D. V. Kulakovskii

Collective excitations in exciton crystal

Crystal phase of indirect excitons formed by spatially separated electrons and holes in coupled quantum wells is analyzed. The collective mode spectrum of the exciton crystal at zero and nonzero agnetic fields is found. The spectrum consists of two optical and two acoustical modes (transverse and longitudinal in each case). We also study changes of the dipole crystal collective excitations at the transition exciton crystal-electron-hole plasma.

Effect of screening by two-dimensional charge carriers on the binding energy of excitonic states in GaAs/AlGaAs quantum wells

The spectrum of excitonic excited states in GaAs/AlGaAs quantum wells of different width is studied together with its change due to the screening of electron-hole interaction by two-dimensional electrons. The exciton binding energy decreases sharply with an increase in the concentration of two-dimensional electrons. The temperature dependence of screening parameters is studied for the ground and excited excitonic states down to ultralow temperatures T=50 mK.

Charged many-particle complexes: Properties of an indirect trion

A system of spatially separated interacting excitons and electrons is considered. The formation of a spatially indirect trion in this system is investigated. The asymptotic behavior of the binding energy of this charged many-particle complex is determined in the limits of large and small layer separations. The variation of the binding energy due to screening by the 2D electron gas is calculated. The bound state of the spatially separated exciton and electron is shown to disappear from the collective excitation spectrum at a certain threshold concentration of 2D electrons.

Screening and rearrangement of excitonic states in double layer systems

Screening and rearrangement of excitonic states in double layer systems were considered. Binding energy changes as a result of screening by spatially separated electrons were calculated for both direct and spatially indirect excitons. The critical electron concentrations at which screening effects cause a sharp decrease in the exciton binding energy and an increase in the exciton radius depending on the interlayer distance were determined. For a spatially indirect exciton, this dependence had a maximum caused by the competition of two effects as the interlayer distance grew, namely, a decrease in the seed interaction between electrons and holes and a weakening of the effect of hole screening by spatially separated electrons. The effect of spatially separated electron-exciton drag in a double layer system was studied in the Debye-Hückel approximation taking into account screening of interlayer electron-exciton interaction.

Observation of plasmon-polariton modes in two-dimensional electron systems

A manifestation of retardation effects, which were predicted theoretically more than 35 years ago, is revealed for the first time in the plasma excitation spectrum of a two-dimensional electron system with a high electron mobility. It is shown that a significant decrease in the resonant plasma frequency due to a hybridization of the plasma and light modes is observed in zero magnetic field. An unusual dependence of the frequency of the hybrid cyclotron-plasmon mode on the magnetic field has been observed in a perpendicular magnetic field. The experimental results are in good quantitative agreement with the theory.

Properties of exciton states in GaAs/AlGaAs quantum wells in the presence of a quasi-two-dimensional electron gas

The changes in binding energy and oscillator strength of the exciton state due to the screening by a quasi-two-dimensional electron gas are calculated self-consistently in the approximation of noninteracting electrons and in the local field approximation. It is shown that the collapse of the bound state occurs at very low concentrations, Ns≈5×109 cm−2, which is a consequence of the inclusion of the nonlinearity of the response of the system to a Coulomb perturbation. The temperature dependence of the exciton collapse is investigated. The phase diagram of the dissociation of the given bound state is constructed, and the region in which it is possible to observe experimentally the temperature dependence of the exciton collapse is indicated.

Excitonic representation: Collective excitation spectra in the quantized Hall regime and spin biexciton

The excitonic representation method for describing collective excitations in the quantized Hall regime makes it possible to simplify analysis of the spectra and to obtain new results in the strong magnetic field limit, when EC≪ħωc (ωc is the cyclotron frequency and EC is the characteristic Coulomb energy). For an integer odd filling factor ν greater than unity (i.e., for ν = 3, 5, 7,...), the spectra of one-cyclotron magneto-plasma excitations are calculated. For unit filling factor, the existence of a spin biexciton (bound state of two spin waves) corresponding to excitation with a spin change (δS = δSz = −2) is proved. The exact equation determining the ground state of the biexciton is derived in the thermodynamic limit NΦ → ∞ (Nϕ is the system degeneracy). The exchange energy of this state is lower than for a single spin wave (with δS = δSz = −1) for the same value of the 2D wavevector q. In the limit q → ∞ corresponding to the decay of a biexciton into a pair of quasiparticles one of which is a trion with a spin of −3/2, the energy is found to be lower than the energy (e2/εlB)√π/2 required for exciting an electron-hole pair in the strictly 2D case (lB is the magnetic length and ε is the dielectric constant), although this energy is higher than another “classical” result (e2/εlB)√π/2, corresponding to the excitation of a skyrmion-antiskyrmion pair (|δS|=|δSz|≫1). The solution of the exact equation gives the trion binding energy and the activation gap for quasiparticles whose excitation corresponds to a change in the total spin by δS = δ Sz =−3. The energy of a spin biexciton is calculated for values of the wavevector such that qlB≫1.

Screening of exciton states by quasi-two-dimensional electron gas in quantum wells

Changes in the binding energy and oscillator strength of an exciton state due to screening by a quasi-two-dimensional electron gas were calculated self-consistently in a nonlinear approximation. It was shown that the collapse of the bound state proceeds at very small concentrations Ns≃5×109 cm−2, which is a consequence of taking into account the nonlinearity of the system response to the Coulomb perturbation.

Bistable charge states in a photoexcited quasi-two-dimensional electron-hole system

The luminescence spectra of GaAs/AlGaAs quantum wells (QWs) with low-density quasi-two-dimensional electron and hole channels were studied. It was demonstrated that, at temperatures below some critical value (Tc∼30 K) and for an excitation power lying in a certain temperature-dependent range, two metastable charge states with two-dimensional charge densities differing in both magnitude and sign can occur in the system under the same conditions. The obtained experimental data agree well with the mathematical model allowing for the transfer of photoexcited carriers to the barrier followed by their tunneling into QW.

Collective magnetoplasma excitations in two-dimensional electron rings

The spectra of magnetoplasma excitations in two-dimensional electron disks and rings are studied by optical detection of resonance microwave absorption. For ring-shaped structures, two types of edge magnetoplasma modes localized along the inner and outer boundaries of the ring are observed. It is shown that the interaction between these modes leads to a strong modification of their magnetic-field dependences as compared to disks. In addition to the longitudinal edge magnetoplasma excitations, transverse plasma modes associated with the electron density oscillations along the ring radius are revealed. The spectra of magnetoplasma excitations are calculated in terms of the electrodynamic theory for both ring-shaped and disk-shaped structures. The classification of all modes of collective magnetoplasma excitations observed in the experiment is performed on the basis of the comparison between experimental and theoretical results.