Search Results (75 total)

We study the size of polyelectrolyte multilayer capsules as a function of ionic strength, temperature, and time. A dynamic micromechanical model is developed which successfully describes the experiments. The model includes the polymer-solvent surface tension, an electrostatic force which is strongly ionic strength dependent, and a temperature-dependent mobility parameter. The activation of >50  kT suggests that multiple ion pairs must be broken simultaneously in the process of chain rearrangement. In support of our physical model capsules can repeatedly swell and shrink by varying ionic strength.

The current flowing across a semiconductor superlattice in tilted electric and magnetic fields is known to exhibit resonant enhancement, when Landau states of neighboring wells align at certain ratios of the field strengths. We show that the ultrafast version of this effect, in which coherent electron wave packets are involved, has a profound analogy to the Fiske effect in superconductor Josephson junctions and superfluid weak links, in that the coupling of the tunneling-induced charge oscillations (magneto-Bloch versus Josephson oscillations) to another oscillator (in-plane cyclotron oscillations versus external oscillator modes) opens an elastic rectifying transport channel. We explore the superlattice effect both theoretically and experimentally, and find that the transient self-induced current can be adequately modeled if the damping of both types of coupled electron oscillations is properly taken into account.

An optically detected magnetic resonance (ODMR) study of Ga(Al)NAs alloys grown by molecular beam epitaxy on GaAs substrates is presented. A number of grown-in defects were observed which act as nonradiative recombination centers. A detailed analysis of experimental data using a spin Hamiltonian leads to the identification of two Ga_i defects. A comparison with similar defects in other phosphide-based diluted nitride III-V compounds, such as GaAlNP and GaInNP, allows us to obtain additional information about the nearest surrounding of the defects. A discussion of possible models for other defects observed in the experiments is also presented.

The two transverse polarization components P_T1 and P_T2 of the e⁺ from the decay of polarized μ⁺ have been measured as a function of the e⁺ energy. Their energy averaged values are ⟨P_T1⟩=(6.3±7.7±3.4)×10^-3 and ⟨P_T2⟩=(-3.7±7.7±3.4)×10^-3. From the energy dependence of P_T1 and P_T2 the decay parameters η,η^′′ and α^′/A,β^′/A are derived, respectively. Assuming only one additional coupling besides the dominant V-A interaction one gets improved limits on η, β^′/A, and the scalar coupling constant g_RR^S:  η=(-2.1±7.0±1.0)×10^-3, β^′/A=(-1.3±3.5±0.6)×10^-3, Re{g_RR^S}=(-4.2±14.0±2.0)×10^-3, and Im{g_RR^S}=(5.2±14.0±2.4)×10^-3.

For high electric fields, the lifetime of Wannier-Stark ladder states in a periodic potential is reduced by the fundamental process of Zener tunneling. We report on the analysis of the coherence lifetime of such states in semiconductor superlattices by interband spectroscopy. The reduction of lifetime by strong coupling between bands can only in the first approximation be described by the well-known Zener theory. A recently developed theoretical model is applied to calculate directly the tunneling probability of Wannier-Stark states as a function of the electric field. The theoretical results compare well with experiment, reproducing the complex interplay of both nonresonant and resonant Zener tunneling to higher bands. By comparing experiment and theory for a superlattice with a symmetric and one with a nonsymmetric potential, we can draw conclusions on a very general basis about the sensitive dependence of Zener tunneling on the specific dispersion relation of the carriers.

We present experimental and theoretical photocurrent spectra of a GaAs/AlGaAs superlattice under the influence of electric and magnetic fields parallel to the growth axis. In this field configuration, the charge carriers are subjected to zero-dimensional confinement. Depending on the fields, we observe a complicated interplay of Wannier-Stark and Landau levels. The experimental spectra are found to be in good agreement with numerical results.

We present a study of the Zener effect in the optical absorption of strongly coupled superlattices with both a magnetic and an electric field in growth direction. The in-plane continuum of electron states is discretized due to Landau quantization, which allows to directly observe the transition from discrete to continuum states due to Zener tunneling in a true 1D system.

We investigate the dynamics of a Bloch-oscillating wave packet in the presence of strong coupling to delocalized above barrier states (Zener tunneling), using time-resolved intraband polarization-sensitive measurements. At a threshold electric field, the resonance of localized and delocalized states causes a quantum beating which is observed as a revival in the intraband polarization. Our numerical simulation visualizes the spatial wave packet decomposition and reformation. The wave packet moves on a ps time scale over a distance of more than 100 nm and sequentially undergoes Bloch oscillations in the below- and above-barrier bands.

The coherent Hall effect denotes the transient Hall response of impulsively excited coherent charge-carrier wave packets in a solid. We report the first experimental study of this phenomenon (i) using a semiconductor superlattice in crossed electric and magnetic fields as a model for three-dimensional materials and (ii) employing a contactless optoelectronic technique to probe the transient currents. Two field regimes with distinctly different oscillatory wave packet dynamics are found, separated from each other by a transition region where all oscillations are suppressed.

Cyclotron resonances of electron space-charge layers in GaAs were studied at ³He temperatures covering a density regime from 2 to 13×10¹¹ cm^-2. For densities higher than about 6×10¹¹ cm^-2 many-body influences are sufficiently reduced such that line splittings due to band-structure influences were involved. At integer filling factors the spin-up and spin-down electrons are not completely decoupled as predicted by theory. Inter Landau-level optical gaps are renormalized at odd fillings, and there is a correlation gap between spin- and charge-density excitations in the long-wavelength limit.

We investigate the energy spectrum and the electron dynamics of a band in a semiconductor superlattice as a function of the electric field. Linear optical spectroscopy shows that, for high fields, the well-known localization of the Bloch states is followed by a field-induced delocalization, associated with Zener breakdown. Using time-resolved measurements, we observe Bloch oscillations in a regime where they are damped by Zener breakdown.

We present a detailed experimental study on photoluminescence quenching due to exciton and donor impact ionization by accelerated electrons under an in-plane nanosecond duration electric field created in GaAs/Al_0.35Ga_0.65As quantum wells. From the photoluminescence transients measured by the time-correlated single-photon counting technique, we have determined the experimental conditions under which donor impact ionization can have an influence on quenching of the excitonic photoluminescence. The coefficient of two-dimensional exciton impact ionization has been estimated; its dependences on the applied electric field, lattice temperature, and width of the quantum wells are given.

We observe that the oscillatory motion of photoinjected electron-hole pairs in a biased semiconductor superlattice (Bloch oscillation) is accompanied by a coherent quasi-dc current that is generated by the interaction of the carriers with the self-induced oscillating field. It is shown that this novel macroscopic quantum effect, which is a coherent analog of the Shapiro effect observed in Josephson junctions, can be controlled by changing the spectral position of the exciting laser pulse, which in turn determines the amplitude and phase of the wave packet oscillations. It is thereby possible to coherently drive the electrons either downwards or upwards in the potential of the static field.

The concentration p and the mobility μ of holes in metal-organic chemical vapor deposition (MOCVD) GaN:Mg layers were studied by room temperature Hall-effect measurements as a function of the Mg concentration N_A in the range 3×10¹⁸ cm^-3<~N_A<~1×10²⁰ cm^-3. The hole density first increases with increasing N_A, reaches a maximum value p_max≈6×10¹⁷ cm^-3 at N_A≈2×10¹⁹ cm^-3, decreases for larger N_A values, and drops to very small values at N_A≈1×10²⁰ cm^-3. The hole mobility decreases monotonically with increasing N_A. The p(N_A) data provide strong evidence for self-compensation, i.e., for a doping driven compensation of the Mg acceptor by intrinsic donor defects. This effect becomes significant when N_A exceeds a value of 2×10¹⁹ cm^-3. A semiquantitative self-compensation model involving nitrogen vacancies is developed. It accounts satisfactorily for the measured p(N_A) dependence and suggests that self-compensation limits the hole conductivity in bulklike MOCVD GaN:Mg layers grown near 1300 K to about 1.2 (Ω cm)^-1.

We have investigated the photon drag current that is excited by an infrared laser beam in the plane of the two-dimensional electron gas of GaAs/Al_0.35Ga_0.65As multiple-quantum-well systems. An analysis of the spectral response, measured with the picosecond infrared pulses of the wavelength-tunable free electron laser source FELIX, is presented for different doping schemes and examined as a function of temperature and intensity. The influence of the subband-selective scattering by a δ doping is explored, which demonstrates that the photon drag spectral response allows the determination of the momentum relaxation time ratio, R=τ₁/τ₂, of the electrons in the ground and excited subbands. The relaxation time ratio is found to be surprisingly constant over a large temperature range. The variation of the ratio with intensity can be attributed to heating of the electron gas, whose temperature exceeds 1000 K at saturation intensity.

We investigate coherent Bloch oscillations in GaAs/Al_xGa_1-xAs superlattices with electronic miniband widths larger than the optical phonon energy. In these superlattices the Bloch frequency can be tuned into resonance with the optical phonon. Close to resonance a direct coupling of Bloch oscillations to LO phonons is observed which gives rise to the coherent excitation of LO phonons. The density necessary for driving coherent LO phonons via Bloch oscillations is about 2 orders of magnitude smaller than the density necessary to drive coherent LO phonons in bulk GaAs. The experimental observations are confirmed by the theoretical description of this phenomenon [A.W. Ghosh et al., Phys. Rev. Lett. 85, 1084 (2000)].

We investigate the influence of scattering and coherent plasmon coupling on the temporal and spatial dynamics of Bloch-oscillating electrons in a semiconductor superlattice. We demonstrate that the dynamics are strongly influenced due to the scattering processes. For higher carrier densities, coupling to coherent plasmons leads to anharmonic Bloch oscillations since the static bias field is considerably changed by the oscillating carriers.

Bloch oscillations excited in a biased GaAs/Al_xGa_1-xAs superlattice are investigated in a time-resolved two-color electro-optic detection scheme. The detection of resonantly excited Bloch oscillations is based on the Pockels effect probed at a wavelength in the center of the band gap. The observed birefringence is induced by the macroscopic polarization of the electronic wave packets relative to the localized holes. The off-resonant detection away from optical transitions directly monitors the spatial dynamics of the electrons in amplitude and phase. The dependence of the amplitudes of the Bloch oscillating electrons on the applied electric fields are in good agreement with the electron-hole dipole lengths calculated by a quantum-mechanical model.

We have studied the intersubband resonance of GaAs/AlGaAs multi-quantum-well systems by comparing photon drag and absorption spectra obtained by in-plane photocurrent and photoconduction measurements. The peak absorption at room temperature is found to be blueshifted from the photon drag resonance by as much as 33 cm^-1. We argue that this difference gives directly the depolarization shift, since the resonant photon drag current is driven by the Doppler effect, which is a k-vector dependent single particle process.

Near infrared Brillouin scattering and high resolution x-ray diffraction is used for a precise determination of the elastic constants and the relaxed lattice parameters of Al_xGa_1-xAs epitaxial layers (0.1<~x<~1.0). The composition of the layers is specified by inductively coupled plasma atomic emission spectroscopy, photoluminescence, and Raman spectroscopy. For the elastic constants we get a composition independent value of 118.9±0.7 GPa for C₁₁, a nonlinear increase in C₁₂ and a linear decrease in C₄₄ with increasing Al composition. The Poisson ratio shows a linear increase for x<0.8 and a downward bowing for higher Al concentrations to the AlAs value of ν=0.325±0.004. The effect of lattice mismatch induced strain on the elastic properties is investigated on free standing epitaxial layers. The trend in ionicity from the GaAs to the AlAs bonds are deduced from phenomenological expressions for the bond-bending and bond-stretching forces which are calculated from the elastic constants. The lattice parameters of the unstrained crystals are obtained from the measured full metric of the tetragonally strained layers and the Poisson ratios. The combined results of Brillouin scattering, x-ray diffraction, and compositional analysis confirm the deviation of the Al_xGa_1-xAs lattice parameter from Vegard’s law, and provides the first direct and accurate determination of the quadratic bowing parameter.

Coherent zone-folded acoustic phonons are excited in GaAs/AlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions.

We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling.

We use a technique to measure the spatial dynamics of Bloch wave packets in semiconductor superlattices and investigate the dependence of the dynamics on the optical excitation conditions. For excitations well above or below the center of the Wannier-Stark ladder (WSL), the wave packets perform harmonic oscillations following the prediction of Zener; for excitations near the center of the WSL, the wave packets undergo a symmetric oscillation with virtually zero center-of-mass amplitude.

We demonstrate a new fast-scan ultrafast coherent reflectivity technique that tracks the amplitude and phase of excitons in GaAs heterostructures at densities down to 4×10⁶ cm^-2. Observed growth in the exciton coherence over the first 1.5 ps is direct evidence for the presence of interfering polariton modes. Polariton theory provides a good account of the data as the number of quantum wells is increased. Polariton motional narrowing of spectral lines from multiple quantum wells is demonstrated both experimentally and theoretically, annealing out the inhomogeneous broadening of the exciton energies.

The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.