier effective mass and will contribute to the development of high-performance HgTe/CdTe SL-based IR photo-voltaic devices and lasers, in which tunneling noise due to diffusion currents can be considerably reduced.

Coupling of confined impurity states in doped double-quantum-well structures.

Far-infrared magnetotransmission investigations have revealed coupling effects on hydrogenic impurity states in doped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As coupled double quantum wells. Impurity transitions of the well-center donors were measured in several samples with identical well widths but different narrow barrier widths. Results agree with variational calculations for all samples when intersubband mixing and the difference in the masses in the well and barrier are taken into account. Evidence for intersubband mixing in double wells is also provided.

Electron-phonon interaction, dynamical screening and collective excitations in heterostructures: III. Coupled intra- and intersubband plasmon-phonons

The complete spectrum of elementary excitations for the quasi two-dimensional electron gas interacting with the optical phonons of semiconductor heterostructures is calculated within the random-phase approximation. The dispersion curves of the coupled intra- and intersubband plasmon-phonons are plotted in graphical form for various cases. Considering the intersubband mixing we found that this yields only a very weak contribution to the dispersion curves of the plasmon-phonon modes. The theoretical results for the dispersion curve of the (0-0) intrasubband plasmons obtained, including exchange and correlation effects, coupling to the long-wavelength optical phonons, intersubband mixing and screening effects of the grating on top of the sample agree very well with experimental results from far-infrared transmission spectroscopy.

Exciton binding energy and external-field-induced blue shift in double quantum wells.

We calculate the intersubband mixing, brought about by the Coulomb interaction of electron-hole pairs in symmetric double quantum wells. On increasing the barrier thickness the exciton binding energy at first decreases because of the extra electron-hole separation. A further increase in the barrier thickness allows the symmetric and antisymmetric electron and hole subbands to mix, thereby localizing the exciton in one of the two wells. This leads to a rise in the exciton binding energy. We find a minimum in the binding energy as a function of barrier width. Additionally, a novel blue shift in the optical transition energy is predicted for small values of the applied electric field. The mechanism for this can also be related to intersubband mixing.

Intersubband Mixing of Coupled Plasmon‐Phonons in Double Heterostructures

Intersubband Mixing of Coupled Plasmon‐Phonons in Double Heterostructures