Excitation spectra of a system of two coupled quantum wells

Quasi-particle spectra, reconstructed by e-h pairing, have been calculated for a system of spatially separated electrons (e) and holes (h) in ground state. The regions of strong pairing interaction and significant correlation effects are reached by abandoning the BCS approximation and using instead interpolation expressions for correlation energies, which depend functionally on the coefficients of the u-v transformation, with subsequent minimization of the total energy of the reconstructed state with respect to the parameters of the u-v transformation. The dependence of the spectra on quantum-well separation and particle concentration in a system of two coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 2226–2228 (December 1998)     

Phase transitions of indirect excitons in coupled quantum wells: The role of disorder

The theory of what happens to a superfluid in a random field, known as the “dirty boson” problem, directly relates to a real experimental system presently under study by several groups, namely excitons in coupled semiconductor quantum wells. We consider the case of bosons in two dimensions in a random field, when the random field can be large compared to the repulsive exciton–exciton interaction energy, but is small compared to the exciton binding energy. The interaction between excitons is taken into account in the ladder approximation. The coherent potential approximation (CPA) allows us to derive the exciton Green's function for a wide range of the random field strength, and in the weak-scattering limit CPA results in the second-order Born approximation. For quasi-two-dimensional excitonic systems, the density of the superfluid component and the Kosterlitz–Thouless temperature of the superfluid phase transition are obtained, and are found to decrease as the random field increases.     

Intraband transitions in magnetoexcitons in coupled double quantum wells

A theory of far-infrared (FIR) magneto-optical intraband s→p ^± transitions of direct and indirect excitons in semiconductor coupled double quantum wells has been developed. The case of symmetric strained In_xGa_1−x As/GaAs quantum wells with nondegenerate valence band in the regime of both narrow and wide barriers has been analyzed. The energies and dipole matrix elements of transitions between the ground s and excited p ^± states in a quantizing magnetic field B>2 T and electric field ℰ perpendicular to the quantum well plane have been studied. The regimes of direct (in a weak electric field) and indirect (in a strong electric field) transitions, and the transition between the direct and indirect regimes, have been investigated. Zh. éksp. Teor. Fiz. 113, 1446–1459 (April 1998)     

Intersubband transitions from the second subband in a coupled quantum wells structure

Asymmetrical coupled quantum wells structures with energy separation between the first two subbands of the order of 10–50 meV are key structures in the design of optically pumped intersubband lasers. In these structures the population of the second subband is not negligible and intersubband transitions from the second to higher excited subbands can be observed. In this work we investigate the temperature dependency of the intersubband transitions from the second subband in an asymmetrical coupled quantum wells structure. We show that this approach provides a direct way to measure the energy separation between the second subband and the Fermi energy which is a crucial parameter in the design of optically pumped intersubband lasers.     

Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells

Two schemes for steady stimulated phonon generation (saser, i.e., phonon laser) are proposed. The first scheme exploits a narrow-gap indirect semiconductor or analogous indirect gap semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron–hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. In the second scheme the magnitude of phonon wave vector is determined by magnitude of in-plane magnetic field and, therefore, such a saser is tunable. Both schemes are analyzed and estimated numerically.     

Strain dependence of valence band structure and intersubband transitions in coupled quantum wells

We have calculated the structure of the valence bands and its dependence on strain in a lattice mismatched quantum well, and in double quantum wells with equal and unequal well thicknesses. We have used the 4×4 Luttinger-Kohn Hamiltonian and the envelope function approximation. Further the character of the strain dependence of optical transitions in valence band was analysed in all of these structures. It has been found that the strain dependence of the valence band structure and optical momentum matrix elements is stronger in the double quantum well than in the single quantum well. This result is potentially useful in valence band engineering and device applications.     

Phase transitions in a system of coupled Boson oscillators on a sphere

The solutions of mean-field equations for a system of coupled Boson oscillators on an infinite k-dimensional sphere are discussed in the low density - high temperature region and high density — low temperature region. It is shown that for k = 2 the system exhibits only spatial condensation, whereas for k ⩾ 3 both spatial condensation and Bose-Einstein condensation.     

Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Magnetoexciton absorption in coupled quantum wells

A Mott exciton in coupled quantum wells in a transverse magnetic field H is considered. An expression for the exciton spectrum in an arbitrary magnetic field for large separations D between quantum wells containing an electron (e) and a hole (h) is given. The exciton spectrum in a strong magnetic field for different Landau levels at arbitrary D has been calculated. Changes in the parameter D/l, where is the magnetic length, cause rearrangement of the magnetoexciton dispersion curves ℰ(P), where P is the conserved “magnetic” momentum, which is a function of the separation between the electron and hole in the plane of the quantum wells. Off-center (“roton”) extrema occur only for D/l,<(D/l)_cr, where (D/l)_cr is a function of the exciton quantum numbers n and m. The magnetoexciton effective mass in states with magnetic quantum number m=0 monotonically increases with H and D, while in states with m≠0 it is a nonmonotonic function of D/l. The probability of generating an exciton in coupled quantum wells increases with H. Absorption of electromagnetic radiation due to transitions between excitonic levels in coupled quantum wells is discussed. For an exciton containing a heavy hole the oscillator strengths increase with H and the oscillator strengths decrease. Zh. éksp. Teor. Fiz. 112, 1791–1808 (November 1997)     

Spin-orbit interaction in coupled quantum wells

We theoretically investigate the spin-orbit interaction in GaAs/AlxGa1 x As coupled quantum wells. We consider the contribution of the interface-related Rashba term as well as the linear and cubic Dresselhaus terms to the spin splitting. For the coupled quantum wells which bear an inherent structure inversion asymmetry, the same probability density distribution of electrons in the two step quantum wells results in a large spin splitting from the interface term. If the widths of the two step quantum wells are different, the electron probability density in the wider step quantum well is considerably higher than that in the narrower one, resulting in the decrease of the spin splitting from the interface term. The results also show that the spin splitting of the coupled quantum well is not significantly larger than that of a step quantum well.     

Exciton-exciton interaction in coupled quantum wells

The exciton-exciton interaction is investigated for spatially indirect excitons in coupled quantum wells. The Hartree-Fock and Heitler-London approaches are improved by a full two-exciton calculation including the van der Waals effect. Using these potentials for the singlet and triplet channel, the two-body scattering matrix is calculated and employed to derive a modified relation between exciton density and blue shift. Such a relation is of central importance for gauging exciton densities on the way toward Bose condensation.     

Finite motion of a particle in coupled quantum wells

We examine the nonstationary state of a particle in two or three coupled quantum wells by introducing quasiseparate potentials. We obtain analytic expressions for the tunneling time and the single-particle quantum current in the WKB approximation. The general result is applied to the case of a triangular barrier.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 87–91, September, 1989.     

Phase transitions in quantum plasmas

We present a theory for a critical point and phase transitions in quantum plasmas (QPs). We use a newly obtained Lennard-Jones-type interaction potential or an oscillating exponential-screened Coulomb potential around ions that are screened by degenerate electrons in an unmagnetized QP. Expressions for the free energy and an equation of state for non-degenerate ions are obtained. It is found that the existence of a critical point and phase separations in our QPs depends on the Wigner–Seitz electron radius and the equilibrium ion temperature. Our results are relevant for understanding superdense astrophysical objects such as the cores of white dwarfs.     

Geometric phase for a coupled two quantum dot system

The adiabatic geometric phase is calculated in a coupled two quantum dot system, which is entangled through Förster interaction. This phase is then utilized for implementing basic quantum logic gate operation useful in quantum information processing. Such gates based on geometric phase provide fault-tolerant quantum computing.     

Propagation of electrons in coupled quantum wells

Summary We study propagation of an electron wave in a double-quantum-well structure formed by alternate layers of GaAlAs and GaAs. In such a structure, electron states parallel to the layers are described by 2D plane waves and in the perpendicular direction by the bound states of the confining potential. We show that an electron, initially introduced in one well, will execute oscillations between the two wells of the structure. Although the frequency of oscillations depends primarily on the distance separating the wells and the confining potential, it is shown in this paper that the frequency also depends on the effective mass of the electron, if it is different within and outside the well. Expressions are derived for the frequency of oscillations, taking into account the difference in the effective mass of the electron.     

Polarization dependence of two-photon transitions in quantum wells

Summary We give explicitly the polarization dependence of two-photon subband-subband transitions in semiconductor quantum wells. We consider transitions from heavy-hole subbands as well as from light-hole subbands. We study the polarization dependence in the case of absorption of one photon having an energy of the order of the band gap and one having an energy of the order of the subband separation. We show that the absorption structure depends on the polarization of the low-energy photon. We also give, in the case of equal photons with in-plane linear polarizations, the dependence of the transition rate on the angle between the polarizations.