Conductance of a quantum wire in a longitudinal magnetic field

We examine the ballistic conductance of a quantum wire in a parallel magnetic field in the presence of several impurities and derive analytic expressions for the transmission coefficient and the conductance in such a system. We show that scattering by impurities leads to a number of sharp peaks near the thresholds of the conductance quantization steps. The number of such peaks is determined by the distance between the impurities and the energy of the scattered particle. We also study the conductivity of a quantum wire in the region where the transport mechanism is diffusive. The conductivity is examined for the case in which charge carriers are scattered by randomly distributed point impurities. We study Shubnikov-de Haas oscillations in such a system. The general oscillation pattern consists of broad minima separated by irregularly spaced sharp peaks of the burst type. Zh. éksp. Teor. Fiz. 113, 1376–1396 (April 1998)     

Screening of the electron-hole interaction in quantum well structures

Recent optical non-linearities in GaAs/Ga_1?xAl_xAs quantum well structures have been attributed to the screening of the electron-hole interaction in such structures by the free carriers created. We here present the results of a variational calculation of the ground state energy of an electron-hole pair confined to move in two dimensions when screening of the Coulomb interaction between the electron-hole pair by the free carriers present is taken into account by using the screened potential obtained by Stern and Howard for hydrogenic impurities in semiconducting inversion layers. It is found that the binding energy of the 2D exciton decreases less rapidly with the screening parameter than is the case for a 3D exciton and that the 2D exciton remains bound even for large values of the screening parameter. This is in contrast to the case in bulk semiconductors where the electron-hole pair no longer bind into an exciton when the screening length of the free carriers becomes less than the Bohr radius of the exciton.     

Ferromagnetism of semiconducting structures with the indirect interaction of magnetic impurities through a quasi-two-dimensional conduction channel

Ferromagnetism in a layer of magnetic impurity atoms indirectly interacting through a quasi-two-dimensional conduction channel in the nearest quantum well is considered in the mean-field model. The interaction appears through the RKKY mechanism due to the extension of the wavefunction of charge carriers localized in a model triangular well to the region where the impurities are localized. The Curie temperature and its dependence on the carrier density in the well and well depth are determined. The results are compared with the experimental data concerning the structures based on the diluted magnetic semiconductors Ca_1?x Mn _x As.     

Quantum Discord in Scattering Processes by Fixed Spin Impurities

The dynamic behavior of the quantum discord in one-dimensional scattering of a qubit (a spin-1/2 particle) by single and double well-localized fixed spin impurities is investigated theoretically. It is assumed that the incident particle is scattered by the spin impurities through the Ising and/or Heisenberg interactions. These potentials create quantum mechanical correlation between the reflected and transmitted parts of the scattered system and the impurities. It is shown that the incident momentum, strength of the interaction potentials, and the separation between the impurities can be regarded as the control parameters for the quantum discord and concurrence manipulations. In particular, it has been found that the correlations are periodic functions of the wavelength of the incident particle when it is scattered by the double spin impurities.     

Barrier height effect on binding energies of shallow hydrogenic impurities in coaxial GaAs/ AlxGa1−xAs quantum well wires under a uniform magnetic field

The ground state binding energies of axial hydrogenic impurities in a coaxial cylindrical quantum well wire are reported as a function of the barrier height and the radius of wire in the presence of a uniform magnetic field applied parallel to the wire axis. The quantum well wire (QWW) is assumed to be an infinitely long cylinder of GaAs material surrounded by Al_xGa_1−xAs (for finite case and vacuum for infinite case). Binding energy calculations were performed with the use of a variational procedure in the effective mass approximation. We observed that the binding energy is sensitive to well radius only for both larger R$R$ values and small magnetic fields. We also compared the infinite and finite case binding energies and showed that increasing the Al concentration in the finite barrier case, binding energies are increased as expected. Our results are in good agreement and complementary with the previous theoretical works.     

Ferromagnetism in heterostructures based on a dilute magnetic semiconductor

Ferromagnetism of magnetic impurity atoms located in the barrier regions of various heterostructures (solitary heterojunction, single quantum well, double quantum well, or superlattice) is considered theoretically. The indirect magnetic interaction of impurities occurs via charge carriers localized in quasi-two-dimensional conducting channels of these structures due to “penetration” of the wavefunction of charge carriers into the barrier regions. The wavefunctions defined analytically in the triangular potential model are virtually the same as in “exact” numerical calculations (joint solution of the Poisson and Schrödinger equations). The corresponding Curie temperatures are determined, which may attain approximately 500 K in Ga_{1 ? x} Mn _x As-based structures according to calculations.     

Reduction in relaxation time due to ionized impurities in GaAs/AlGaAs quantum well structures

Time-resolved photoluminescence measurements in δ -doped GaAs/AlGaAs on the quantum well structures are performed to study effects of ionized impurities relaxation process of photoexcited carriers. It is theoretically shown that a thin quantum well with a δ -doping layer inserted in the barrier layer of double quantum wells enhances the impurity scattering rate significantly. Photoluminescence decay time in the δ -doped samples is found to decrease compared with the undoped samples.     

Effect of Magnetic-Impurity Scattering in Disordered Two-Dimensional Square Lattices Around Half Filling

Weak-localization effect in the presence of magnetic impurities is studied in disordered two-dimensional tight-binding square lattices around half filling. Both the magnetic and nonmagnetic impurities are assumed to be randomly distributed on small fractions of the sites, while the nonmagnetic impurities have a strong potential yielding a unitary-limit scattering. We derive in details the expressions of diffusive π modes in the retarded-retarded (or advanced-advanced) channel, which result from the existence of particle-hole symmetry. The quantum interference correction to the density of states is calculated. While the magnetic-impurity scattering suppresses the quantum correction from π-mode cooperon, it does not affect the contribution of π-mode diffuson.     

Photoluminescence studies of GaAs/GaAlAs multiple quantum well heterostructures

The photoluminescence excited by He:Ne and Nd:YAG lasers of GaAs/Ga_0.75Al_0.25As multiple quantum well heterostructures grown by MBE was measured as a function of temperature from 4.2 K up to room temperature and for different pumping powers at constant temperature. The excitonic transitions associated with carriers confined in the quantum wells as well as other transitions associated with impurities either already present in the substrates or introduced into the samples during growth are identified in the spectra and fully characterized. From Arrhenius plots of the photoluminescence peak integrated intensities versus inverse temperature, activation energies are estimated for acceptor defects in the samples as well as for quantum well related excitonic transitions. Photoluminescence polarization experiments demonstrate a dramatic manifestation of the selection rules governing heavy hole and light hole optical transitions in quantum wells.     

The role of spatial dispersion of an electromagnetic wave in its penetration through a quantum well

The theory of light penetration through a quantum well in a strong magnetic field perpendicular to the well plane is developed under the conditions where interband transitions occur in the well. The light wavelength is assumed to be comparable to the well width. The relationships for the reflection, absorption, and transmission are derived with due regard for the spatial dispersion of a monochromatic light wave and the difference between the refractive indices of the quantum well and the barrier. The normal incidence of light with respect to the well plane is considered, and one excited level is taken into account. It is demonstrated that the above two factors most strongly affect the reflection, because the reflection from the well boundaries appears in addition to the reflection caused by interband transitions in the quantum well. The most radical changes in the reflection are observed in the case when the reciprocal radiative lifetime of the excited state in the quantum well is short compared to the reciprocal nonradiative lifetime. In the range of large well widths, the applicability of the theory is limited by the existence condition of quantum well levels.     

Measurement-induced nonlocality and geometric quantum discord in two SC-charge qubits

By using geometric quantum discord and measurement-induced nonlocality, quantum correlations are investigated for two superconducting (SC) charge qubits that share a large Josephson junction where the field is assumed to be prepared initially in a coherent state. It is found that the difference between measure measurement-induced nonlocality and geometric quantum discord, of the final state of the two SC-charge qubits system which is especial case of X-states, is equal to a constant value. It is found that the quantum correlations and entanglement of the qubits are very sensitive to the mean number of the coherent photons. The entanglement exists in small intervals of death quantum discord and measurement-induced nonlocality. This is further evidence in support of the fact that quantum correlation and entanglement are not synonymous.     

Free-carrier absorption in quantum well structures for acoustic phonon scattering

Free-carrier absorption has been studied for quantum well structures fabricated from III-V semiconducting materials where the acoustic phonon scattering is important. The energy band of carriers is assumed to be nonparabolic. We discuss the effect of acoustic phonon scattering on the free-carrier absorption for both deformation-potential coupling and piezoelectric coupling. It is found that the free-carrier absorption coefficient depends upon the polarization of the electromagnetic radiation relative to the layer plane or quantum well, the photon frequency, and the temperature. When the deformation-potential coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane or perpendicular to the layer plane. However, when the piezoelectric coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane, but for photons polarized perpendicularly to the layer plane, the free-carrier absorption coefficient decreases with increasing temperature. Moreover, at high temperatures such as T = 300 K, the free-carrier absorption coefficient oscillates with the film thickness in a small quantum well region and then decreases monotonically with increasing the film thickness. This is different from the result for three-dimensional semiconducting solids.     

Effect of spatial dispersion on the shape of a light pulse propagating through a quantum well

The reflection, transmission, and absorption of a symmetric electromagnetic pulse are calculated. The carrier frequency of the pulse is close to the frequency of direct interband transitions in a quantum well (QW). The QW energy levels are assumed to be discrete, with two closely spaced excited levels being taken into account. The QW width is assumed to be sufficiently large and comparable to the light wavelength corresponding to the pulse carrier frequency. In this case, the dependence of the momentum matrix element for the interband transition on the light wave vector should be taken into account. The refractive indices of the QW and barriers are assumed to be equal. The problem is solved for an arbitrary relation between the radiative and non-radiative lifetimes of the excited electronic states. It is shown that spatial dispersion considerably affects the shapes of the reflected and transmitted pulses. The greatest changes occur in the case where the inverse radiative lifetime is close to the difference between the frequencies of the interband transitions considered.     

Optical detection of cyclotron resonance for characterization of recombination processes in semiconductors

Novel applications of the technique of optically detected cyclotron resonance (ODCR) are discussed. This method is an extension of the conventional cyclotron resonance investigations and shows important advantages when applied to characterization of semiconductor materials. These advantages are due to a higher sensitivity and a longer momentum relaxation time caused by photoneutralization of ionized impurities. This in turn enables experiments at lower magnetic fields and lower microwave radiation frequency. Photoexcitation used in ODCR often results in a simultaneous observation of electron and hole cyclotron resonances in the same sample, which is a rare case in a conventional CR study. High magnetic field far infrared ODCR experiments utilize all these advantages of the method. For the most common X-band (10 GHz) microwave setups, the ODCR resolution often is too low to allow accurate CR determination of the band structure parameters of the material studied. In that case, ODCR may be used for high-resolution photoluminescence experiments and for identification of carrier capture and recombination paths at impurities, as was proposed recently. These new and important applications of the ODCR technique are described here, and documented by recent experimental results. The mechanism of ODCR detection for pure and doped semiconductors is discussed, and a prominent role of the impact ionization mechanism is demonstrated. This is followed by a discussion of recent applications of ODCR for identification of recombination mechanisms. Then high spectral resolution ODCR experiments are described, with the example of overlapping free-to-bound and donor-acceptor pair transitions. Some special applications of ODCR are demonstrated. ODCR also can be applied to analyze quantum confinement of carriers in two-dimensional (2D) structures such as heterojunctions and quantum wells. It is shown that useful information can be obtained on the electronic properties of the 2D electron (or hole) gas and the interlayer carrier transfer enhanced by carrier heating at CR absorption in such structures.     

Optical and electronic properties of quantum dots with magnetic impurities

The article discusses some of the recent results on semiconductor quantum dots with magnetic impurities. A single Mn impurity incorporated in a quantum dot strongly changes the optical response of a quantum-dot system. A character of Mn-carrier interaction is very different for II-VI and III-V quantum dots (QDs). In the II-VI QDs, a Mn impurity influences mostly the spin-structure of an exciton. In the III-V dots, a spatial localization of hole by a Mn impurity can be very important, and ultimately yields a totally different spin structure. A Mn-doped QD with a variable number of mobile carriers represents an artificial magnetic atom. Due to the Mn-carrier interaction, the order of filling of electronic shells in the magnetic QDs can be very different to the case of the real atoms. The “periodic” table of the artificial magnetic atoms can be realized in voltage-tunable transistor structures. For the electron numbers corresponding to the regime of Hund's rule, the magnetic Mn-carrier coupling is especially strong and the magnetic-polaron states are very robust. Magnetic QD molecules are also very different to the real molecules. QD molecules can demonstrate spontaneous breaking of symmetry and phase transitions. Single QDs and QD molecules can be viewed as voltage-tunable nanoscale memory cells where information is stored in the form of robust magnetic-polaron states. To cite this article: A.O. Govorov, C. R. Physique 9 (2008).     

Magnetic-field dependence of hydrogenic impurity states in a quantum well wire

Summary The binding energy for on-centre impurities in a rectangular quantum well wire is calculated as a function of the width of the wire and perpendicular magnetic field. The results for zero-magnetic-field case are in perfect agreement with previous calculations. The authors of this paper have agreed to not receive the proofs for correction.     

Entanglement dynamics in two-qubit open system interacting with a squeezed thermal bath via quantum nondemolition interaction

We analyze the dynamics of entanglement in a two-qubit systeminteracting with an initially squeezed thermal environment via aquantum nondemolition system-reservoir interaction, with the systemand reservoir assumed to be initially separable. We compare andcontrast the decoherence of the two-qubit system in the case wherethe qubits are mutually close-by (`collective regime’) or distant(`localized regime’) with respect to the spatial variation of theenvironment. Sudden death of entanglement (as quantified byconcurrence) is shown to occur in the localized case rather than inthe collective case, where entanglement tends to `ring down’.A consequence of the QND character of the interaction is that thetime-evolved fidelity of a Bell state never falls below \(1/\sqrt{2}\),a fact that is useful for quantum communication applicationslike a quantum repeater. Usinga novel quantification of mixed state entanglement, we show thatthere are noise regimes where even though entanglementvanishes, the state is still available forapplications like NMR quantum computation, because of the presenceof a pseudo-pure component.     

Resonant impurity bands in semiconductor superlattices

It is shown that the confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, which also requires reinterpretation of previous data.     

Effect of high magnetic fields on the conductivity of a quantum cylinder under Stark ladder conditions

The conductivity of a quantum cylinder with a parabolic lateral confinement potential and a superstructure is studied under conditions where uniform static quantizing electric and magnetic fields are applied along the cylinder axis. The charge carriers are assumed to be scattered by optical phonons. The dependence of the current density along the superlattice axis on the dc magnetic field is obtained. It is shown that, under certain conditions, the so-called Stark-hybrid-phonon resonance appears due to the hybridization of the electronic energy spectrum. In turn, this gives rise to a sharply nonmonotonic magnetic-field dependence of the current density.