Interwell Coupling Research Articles

Relation between ac Josephson effect and double-well Bose-Einstein-condensate oscillations

In this article we comment on the relation between the ac Josephson effect and the coherent oscillations of a Bose-Einstein condensate confined to a double-well potential. The goal is to elucidate the extent to which the latter is a realization of the former. We detail the correspondence between the two oscillation frequencies, that emerges in the high occupation limit of the double-well potential. We show that in the latter thermodynamic limit the effective one-particle interwell coupling vanishes with the system size, leading to oscillation frequency that depends only on the interwell imbalance, consistent with the ac Josephson effect frequency.

Binding energy of exciton in a nanowire superlattice in magnetic and electric fields

We study the binding energy of excitons in a cylindrical GaAs/Ga1−xAlxAs nanowire superlattice, embedded in Ga1−yAlyAs matrix, in the presence of magnetic and electric fields applied parallel to the growth direction. We express the exciton trial function as a product of one-particle wave functions of the electron and the hole with variationally determined envelope function, which describes the exciton intrinsic properties and depends only on the electron-hole separation. By using a functional derivative technique, we derive a differential equation for this envelope function, which we solve numerically. By varying the wire radius, interwell barrier width and well sizes we obtain binding energies ranging in character from one for strongly coupled superlattice to that for a system of stack of isolated disks. The behaviour of the binding energies and the charge distributions as functions of the interwell coupling, well sizes, and the external fields is consistently described with our simple formalism.

Energy spectrum of excitons in single and double narrow wells in a magnetic field

A theoretical investigation is presented of the properties of heavy- and light-hole excitons in multiple GaAs/Ga1−xAlxAs quantum wells in the presence of a magnetic field directed along the growth axis. The low-lying energy levels are calculated as functions of the barrier width, the well size and the magnetic field strength. Effects of interwell coupling on the energies of different exciton levels are consistently included in our calculations. The calculation is realized by means of the diagonalization method.

Vortex qubit based on an annular Josephson junction containing a microshort

We report theoretical and experimental work on the development of a vortex qubit based on a microshort in an annular Josephson junction. The microshort creates a potential barrier for the vortex, which produces a double-well potential under the application of an in-plane magnetic field; The field strength tunes the barrier height. A one-dimensional model for this system is presented, from which we calculate the vortex depinning current and attempt frequency as well as the interwell coupling. Implementation of an effective microshort is achieved via a section of insulating barrier that is locally wider in the junction plane. Using a junction with this geometry we demonstrate classical state preparation and readout. The vortex is prepared in a given potential well by sending a series of "shaker" bias current pulses through the junction. Readout is accomplished by measuring the vortex depinning current.

Resonant modes and inter-well coupling in photonic quantum well with negative index materials

The transfer matrix method was used to study the resonant modes in photonic quantum well by stacking different photonic crystals consisting of positive index materials and negative index materials. The eigenfrequency equation for the resonant modes is derived. It is found that these resonant modes are omnidirectional, and the number of resonant modes is equal to the period number of photonic quantum wells. Moreover, the resonant modes become N-fold splitting in the N photonic quantum wells. The splitting intervals increase with the deceasing of photonic barrier thickness due to the coupling among the wells.

Design and epitaxy of structural III-nitrides

We designed Mg-doped InGaN/GaN quantum well (QW), strained ultrathin InN/GaN QWs, and Mg and Si δ-doped AlGaN/GaN superlattices (SLs) by using first-principles simulations. The designed structures were grown by metal–organic vapor phase epitaxy (MOVPE) on high-quality thick GaN using an interruption technique. The injection-current-dependent electroluminescence characteristics of Mg-doped QW exhibit smaller injection-current-induced blueshift and energy separation, which indicate that modification of quantized levels has occurred in Mg-doped QW due to the reduction of polarization field. The cathodoluminescence intensity of the strained ultrathin InN/GaN QWs is enhanced with increasing barrier thickness due to the suppression of interwell coupling, while the emission wavelength redshifts significantly as the well width increases due to reduction of strain in the QW. Higher hole concentration and mobility are achieved in Mg and Si δ-doped p-type SLs compared to those in modulation-doped SL. These results are in excellent agreement with those of theoretical designs.

Field-induced energy band and effective mass anisotropy in coupled double quantum wells

The effects of the field-induced conduction band anisotropy on the electron effective mass and electrical transport properties of coupled double quantum well structures are discussed. This anisotropy is due to the formation of a partial energy gap in the dispersion curves of coupled double quantum wells by an in-plane magnetic field. The reported data show that the magnitude of this gap is mainly dependent on the interwell coupling, although it also depends on the magnitude of the applied magnetic field. A field-induced anisotropy of the electron effective mass and the in-plane electrical transport properties, due to the formation of a saddle point at the lower edge of this gap, is predicted. Further, a simple model to estimate the critical field at which a saddle point appears is also discussed.

Microscopic Theory of Coupled Quantum Well Structures in Photovoltaics

AbstractWe use our recently developed microscopic approach to a quantum theory of photovoltaic processes in nanostructures [1]to investigate geometry effects in quantum well photovoltaics, focussing on the role of asymmetry and inter-well coupling in double quantum well (DQW) systems. For that purpose, the IV and power characteristics for DQW systems with different asymmetry and degree of coupling are calculated numerically in the vicinity of the maximum power point. In order to assess the transport properties of a specific QW structure, we isolate these from the effects of absorption by normalizing to the absorptivity. The results obtained from this procedure reveal the escape regime dominating at room temperature and confirm previous experimental observations.

Intersubband absorption in AlN∕GaN∕AlGaN coupled quantum wells

Al N ∕ Ga N ∕ Al Ga N coupled quantum wells grown by molecular beam epitaxy have been developed and characterized via intersubband absorption spectroscopy. In these structures, an AlGaN layer of sufficiently low Al content is used to achieve strong interwell coupling without the need for ultrathin inner barriers. At the same time, AlN is used in the outer barriers to provide the large quantum confinement required for near-infrared intersubband transitions. The composition of the inner barriers also provides a continuously tunable parameter to control the coupling strength. Double intersubband absorption peaks are measured in each sample, at photon energies in good agreement with theoretical expectations.

Electron magnetotransport in GaAs/AlGaAs superlattices with weak and strong inter-well coupling

We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.

Interwell coupling effect in Si/SiGe quantum wells grown by ultra high vacuum chemical vapor deposition

Si/Si0.66Ge0.34coupled quantum well (CQW) structures with different barrier thickness of 40, 4 and 2 nm were grown on Si substrates using an ultra high vacuum chemical vapor deposition (UHV-CVD) system. The samples were characterized using high resolution x-ray diffraction (HRXRD), cross-sectional transmission electron microscopy (XTEM) and photoluminescence (PL) spectroscopy. Blue shift in PL peak energy due to interwell coupling was observed in the CQWs following increase in the Si barrier thickness. The Si/SiGe heterostructure growth process and theoretical band structure model was validated by comparing the energy of the no-phonon peak calculated by the 6 + 2-bandk·pmethod with experimental PL data. Close agreement between theoretical calculations and experimental data was obtained.

Coupling of one-dimensionally confined optical modes in porous silicon: Spectral observation and photonic-quantum-well description

Single and double photonic-quantum-wells (PQWs) have been fabricated, using refractive-index-modulated porous silicon multilayer structures. The one-dimensionally confined optical modes observed are consistent with those calculated using the transfer matrix method. While single PQWs exhibit atom-like discrete spectral features, molecule-like energy level splitting is accessible by combining two single PQWs into a double PQW structure. Similar to an electron in a double-quantum-well structure, the energy separation between the split ‘anti-bonding’ and ‘bonding’ photonic states can be ‘tuned’ by changing the strength of inter-well coupling.

Giant Zeeman Splitting of Excitons in a III–V Nonmagnetic Quantum Well Electronically Coupled With a II–VI Semimagnetic Quantum Well

We report on design, fabrication by molecular beam epitaxy, and photoluminescence (PL) studies of GaAs/AlGaAs/ZnSe/ZnCdMnSe double quantum wells (QWs), where resonant electronic coupling occurs through a heterovalent interface. The resonant conditions achieved in the properly designed sample facilitate penetration of the electron wave function from the nonmagnetic GaAs QW into the diluted magnetic semiconductor ZnCdMnSe QW. It results in the sign reversal and drastic increase of a GaAs QW excitonic g factor. The exciton spin splitting observed in the magneto-PL spectra is in general agreement with the calculation performed within the envelope function approximation, taking into account both the inter-well electron coupling and Brillouin-like paramagnetic behavior of the Mn2+ ions.

Confinement and interwell coupling effects in Ge double quantum wells pseudomorphic to a Si (001) substrate

We present a theoretical study of the electronic properties of two coupled Ge quantum wells separated by a variable number of Si monolayers, epitaxially grown on a Si (001) substrate. We adopt the real space decimation-renormalization method and a $s{p}^{3}{d}^{5}{s}^{*}$ nearest neighbors tight binding Hamiltonian for the description of the electronic states of bulk Si and Ge crystals. Strain, band offsets and spin-orbit interactions are properly taken into account. From the Green's function of the multilayer structure considered, the energy spectrum and partial and total densities of states projected on each layer and orbital are obtained. This has allowed us to investigate the nature of the valence and conduction confined states and the effect of interwell coupling on the optical properties. In particular we show that position and red shift with Ge width of the experimental no phonon luminescence line of these structures are well interpreted by the present calculation and that additional luminescence lines are predicted.

Spin transfer and coherence in coupled quantum wells

Spin dynamics of optically excited electrons confined in asymmetric coupled quantum wells are investigated through time resolved Faraday rotation experiments. The inter-well coupling is shown to depend on applied electric field and barrier thickness. We observe three coupling regimes: independent spin precession in isolated quantum wells, incoherent spin transfer between single-well states, and coherent spin transfer in a highly coupled system. Relative values of the inter-well tunneling time, the electron spin lifetime, and the Larmor precession period appear to govern this behavior.

Linear and nonlinear pulse propagation in a multiple-quantum-well photonic crystal

We investigate the temporal and spectral properties of subpicosecond pulses transmitted on the heavy-hole exciton transition through a multiple-quantum-well Bragg structure, exhibiting a one-dimensional photonic band gap. At low light intensities, a temporal propagation beating is observed. This beating is strongly dependent on the optical dephasing time ${T}_{2}$ which is dominated by the radiative interwell coupling. In an intermediate intensity regime, the Pauli-blocking nonlinearity leads to gradual suppression of the photonic band gap and vanishing of the linear propagation beating. For highly nonlinear excitation, we find signatures of self-induced transmission due to Rabi flopping and adiabatic following of the carrier density. Numerical simulations using the semiconductor Maxwell-Bloch equations are in excellent agreement with the experimental data up to intensities for which higher many-particle correlations become more important and self-phase modulation occurs in the sample substrate.

Spin-polarized ballistic transport in diluted magnetic semiconductor quantum wire systems

The energy dispersion of an electron in a double quantum wire with a diluted magnetic semiconductor barrier in between is calculated. An external magnetic field modifies significantly the energy dispersion of the electron which is different for the two spin states. The conductance exhibits many interesting peaks and dips which are directly related to the energy dispersions of the different electron spin states. These phenomena are attributed to the interwell coupling which can be tuned by the magnetic field due to the $s\ensuremath{-}d$ exchange interaction.

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.

Thermopower of a double quantum well based on GaAs

The resistivity and thermopower of a double quantum well have been investigated in the temperature range $0.3\ensuremath{-}4.2\mathrm{K}$ as a function of voltage applied to a top gate. As in previous studies, the resistivity showed a strong resonance when the carrier densities in each well were approximately the same. In contrast, neither diffusion nor phonon-drag thermopower showed any sensitivity to the resonance. We have calculated the thermopower based on a model of two independent two-dimensional electron gases (2DEG's) connected in parallel. In the case of phonon-drag thermopower, ${S}^{g},$ it was necessary to take into account the mutual screening of the electron-phonon interaction by electrons in each layer and also the local-field correction to the static dielectric function. We find that ${S}^{g}$ exhibits a ${T}^{5}$ dependence at low temperatures instead of the standard ${T}^{4}$ expected for single GaAs quantum wells. We also find that, for the lowest densities examined, the local-field correction enhances the magnitude of the calculated ${S}^{g}$ by over a factor of 2, in good agreement with experiment. As a check on the model of two independent 2DEG's we have also calculated ${S}^{g}$ at resonance taking into account interwell coupling. The calculated values of ${S}^{g}$ so obtained are in good agreement with those obtained for uncoupled wells. This confirms the experimental result that ${S}^{g}$ is insensitive to the resonance condition.

Variation of Inter-Well Coupling in Magnetically Tunable Multiple Quantum Wells

We have investigated the problem of inter-well coupling in multiple quantum well (QW) systems using magnetooptical experiments. A series of multi-QW structures consisting of II-VI-based diluted magnetic semiconductor (DMS) and non-magnetic semiconductor wells were used for this purpose. In one study, we used a double quantum well structure consisting of non-magnetic quantum wells (Zn 1-y Cd y Se) separated by DMS barriers (Zn 1-x Mn x Se). The dependence of coupling strength on inter-well barrier height was studied by examining the energy splitting of the lowest multiplet of states. We have also investigated inter-well coupling in triple quantum well structures. For this purpose we designed symmetric triple quantum wells consisting of DMS (Zn 1 -x-y Cd y Mn x Se) and non-DMS (Zn 1-y Cd y Se) semiconductor wells, separated by non-magnetic ZnSe barriers. Transitions involving the lowest multiplet of states were clearly observed and well resolved at zero magnetic field. The strongest interaction between the states in such triple quantum wells occurs when the wells are identical (i.e., when they are in resonant condition). As soon as the depths of the wells become unequal (achieved by Zeeman splitting in the presence of an external magnetic field), the coupling between the wells is dramatically reduced.