Symmetric Quantum Wells Research Articles

Spin polarization of composite fermions and particle-hole symmetry breaking

We study the critical spin-polarization energy ($\alpha_{\rm C}$) above which fractional quantum Hall states in two-dimensional electron systems confined to symmetric GaAs quantum wells become fully spin-polarized. We find a significant decrease of $\alpha_{\rm C}$ as we increase the well-width. In systems with comparable electron layer thickness, $\alpha_{\rm C}$ for fractional states near Landau level filling $\nu=3/2$ is about twice larger than those near $\nu=1/2$, suggesting a broken particle-hole symmetry. Theoretical calculations, which incorporate Landau level mixing through an effective three-body interaction, and finite layer thickness, capture certain qualitative features of the experimental results.

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in InGaAs/GaAs/AlGaAs step quantum wells

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect (CPGE) at inter-band excitation have been experimentally investigated in InGaAs/GaAs/AlGaAs step quantum wells (QWs) at room temperature. The Rashba- and Dresselhaus-induced CPGE spectra are quite similar with each other during the spectral region corresponding to the transition of the excitonic state 1H1E (the first valence subband of heavy hole to the first conduction subband of electrons). The ratio of Rashba- and Dresselhaus-induced CPGE current for the transition 1H1E is estimated to be 8.8±0.1, much larger than that obtained in symmetric QWs (4.95). Compared to symmetric QWs, the reduced well width enhances the Dresselhaus-type spin splitting, but the Rashba-type spin splitting increases more rapidly in the step QWs. Since the degree of the segregation effect of indium atoms and the intensity of build-in field in the step QWs are comparable to those in symmetric QWs, as proved by reflectance difference and photoreflectance spectra, respectively, the larger Rashba-type spin splitting is mainly induced by the additional interface introduced by step structures.

27aAW-5 zinc-blende(110)対称量子井戸におけるスピン緩和とLOフォノン散乱(27aAW 半導体スピントロニクス,領域4(半導体,メゾスコピック系・局在))

27aAW-5 zinc-blende(110)対称量子井戸におけるスピン緩和とLOフォノン散乱(27aAW 半導体スピントロニクス,領域4(半導体,メゾスコピック系・局在))

Rashba spin splitting and cyclotron resonance in strained InGaAs/InP heterostructures with a two-dimensional electron gas

Cyclotron resonance and magnetic transport in InP/InGaAs/InP heterostructures with axially symmetric quantum wells are studied experimentally at 4.2 K. An increase in the cyclotron mass at the Fermi level from 0.047m 0 to 0.057m 0 with an increase in the concentration of two-dimensional electrons from 5.5 × 1011 to 2.1 × 1012 cm−3 is shown. The values of the Rashba spin splitting at the Fermi level are determined from Fourier analysis of the beats of Shubnikov-de Haas oscillations. The obtained experimental data are compared with the theoretical results of self-consistent calculations of the energy spectrum and cyclotron masses of 2D electrons performed using the eight-band k · p Hamiltonian.

Anisotropy of the electron g factor in quantum wells based on cubic semiconductors

A new mechanism for the spin splitting of electron levels in asymmetric quantum wells based on GaAs-type semiconductors relative to rotations of the magnetic field in the well plane is suggested. It is demonstrated that the anisotropy of the Zeeman splitting (linear in a magnetic field) arises in asymmetric quantum wells due to the interface spin-orbit terms in the electron Hamiltonian. In the case of symmetric quantum wells, it is shown that the anisotropy of the Zeeman splitting is a cubic function of the magnitude of the magnetic field, depends on the direction of the magnetic field in the interface plane as the fourth-order harmonic, and is governed by the spin-orbit term of the fourth order by the kinematic momentum in the electron Hamiltonian of a bulk semiconductor.

Linear and non-linear optical properties in symmetric and asymmetric double quantum wells

The effect of well and barrier widths on optical properties of intersubband transitions in asymmetric (CdS/ZnSe) quantum wells have been investigated theoretically. The electronic states in these structures are calculated using both the envelope wave function approximation and the compact-density matrix formalism. The obtained results reveal that the linear and nonlinear properties have non-monotonic behavior with these parameters. It is found that the optical absorption and the refractive index change are greater in asymmetric double quantum well than that of symmetric heterostructure. These results suggest that the absorption process can be easily controlled by the structure parameters of an asymmetric rectangular quantum well. Thus, our results can be useful for electro-optical modulators and photodetectors in the infrared region.

Exchange enhancement of quasiparticle and ESR spin-gap in symmetric and asymmetric narrow-gap quantum wells

We report a study of electron-electron (e-e) interaction effect on spin-gap in n-type narrow-gap quantum well (QW) heterostructures. By using the Hartree-Fock approximation (HFA) and generalized single-mode approximation (GSMA) based on the 8-band k·p Hamiltonian we demonstrate that spin-orbit interaction and the mixing between |S>- and |P>-states in the conduction and valence bands affect significantly many-body corrections to the spin-gap in symmetric and asymmetric QWs in the integer and fractional Quantum Hall regime. The spin-gap values estimated for 2D electron gas (2DEG) in InAs/AlSb QWs are compared with the experimental results obtained by magnetotransport and by electrically detected electron spin resonance (ESR).

Effects of Rashba spin splitting and exchange interaction in electron spin resonance in narrow-gap quantum well heterostructures

We report an electron spin resonance (ESR) study in n-type narrow-gap quantum well (QW) heterostructures. By using the Hartree-Fock approximation based on the 8 k·p Hamiltonian the many-body theory of the ESR in narrow-gap QWs is developed. We have discovered significant enhancement of the ESR g-factor and its low-magnetic-field divergence in both in asymmetric and symmetric QWs which is caused by the exchange interaction in 2D electron gas (2DEG). The ESR energies estimated for 2DEG in asymmetrical InAs/AlSb QWs are compared with the experimental results obtained by electrically detected ESR technique.

Erratum: Spin-wave excitations and electron spin resonance in symmetric and asymmetric narrow-gap quantum wells [Phys. Rev. B87, 155113 (2013)

Erratum: Spin-wave excitations and electron spin resonance in symmetric and asymmetric narrow-gap quantum wells [Phys. Rev. B<b>87</b>, 155113 (2013)

Radiative lifetime of barbell excitons in semiparabolic double quantum wells under intense laser fields

The binding energy and optical properties of barbell excitons in GaAs–Ga1−xAlxAs semiparabolic double quantum wells under intense laser fields are investigated. Calculations are performed within the effective mass and envelope-function approximations, including the conduction band nonparabolicity. The dependence of the binding energy, oscillator strength and exciton absorption spectrum on the laser field in symmetric and asymmetric quantum wells is studied by using a finite difference method. It is shown that the exciton radiative lifetime can be tuned to a large extent by a proper choice of the structure design (double well size, middle barrier position and its thickness) as well as by varying the laser field intensity.

Spin-wave excitations and electron spin resonance in symmetric and asymmetric narrow-gap quantum wells

We report a theoretical study of spin-wave excitations and electron spin resonance (ESR) in a perpendicular magnetic field in $n$-type narrow-gap quantum well (QW) heterostructures. Using the Hartree-Fock approximation, based on the 8 $\ifmmode\times\else\texttimes\fi{}$ 8 k$\ifmmode\cdot\else\textperiodcentered\fi{}$p Hamiltonian, the many-body corrections to the ESR energy are found to be nonzero in symmetric and asymmetric QWs. We demonstrate a significant enhancement of the ESR energy in the asymmetric QWs, induced by the Bychkov-Rashba spin splitting and exchange interaction in weak and moderate magnetic fields, as well as the exchange-induced divergence of the ESR $g$ factor in the symmetric QWs in weak magnetic fields. We calculate the spin-wave dispersions at odd-valued filling factors of the Landau levels for different values of 2D electron concentration in the symmetric and asymmetric InAs/AlSb QW heterostructures. We predict a gap for the long wavelength spin-wave excitations in symmetric and asymmetric narrow-gap QWs induced by the spin-orbit interaction and subband nonparabolicity. The many-body renormalization of electron spin resonance energies in HgTe/CdHgTe QWs is also qualitatively discussed.

Mesoscopic spin-orbit effect in the semiconductor nanostructure electrongfactor

The renormalization of the electron $g$ factor by the confining potential in semiconductor nanostructures is considered. A new effective $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Hamiltonian for the electronic states in III--V semiconductor nanostructures in the presence of an external magnetic field is introduced. The mesoscopic spin-orbit (Rashba type) and Zeeman interactions are taken into account on an equal footing. It is then solved analytically for the electron effective $g$ factor in symmetric quantum wells (${g}_{\mathrm{QW}}^{*}$). Comparison with different spin quantum beat measurements in GaAs and InGaAs structures demonstrates the accuracy and utility of the theory. The quantum size effects in ${g}_{\mathrm{QW}}^{*}$ are easily understood and its anisotropy $\ensuremath{\Delta}{g}_{\mathrm{QW}}^{*}$ (i.e., the difference between the in-plane and perpendicular configurations) is shown to be given by a mesoscopic spin-orbit effect having the same origin as the Rashba one.

BUILT-IN ELECTRIC FIELD EFFECT ON DONOR IMPURITIES IN STRAINED WURTZITE GaN/AlGaN ASYMMETRIC DOUBLE QUANTUM WELLS

The ground state binding energies of donor impurities in strained [0001]-oriented wurtzite GaN / Al x Ga 1-x N asymmetric double quantum wells are investigated using a variational method combined with numerical computation. The built-in electric field due to the spontaneous and strain-induced piezoelectric polarization and the strain modification on material parameters are taken into account. The variations of binding energies versus the width of central barrier, the ratio of two well widths, and the impurity position are presented, respectively. It is found that the built-in electric field causes a mutation of binding energies with increasing the width of central barrier to some value. The results for symmetrical double quantum wells and without the built-in electric field are also discussed for comparison.

Exciton-related energies of the 1s-like states of excitons in GaAs-Ga1−xAlxAs double quantum wells

The dependencies of the binding energies of the lowest four 1s-like exciton states in GaAs-(Ga,Al)As coupled double quantum wells (CDQW) on the geometric parameters of the system are theoretically studied. A variational approach, together with the parabolic band and effective mass approximations, were considered in order to perform the numerical calculations. It is shown that in the case of a symmetric system there is a degeneracy between the heavy-hole even and odd states and this degeneracy can be removed by the presence of a sufficiently narrow middle barrier. In contrast to this fact, the electron even and odd states are never degenerated. It is detected that, if the system is asymmetric, there will appear binding energies anticrossings between the heavy-hole states at the point of the asymmetric → symmetric QW transition.

Optically induced charge conversion of coexistent free and bound excitonic complexes in two-beam magnetophotoluminescence of two-dimensional quantum structures

We report on extensive polarization-resolved photoluminescence (PL) studies of a variety of excitonic complexes formed in high-quality symmetric GaAs quantum wells containing a high-mobility two-dimensional (2D) hole gas in a broad range of magnetic fields from 0 to 23 T and under two-beam illumination, allowing for dynamical control of the hole concentration beyond the point of conversion from $p$- to $n$-type structures. We have demonstrated charge conversion between positive and negative complexes bound to acceptors in the well, differing from the charge conversion of free trions due to charge reflection symmetry breaking by a fixed impurity, leaving a qualitative trace (exchange splitting) in the PL spectrum. The effect of switching between the electron and hole gases (in the same well) on different emission lines has also allowed us to distinguish the (direct and cyclotron-satellite) emission lines from positive trions moving almost freely in the quantum well and bound to nearby ionized acceptors in the barrier, thus demonstrating their coexistence in high-quality structures.

Effective Hamiltonian, energy spectrum, and phase transition induced by in-plane magnetic field in symmetric HgTe quantum wells

The effective $6\ifmmode\times\else\texttimes\fi{}6$ matrix Hamiltonian for two-dimensional states in HgTe/CdTe quantum wells is derived. The use of the extended basis (in contrast to the previously studied $4\ifmmode\times\else\texttimes\fi{}4$ matrix Hamiltonian) allows us to describe quantum wells with arbitrary orientation of interfaces and investigate the influence of in-plane magnetic field. The Hamiltonian is applied to calculation of energy spectra of both two-dimensional subbands and edge states in the range of well widths corresponding to the topological insulator phase. It is found that the in-plane magnetic field opens a gap in the edge-state spectrum, and the increase in the field causes disappearance of one of the edge-state branches. A strong (about 10 T) magnetic field induces a phase transition from the gapped state to a gapless two-dimensional state, with energy spectrum similar to that of bulk HgTe. An analytical expression for the transition field through the parameters of the effective Hamiltonian is obtained.

Electric field effects on the linear and nonlinear intersubband optical properties of double semi-parabolic quantum wells

In this work, the effects of the electric field on the optical properties of the symmetric and asymmetric double semi-parabolic quantum wells (DSPQWs) are investigated numerically for typical GaAs/Al x Ga 1− x As. Optical properties are obtained using the compact density matrix approach. Our calculations for the asymmetric DSPQW show that the resonant peak values of the total refractive index change and total optical absorption coefficient are maximum for a certain value of the applied electric field, due to the anti-crossing effect. However, for the symmetric DSPQW, the resonant peak values of these optical properties decrease monotonically with increasing the applied electric field. Also, our results indicate that a larger value of the optical rectification coefficient of the symmetric DSPQW can be induced by applying a small electric field.

Resonant subband Landau level coupling in symmetric quantum well

Subband structure and depolarization shifts in an ultrahigh mobility GaAs/Al0.24Ga0.76As quantum well are studied using magnetoinfrared spectroscopy via resonant subband Landau level coupling. Resonant couplings between the first and up to the fourth subbands are identified by well-separated antilevel-crossing split resonance, while the hy-lying subbands were identified by the cyclotron resonance linewidth broadening in the literature. In addition, a forbidden intersubband transition (first to third) has been observed. With the precise determination of the subband structure, we find that the depolarization shift can be well described by the semiclassical slab plasma model and the possible origins for the forbidden transition are discussed.

Pulsed four-wave mixing in intersubband transitions of a symmetric semiconductor quantum well

We study theoretically the phenomenon of pulsed four-wave mixing in intersubband transitions of a symmetric semiconductor double quantum well structure. In the theoretical model we consider two quantum well subbands that are coupled by a strong pump electromagnetic pulse and a weak probe electromagnetic pulse and take into account the effects of electron–electron interactions. For the description of the system dynamics we use the density matrix equations obtained from the effective nonlinear Bloch equations. These equations are solved numerically for a specific double GaAs/AlGaAs quantum well structure. We show that the time-integrated four-wave mixing spectrum depends strongly on the frequency and the intensity of the pump field, on the time-delay between the two electromagnetic pulses and on the electron sheet density.

Linear and nonlinear optical properties of a two-subband system in a symmetric semiconductor quantum well

We study the linear and nonlinear optical response of intersubband transitions in a semiconductor quantum well. We describe the coupling of the quantum well structure with the electromagnetic field by using the nonlinear density matrix equations, in the two-subband approximation. We provide proper approximate analytical solutions to these equations that are used for the closed-form determination of the optical susceptibilities χ(1), χ(3), and χ(5). We also explore the dependence of χ(1), χ(3), and χ(5) on the electron sheet density for a specific double GaAs/AlGaAs quantum well.