Quantum Hall effect in back-gated InAs/GaSb heterostructures under a tilted magnetic field

We investigate the quantum Hall effect (QHE) in the InAs/GaSb hybridized electron–hole system grown on a conductive InAs substrate which act as a back-gate. In these samples, the electron density is constant and the hole density is controlled by the gate-voltage. Under a magnetic field perpendicular to the sample plane, the QHE appears along integer Landau-level (LL) filling factors of the net-carriers, where the net-carrier density is the difference between the electron and hole densities. In addition, longitudinal resistance maxima corresponding to the crossing of the extended states of the original electron and hole LLs make the QHE regions along integer-ν_net discontinuous. Under tilted magnetic fields, these R_xx maxima disappear in the high magnetic field region. The results show that the in-plane magnetic field component enhances the electron–hole hybridization and the formation of minigaps at LL crossings.     

Relaxation time for a charge carrier due to its scattering from other charge carriers in superlattices

A calculation of relaxation time for (i) electron–electron scattering in a modulation-doped superlattice of type-I and (ii) electron–electron, hole–hole and electron–hole scattering processes in a compositional superlattice of type-II has been performed, using Fermi's golden rule. As compared to a two-dimensional electron gas system, both intralayer and interlayer interactions, between charge carriers in a superlattice, contribute to relaxation time. It is found that scattering processes at all possible value of momentum transfer contribute to relaxation time, for a given value of temperature and carrier density. We further find interlayer interactions in a superlattice make a significant contribution to relaxation time. Relaxation time is found to decrease on increasing temperature, carrier density and single particle energy, in a superlattice. The computed relaxation time for an electron (hole) in a superlattice enhances on increasing the width of layer consisting of electrons (holes). The electron–hole (hole–electron) scattering process in a type-II superlattice yields maximum contribution to the relaxation time when a hole layer lies exactly in between two consecutive electron layers.     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Mean-field dynamics of a quantum dot-microcavity system

Mean-field evolution equations for the exciton and photon populations and polarizations (Bloch–Lamb equations) are written and numerically solved in order to describe the dynamics of electronic states in a quantum dot coupled to the photon field of a microcavity. The equations account for phase space filling effects and Coulomb interactions among carriers, and include also (in a phenomenological way) incoherent pumping of the quantum dot, photon losses through the microcavity mirrors, and electron–hole population decay due to spontaneous emission of the dot. When the dot may support more than one electron–hole pair, asymptotic oscillatory states, with periods between 0.5 and 1.5 ps, are found almost for any values of the system parameters.     

Micropillar resonator in a magnetic field: Zero and finite temperature cases

In this work, we present a theoretical study of a quantum dot–microcavity system which includes a constant magnetic field in the growth direction of the micropillar. First, we study the zero temperature case by means of a self-consistent procedure with a trial function composed of a coherent photon field and a BCS function for the electron–hole pairs. The dependence of the ground state energy on the magnetic field and the number of polaritons is found. We show that the magnetic field can be used as a control parameter for the photon number, and we make explicit the scaling of the total energy with the number of polaritons. Next, we study this problem at finite temperatures and obtain the scaling of the critical temperature with the number of polaritons.     

Composite fermion picture for multi-component plasmas in 2D electron–hole systems in a strong magnetic field

Low-lying states of a 2D electron–hole system contain electrons and one or more types of charged excitonic complexes. Binding energies and angular momenta of these excitonic ions, and the pseudopotentials describing their interactions with electrons and with one another are obtained from numerical studies of small systems. Incompressible fluid ground states of such multi-component plasmas are found in exact numerical diagonalizations. A generalized composite fermion (CF) picture involving Chern–Simons charges and fluxes of different types is proposed and shown to predict the low-lying states at any value of the magnetic field.     

Electron–hole states in the fractional quantum Hall regime probed by photoluminescence

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling factors.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Magnetoexcitons in unbalanced double layers

We study theoretically correlations of electrons and holes in unbalanced double-layer electronic systems in strong magnetic fields. Calculations are made using the exact diagonalization and the variational wave function. The ground state of an electron–hole pair in quantized cyclotron orbits possesses an in-plane electric dipole moment, when an electron and a hole are in different Landau orbits with different radii. The resulting attractive interactions between pairs creates the possibility of novel states.     

Pseudospin solitons in the coherent stripe phase of a bilayer quantum Hall system

In the Hartree–Fock approximation and at total filling factor ν=4N+1, the ground state of the two-dimensional electron gas in a double quantum well system in a quantizing magnetic field is, in some range of interlayer distances, a coherent striped phase. This stripe phase has one-dimensional coherent channels that support charged excitations in the form of pseudospin solitons. In this work, we compute the transport gap of the coherent striped phase due to the creation of soliton–antisoliton pairs using a supercell microscopic unrestricted Hartree–Fock approach. We study the energy gap as a function of interlayer distance and tunneling amplitude. Our calculations confirm that the soliton–antisoliton excitation energy is lower than the corresponding Hartree–Fock electron–hole pair energy.     

Contact correlations in one-dimensional systems of interacting fermions and the photoluminescence from quantum wires

We calculate the dependence of the carriers lifetime with the wire width in quantum wires by considering a strictly one-dimensional system of interacting electrons and holes. Confinement effects are taken into account through a width-dependent pair-potential proposed by Hu and Das Sarma. The carriers lifetime is then obtained from the inverse of the contact electron–hole correlations. We explain the change in the sign of the derivative at a critical temperature, as it is observed in photoluminescence experiments from In Ga As/InP quantum wires, by taking into account the carriers density dependence with temperature and assuming that the contact correlations are either just a two-body quantity or a many-body one for the lower and higher densities, respectively. In the former case, the system is viewed as an ionized excitonic gas, the pair correlation being the square of the two-body wave function for unbound states. In the latter, we have a metallic electron–hole system and we calculate the contact pair correlation in the many-body ladder approximation.     

Influence of the quantum dot shape on the determination of the electronic structure and electron decoherence

Theoretical calculations of electron–phonon scattering rates in AlGaN/GaN quantum dots (QDs) have been performed by means of effective mass approximation in the frame of finite element method. The influence of a symmetry breaking of the carrier's wave function on the electron dephasing time is investigated for various QDs shapes. In a QD system the electron energy increases when the QD shape changes from a spherical to a non-spherical form. In addition, the influence of the QD shape upon the electronic structure can be modulated by external magnetic fields. We also show that the electron–acoustic phonon scattering rates strongly depend upon both the QD shape and the applied magnetic field. As an additional parameter, the QD shape can be used to modify the electron–acoustic phonon interaction in a wide range. Moreover, the scattering rate of different transitions, such as Δm=0(1), presents distinct magnetic field dependency.     

Ion acceleration by the space charge electric force arising from the radiation pressure in a magnetized electron–positron plasma

It is shown that ions can be accelerated by the space charge electric force arising from the separation of electrons and positrons due to the ponderomotive force of the magnetic field-aligned circularly polarized electromagnetic (CPEM) wave in a magnetized electron–positron–ion plasma. The ion acceleration critically depends on the external magnetic field strength. The result is useful in understanding differential ion acceleration in magnetized electron–positron–ion plasmas, such as those in magnetars and in some laboratory experiments that aim to mimic astrophysical environments.     

Electron–hole complexes in self-assembled quantum dots in strong magnetic fields

We present results of calculations and experiments on electron–hole complexes in InGaAs/GaAs self-assembled quantum dots in high magnetic field (B). Due to hidden symmetries, the chemical potential of an N-exciton system at special B fields becomes insensitive to the exciton number as well as the magnetic field. This results in plateau regions of high intensity in measured magneto-PL spectrum. Theoretical calculations using exact diagonalization techniques successfully explain the measured magneto-photoluminescence spectrum with B fields up to 28 T.     

Coherent localization of two interacting carriers in one-dimensional quantum dot arrays

The coherent dynamics of two interacting carriers in one-dimensional quantum dot arrays driven by oscillating electric fields is theoretically investigated with the help of numerical calculations. The coherent localization of two electrons and that of an electron–hole pair are studied in this paper. For the two-electron case, the dynamic localization of the electrons is achieved when the Coulomb interaction is large enough. In this coherent localization, the Coulomb repulsion helps the electrons to be localized. For an electron–hole pair, although the dynamic localization of the composite particle does not occur due to charge neutrality, a different type of coherent localization can occur. These phenomena are explained by the quasienergy spectra based on Floquet analysis.     

Effect of electron–phonon interaction on the energy spectrum of a quantum antidot in the presence of a uniform strong magnetic field

We have studied the effect of electron–phonon interaction for small electron–phonon coupling on the electronic energy spectrum of an electron confined by a parabolic potential and a repulsive antidot potential in the presence of a uniform strong magnetic field and an Aharonov–Bohm flux field by using a variational procedure. We have shown that the presence of the antidot potential removes degeneracy of the Landau levels and electron–phonon interaction has nonnegligible effects on these levels.     

On the possibility of excitonic phases at the semiconductor–semimetal transition

Motivated by recent experimental evidences for pressure-induced exciton condensation in intermediate valent Tm[Se,Te] compounds, we re-examine, adopting a BEC–BCS crossover scenario, the formation and stability of exciton insulator versus electron–hole liquid phases.     

Theory of the excitonic Mott transition in quasi-two-dimensional systems

The excitonic Mott transition in single and double quantum wells is studied using the Green’s function technique. An abrupt jump in the value of the ionization degree, which happens with an increase of the carrier density or temperature, is found in a certain density–temperature region. The opposite effect–the collapse of the electron–hole plasma into an insulating exciton system–is predicted to occur at lower densities. The critical density of the Mott transition for spatially indirect excitons may be much smaller than that for direct excitons.     

Electron and phonon relaxation in metal films perturbed by a femtosecond laser pulse

A theoretical investigation of the electron and phonon time-dependent distributions in an Ag film subjected to a femtosecond laser pulse has been carried out. A system of two coupled time-dependent Boltzmann equations, describing electron and phonon dynamics, has been numerically solved. In the electron Boltzmann equation, electron–electron and electron–phonon collision integrals are considered together with a source term for laser perturbation. In the phonon Boltzmann equation, only electron–phonon collisions are considered, neglecting laser perturbation and phonon–phonon collisions. Screening of the interactions has been accounted for in both the electron–electron and the electron–phonon collisions. The results show the simultaneous electron and phonon time-dependent distributions from the initial non-equilibrium behaviour up to the establishment of a new final equilibrium condition. PACS 72.10.-d; 71.10.Ca; 63.20.Kr     

Electronic excitations of CO adsorbed on MgO(001)

We report ab initio calculations of the quasiparticle band structure and the optical excitation spectrum of bulk MgO, the MgO(001) surface, and CO molecules adsorbed on MgO(001). Many-body exchange and correlation effects are included within the GW approximation of the electron self-energy operator and the corresponding electron–hole interaction. The excited electron–hole states are obtained from the Bethe–Salpeter equation. At the clean MgO(001) surface exciton states are found with binding energies that are significantly stronger than in the bulk. The exciton spectrum of the adsorbate system CO:MgO is dominated by charge-transfer excitons, which couple strongly to the molecular excitations of CO. PACS 73.20.At; 73.20.Hb; 34.70.+e