Showing papers in "Physica Status Solidi B-basic Solid State Physics in 1998"
TL;DR: In this paper, the amplitude and phase of weak ultrashort pulses are characterized using a spetrally resolved cross-correlation signal of a weak test pulse with a fully characterized intensive reference pulse.
Abstract: We present a new method to characterize the amplitude and phase of weak ultrashort pulses. Our method is based on the spetrally resolved crosscorrelation signal of a weak test pulse with a fully characterized intensive reference pulse and requires no spectral overlap between the signal and the reference. To retrieve the amplitude and phase of the test pulse, we use an iterative Fourier transform algorithm with generalized projections.
284 citations
TL;DR: In this paper, the authors present results from solutions of the semiconductor Kadanoff-Baym equations with self-energies in quasistatic Born approximation, for GaAs single quantum wells.
Abstract: We present results from solutions of the semiconductor Kadanoff-Baym equations (full two-time semiconductor Bloch equations) with self-energies in quasistatic Born approximation, for GaAs single quantum wells. We concentrate on memory and correlation effects under fs-pulse excitation conditions. A remarkable feature is the observed kinetic energy increase which is due to the build-up of correlations among the generated carriers. We demonstrate that the two-time approach is (i) very well suited to study correlation phenomena both on short and long times, thereby avoiding well-known problems of one-time kinetic equations, and (ii) that it is becoming practical with the use of efficient integration techniques.
56 citations
TL;DR: In this article, a systematic theoretical investigation of intersubband optical absorption in AlGaAs/AlAs/InGaAs strained double barrier quantum well (SDBQW) structure is presented for the first time.
Abstract: A systematic theoretical investigation of intersubband optical absorption in AlGaAs/AlAs/InGaAs strained double barrier quantum well (SDBQW) structure is presented for the first time. Electron states in a SDBQW are found to be adequately described within the framework of the envelope function approach which includes the effects of subband nonparabolicity and strain. The linear and the third-order nonlinear intersubband optical absorption are calculated using the density matrix formalism in which intrasubband relaxation is taken into account. Subband nonparabolicity and elastic strain are found to significantly influence both electron states and intersubband optical absorption. In the case of small well width, the inner-barrier thickness affects both electron states and optical absorption. ()ur results can be used in design of high performance quantum well infrared photodetectors operating in the important wavelength region between 3 and 5 μm.
46 citations
TL;DR: In this paper, the effect of spatial disorder on the band spectrum of superlattices by means of the transmission coefficient and the Lyapunov exponent was investigated using a KP model with randomly distributed well widths.
Abstract: We use a Kronig-Penney (KP) model with randomly distributed well widths to study the effect of spatial disorder on the band spectrum of superlattices by means of the transmission coefficient and the Lyapunov exponent. It is mainly found that the eigenstates in the allowed miniband become localized while in the gap edge there is a delocalization contributing to the broadening of the allowed miniband of the corresponding ordered superlattice. The effect of disorder on the T-V curves is also examined. We found that the Stark localization behaves differently in the gap and in the allowed band and tends to be suppressed. Further discussion and comparison with other theoretical and experimental investigations are also provided.
43 citations
TL;DR: In this paper, the authors examined X-ray diffraction, photoluminescence and far-infrared reflectivity of two Si-doped GaN layers and found that the incorporation of silicon depends on the side used for deposition.
Abstract: Gallium nitride doped with oxygen (unintentionally), silicon and magnesium was grown by metalorganic chemical vapor deposition on the conductive single crystals of GaN grown at high hydrostatic pressure. The layers were examined using X-ray diffraction, photoluminescence and far-infrared reflectivity. It was found that the incorporation of silicon depends on the side used for deposition. For the two Si-doped layers grown in the same run, the one grown on the (00.1) side (gallium-terminated) had always smaller free electron concentration with respect to the (00._1) side (nitrogen-terminated). This conclusion could be drawn from the lattice expansion by free electrons, the photoluminescence peak shift by Burstein-Moss effect and the position of plasma edge in far-infrared reflectivity.
42 citations
TL;DR: In this paper, the moments of the spectral density of a single-band Hubbard model were analyzed with respect to their compatibility with the moment sum rule up to the order ma 3 for a qualitatively correct description of the quasi-particle band structure in the strong correlation regime.
Abstract: The sum rule for the moments of the spectral density is discussed for the single-band Hubbard model. It is shown that respecting the sum rule up to the order ma 3 is conceptually important for a qualitatively correct description of the quasi-particle band structure in the strong-correlation regime. Different analytical approximations for the self-energy are analyzed with respect to their compatibility with the moment sum rule. To estimate the practical usefulness of the sum rule, correlation functions and dynamical quantities are determined. The results obtained within the various approximation schemes of different complexity are compared with each other and also with essentially exact results available for infinite-dimensional lattices. It turns out that the ma 3 moment is rather unimportant for the paramagnetic phase on the hyper-cubic lattice. Contrary, it decisively influences the magnetic phase boundary as well as the critical temperature for the ferromagnetic phase on an f.c.c.-type lattice.
41 citations
TL;DR: In this paper, the optical transitions associated with localized levels of a rare-earth impurity can be modulated by quantum confinement and the effect of quantum confinement on impurity critically depends on the size of the host crystal.
Abstract: We report for the first time how the optical transitions associated with localized levels of a rare-earth impurity can be modulated by quantum confinement. The effect of quantum confinement on impurity critically depends on the size of the host crystal. The variation of absorption and luminescence efficiency of Tb3+-ion doped Y2O3 is discussed within the framework of the quantum confined atoms.
41 citations
TL;DR: In this paper, the authors obtained qualitative results that imply charge localization around negative curvature disclinations (i.e., rings with more than six carbon atoms) in graphite sheets.
Abstract: Using a continuum model, we obtain qualitative results that imply charge localization around negative curvature disclinations (i.e. rings with more than six carbon atoms) in a graphite sheet. Conversely, it is found that positive curvature disclinations repel charge, independent of its sign.
40 citations
39 citations
TL;DR: In this paper, a complete analysis of the line shape of the excitonic absorption and photoluminescence lines of the yellow series of Cu 2 O is given, and a detailed fit to the absorption lines up to the n = 5 exciton according to Toyozawa's theory is given.
Abstract: A complete analysis of the line shape of the excitonic absorption and photoluminescence lines of the yellow series of Cu 2 O is given. A detailed fit to the absorption lines up to the n = 5 exciton according to Toyozawa's theory gives precise values for the excitonic resonance energies, allowing to calculate the excitonic Rydberg. The fit to the photoluminescence lines shows very good agreement with experimental data at different temperatures. At high excitation, the fit to the excitonic absorption lines reveals the fundamental mechanism for bleaching of exciton absorption lines in Cu 2 O. Based on these findings, a new all-optical method for the observation of exciton transport in Cu 2 O is proposed.
38 citations
TL;DR: In this article, the wave vector dependence of the E 2g optical phonon frequencies and linewidths in the hexagonal close-packed metal, osmium, was studied at 10 K.
Abstract: The wave vector dependence of the E 2g optical phonon frequencies and linewidths in the hexagonal close-packed metal, osmium, was studied at 10 K. A strong anisotropic dispersion, as well as damping thresholds, were observed for two transverse branches in the high-symmetry directions of the Brillouin zone around a phonon wave vector magnitude of 10 6 cm -1 . Simultaneously, a continuum was found in the Raman spectra, presumably of electronic nature, whose q-dependence correlates with that the phonon self-energies. Numerical simulations of the electronic Raman response in terms of an intraband mechanism were performed with a model energy-band spectrum reproducing the main features of the Fermi surface anisotropy of Os. A large discrepancy was found between the calculated and the measured anisotropy of the electronic scattering with respect to momentum direction and polarization configuration. The possible assignment to a contribution of resonant terms to the Raman vertex is discussed.
TL;DR: In this paper, the vibrational modes are classified by the factor group analysis method and the spectral bands are compared with those of parent in order to propose a tentative assignment of the observed modes.
Abstract: Single crystals of bis(thiourea)zinc chloride, Zn[CS(NH 2 ) 2 ] 2 Cl 2 (BTZC), were grown by slow evaporation of an aqueous solution at room temperature. The crystal has been studied at room temperature by Raman spectroscopy between 20 and 1700 cm -1 and 3000 and 3500 cm -1 and by infrared transmittance in the 400 to 4000 cm -1 wavenumber range. The vibrational modes are classified by the factor group analysis method and the spectral bands are compared with those of parent in order to propose a tentative assignment of the observed modes. Preliminary second harmonic generation measurements have allowed to identify the C 2v 9 space group symmetry of the compound.
TL;DR: In this paper, large scale simulations for a model including static interface-related disorder and acoustic phonon scattering are presented, and strong and systematic deviations between calculated photoluminescence excitation spectra and absorption are found.
Abstract: Exciton spectra of narrow quantum wells with non-ideal interfaces are determined by the nature of the disorder-localized quantum mechanical eigenstates. We point out that the level statistics of spatially resolved spectra allows an unexpected approach to estimate the size of the quantum mechanical center-of-mass wavefunctions. In photoluminescence, the exciton relaxation has to be considered, too. We present large scale simulations for a model including static interface-related disorder and acoustic phonon scattering. Strong and systematic deviations between calculated photoluminescence excitation spectra and absorption are found. A sharp drop on the low-energy side of the former is discussed in terms of an effective mobility edge for exciton relaxation below which excitons cannot relax further within their radiative lifetime
TL;DR: In this paper, the authors reported magnetotransport of (Ga, Mn)As below 1 K in the reentrant insulating phase, where the external magnetic field drove the samples from the strongly insulating regime to the variable range hopping one.
Abstract: We report magnetotransport of (Ga, Mn)As below 1 K in the reentrant insulating phase. The external magnetic field drove the samples from the strongly insulating regime to the variable range hopping one. Below 1 K, the resistivity was strongly anisotropic (by about two orders of magnitude). The conduction along the highly resistive direction ([11-0]) was well described by variable range hopping in the soft Coulomb gap regime while that along the lower resistive direction ([110]) seemed to undergo an insulator-to-metal transition by the external magnetic field. The result may be a key to solve the problem of reentrant metal-to-insulator transition in (Ga, Mn)As with increasing Mn content.
TL;DR: In this article, two mechanisms of the inverse population of shallow impurity states in silicon under optical pumping have been proposed and analyzed, using a procedure allowing to reduce the number of required matrix elements of transitions.
Abstract: Two mechanisms of the inverse population of shallow impurity states in silicon under optical pumping have been proposed and analyzed, using a procedure allowing to reduce the number of required matrix elements of transitions. The first mechanism is based on the resonance interaction of the 2p0 state in Si : Bi with optical phonons. The other one is based on the suppression of acous- tic-phonon-assisted relaxation from the 2p0 state in Si : P due to the momentum conservation law. Spontaneous emission was registered from shallow donors in Si : P under photoionization by a CO2 laser. The dependence of the spontaneous emission intensity on the intensity of pumping radiation confirms the possibility of amplification on impurity transitions. Introduction. The interest in far-infrared (FIR) active media based on shallow impu- rity states in silicon is caused by two reasons. The first one is the low level of lattice absorption of FIR radiation in silicon. The second is the cascade character of the main relaxation processes along the excited coulombic impurity states (1), allowing to expect high efficiency of pumping of impurity excited states population, which is important for reaching continuous lasing. Cascade relaxation means that transitions with a small re- duction of carrier energy are predominating, and the probability that a heated carrier takes part in the amplification is rather high when one of the first excited states or the group of excited states within the step of phonon relaxation are inversely populated. On the other hand, the fast acoustical-phonon-assisted relaxation causes the main complication in obtaining the inverse population of impurity states, since it tends to form the equilibrium distribution at lattice temperature. Thus, the inverse population of impurity states implies conditions in which the distribution is formed by processes with threshold character (interaction with optical phonons, optical pumping) and being faster than acoustical-phonon-assisted and Auger processes, or it implies conditions, where the latter are suppressed for particular states. The other complication originates from the fact that absorption for impurity transitions lies in the same frequency region as possi- ble amplification and thus can prevent it. Hence the possibility of amplification depends on the details of nonequilibrium distribution of charge carriers over excited impurity states.
TL;DR: In this article, the authors performed a plane-wave supercell analysis for the neutral monovacancies in silicon, silicon carbide and diamond using non-norm-conserving Vanderbilt pseudopotentials.
Abstract: Ab initio plane-wave-supercell calculations are performed for the neutral monovacancies in silicon, silicon carbide and diamond using ultrasoft non-normconserving Vanderbilt pseudopotentials, We study the structure, the energetics and the single-particle energy spectrum. The local symmetry, the atomic displacements, and the formation energies are found to be sensitive with respect to the numerical details, The convergence of the calculations with the supercell size is demonstrated, We find a tendency for the stabilization of the tetragonal D 2d symmetry for the vacancy in Si and the C vacancy in SiC, whereas the Si vacancy in SiC and the vacancy in diamond nearly conserve the T d symmetry. The structural changes are related to the electronic defect states. Defect levels are derived lor the four vacancies under consideration.
TL;DR: In this article, the binding energy of a shallow hydrogenic impurity on center located in cylindrical shape was calculated as a function of length, radius of the structure, and intensity of a magnetic field applied in axial direction.
Abstract: Using a variational procedure within the effective-mass approximation we calculated the binding energy of a shallow hydrogenic impurity on-center located in cylindrical shape GaAS–(Ga,Al)As low-dimensional systems, such as: quantum-well, quantum-well wires, and quantum dots, considering an infinite confining potential in all surfaces of the structures. The binding energy of the ground state is calculated as function of length, radius of the structure, and intensity of a magnetic field applied in axial direction. We show that the binding energy of the impurity decreases with both the length and radius of the structure, whereas it increases with the applied magnetic field. Finally, we show that our results are in good agreement with those obtained in previous calculations in GaAs–(Ga,Al)As quantum wells, quantum-well wires and quantum dots of comparable dimensions. We consider future understanding of optical phenomena related with donor impurities in GaAs–(Ga,Al)As low-dimensional systems in which the effects of applied magnetic fields compete with the quantum confinement, must take into account these results.
TL;DR: In this article, the authors studied the pressure-volume relationship and pressure induced structural phase transfor-mation in ytterbium pnictide compounds (YbN, YbP and YbAs) for the first time by using an atomic potential approach based on the rigid ion model which considers only Coulomb and Born-Mayer type short-range interactions.
Abstract: Materials Research Laboratory, Department of Physics, University of Bhopal,Bhopal-462026, India(Received July 14, 1997)We have studied the pressure–volume relationship and pressure induced structural phase transfor-mation in ytterbium pnictide compounds (YbN, YbP and YbAs) for the first time by using aninteratomic potential approach based on the rigid ion model which considers only Coulomb andBorn-Mayer type short-range interactions. The theoretically predicted pressure–volume relationshipcurves show that YbN, YbP and YbAs exhibit normal compression and undergo a NaCl to CsCltype structural transition at 164, 115 and 95 GPa, respectively, similar to the europium chalco-genides but contrary to their counterparts Yb chalcogenides for which the compression curve showsan anomalous behaviour due to the change in the valence state. The present study also confirmsthat these compounds behave similar to the simple rare earth europium chalcogenide compoundsrather than the rare earth mixed valence compounds. A similar conclusion is also drawn from ourearlier studies on the phonon properties for these compounds. The nature of the force constants atequilibrium and high pressure is discussed.
TL;DR: The lattice vibrational spectrum of KNSBN, which arises mainly from internal vibrations of the Nb-O octahedra and studied by Raman and infrared reflectivity spectroscopy, typifies a tetragonal TB type structure as mentioned in this paper.
Abstract: Potassium and sodium (K/Na)-modified strontium barium niobate (SBN) of the idealized formula (K x Na 1 x)0.4(SryBa1-y)0.8Nb2O6 (KNSBN). with x = 0.75. y = 0.60, a Curie temperature of (200 ± 2) C, and a laser damage threshold larger than 600 MW/cm 2 at 4 pps, crystallizes with a filled tungsten bronze (TB) type structure in the tetragonal system. The lattice constants are a = (1.2488 ± 0.0001) and c = (0.3955 ± 0.0001) nm at room temperature. There are five formulas per unit cell. The lattice vibrational spectrum of KNSBN, which arises mainly from internal vibrations of the Nb-O octahedra and studied by Raman and infrared reflectivity spectroscopy, typifies a tetragonal TB type structure. The results shown in Figs. 1 to 6 are typical for vibrational spectra, The NbO 6 clusters form the rigid Nb-O octahedra, and the rigidity of the lattice is mainl ascribed to five vibrations, as a non-degenerate symmetric Nb-O stretching vibration (v 1 ), a doubly degenerate symmetric O-Nb-O stretching vibration (v 2 ). >a >triply |>|>>degenerate >antisymmetric >Nb-O>stretching >vibration >(v 3 ). a triply degenerate antisymmetric O-Nb-O bending vibration (v 4 ), and a triply degenerate symmetric O-Nb-O bending vibration (v 5 ). The KNSBN crystals as well as the SRN crystals show good structural quality and mechanical properties, and large linear electro-optical and pyroelectric coefficients. Besides, the nonlinear optical property of KNSBN is better than that of SBN,
TL;DR: In this paper, phase transitions in zinc tris (thiourea) sulfate (ZTS) were investigated by measuring polarized Raman scattering between 10 and 300 K in the 400 to 800 cm -1 range.
Abstract: Phase transitions in zinc tris (thiourea) sulfate (ZTS), occurring below room temperature at (60 ± 2) K and (122 ± 2) K, respectively, are investigated by measuring polarized Raman scattering between 10 and 300 K in the 400 to 800 cm -1 range. A careful examination of the evolution of the wavenumber, intensity, and width of thermosensitive bands, appearing at 520 cm -1 in the spectra at 10 K, near the transition temperature, is helpful in deciding about the type of phase transition. The transitions are found to be of an order-disorder nature. The values of the estimated critical exponents are 2β = 0.57, δ = 0.92 for the transition at 60 K and 2β = 0.92, δ = 0.77 for the transition at 122 K. It is determined that the mechanism of the transition appears to be closely related to the behavior of the thiourea molecule.
TL;DR: In this paper, the critical collapsing of the Coulomb gap in moderately compensated NTD Ge:Ga and the metal-insulator (MI) transition in this system had been investigated and the results obtained are compared with those received for strongly and weakly compensated Ge, where single electron CG takes place far from the MI transition.
Abstract: Critical collapsing of the recently discovered multielectron Coulomb gap (CG) in moderately compensated neutron transmutation doped (NTD) Ge:Ga and the metal–insulator (MI) transition in this system had been under investigation. The results obtained are compared with those received for strongly and weakly compensated Ge, where single electron CG takes place far from the MI transition. Independently of the single- or multielectron origin all the gaps, as deduced by the variable range hopping (VRH) spectroscopy procedure, turned out to be collapsing in the critical point of the MI transition just the same where the low temperature metallic conductivity vanishes. One can consider such MI transition as the CG collapsing phenomenon and describe it in the frame of scaling theory as a second-order phase transition. In compensated semiconductors it is characterised by the critical index for a correlation length nearly equal to unity.
TL;DR: In this paper, a numerical approach for the calculation of thermodynamics and spin correlations for spin-1/2 XY chains is presented, which can be used for the interpretation of dynamic experiments on quasi-one-dimensional crystals that can be treated within the frames of the spin 1/2 transverse Ising model.
Abstract: The paper presents a numerical approach for the calculation of thermodynamics and spin correlations for spin-1/2 XY chains. To illustrate the approach we performed exact finite-chain calculations of all time-dependent two-spin correlation functions for the spin-1/2 Ising chain in the transverse field. The computed correlation functions permit to evaluate the frequency-dependent susceptibilities and structure factors for this model. The obtained results are discussed in detail. They may have an application for the interpretation of dynamic experiments on quasi-one-dimensional crystals that can be treated within the frames of the spin-1/2 transverse Ising model.
TL;DR: In this article, three families of shallow thermal donors have been identified in annealed Czochralski silicon from measurements of their infrared electronic transitions using Fourier transform spectroscopy.
Abstract: Three families of shallow thermal donors have been identified in annealed Czochralski silicon from measurements of their infrared electronic transitions using Fourier transform spectroscopy. Donors produced in Al-doped Si incorporate an Al impurity; centers produced in hydrogenated material contain an H (or D) atom; a third set of donors labeled STD(X)N produced in nitridated or irradiated pre-hydrogenated samples may incorporate a lattice vacancy rather than a nitrogen atom. A critical review of the literature is made and some rationalization has been effected. Comments are included about the formation of donor centers following anneals of heavily damaged float-zone Si that also contains hydrogen.
TL;DR: In this paper, a variant of the 2D Anderson model is studied, where the disorder is represented by randomly chosen hopping terms and the density of states reveals an abnormally strong peak in the band center and an analysis of multifractal properties indicates that localization is less strong at E = 0 than at E ≠ 0.
Abstract: We study a variant of the two-dimensional (2D) Anderson model of localization in which the disorder is represented by randomly chosen hopping terms. The density of states reveals an abnormally strong peak in the band center and an analysis of multifractal properties indicates that localization is less strong at E = 0 than at E ≠ 0. A finite-size-scaling analysis of localization lengths as obtained from the transfer-matrix method, shows that the state at E = 0 exhibits critical behavior up to a strip width M = 180. However, states outside the band center are localized and the critical state vanishes already for very small amounts of onsite potential disorder. Thus, there is no violation of the scaling theory of localization.
TL;DR: In this article, a nonequilibrium Green's functions approach is presented for consistent computation of semiconductor quantum well optical spectra including strong Coulomb correlations within the coupled photon and carrier system.
Abstract: A nonequilibrium Green's functions approach is presented for the consistent computation of semiconductor quantum well optical spectra including strong Coulomb correlations within the coupled photon and carrier system. Bethe-Salpeter-like equations are given for the optical response and recombination rates in the excited medium. Band structure; quantum confinement, many-body and cavity resonator effects are included in the microscopic approach. The theory is applied to the description of absorption/gain, luminescence, single and two-beam photoluminescence excitation spectroscopy for arbitrary temperatures and carrier densities. Numerical results, showing good agreement with recent experiments are presented for III-V and II-VI materials, from the linear regime, characterized by excitonic effects to the high density case in which a strongly interacting electron-hole plasma is proposed as the dominant mechanism.
TL;DR: In this paper, a kinetic theory of low-temperature localized-exciton energy relaxation in the multiple-hopping regime under steady-state and pulsed optical excitation is presented.
Abstract: (a) A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia(b) University of Linkoping, S-58183 Linkoping, Sweden(c) University of Gothenburg/Chalmers University S-41296 Gothenburg, Sweden(Received August 19, 1997)We present a kinetic theory of low-temperature localized-exciton energy relaxation in the multiplehopping regime under steady-state and pulsed optical excitation. The theory permits to relate thelineshape and the temporal redshift of photoluminescence spectra to the exciton kinetic parametersand band-tail density of states. A series of (Zn,Cd)Se/Zn(S,Se) quantum wells of different thicknessand content has been studied using cw and time-resolved photoluminescence techniques. A 20 AZn