[No abstract available]
We observe remarkable, complete suppression of spin generation under optical excitation in a thin InAs/GaAs wetting layer close to the light-hole excitonic resonance, leading to zero electron spin polarization as monitored by adjacent InAs quantum dots. The suppression is attributed to efficient spin relaxation/scattering at the Fano resonance between the light-hole exciton states and the heavy-hole continuum of the wetting layer. The complete suppression is found to remain effective up to temperatures exceeding 100 K.
Some open questions exist with fluctuation-induced forces between extended dipoles. Conventional intuition derives from large-separation perturbative approximations to dispersion force theory. Here, we present a full non-perturbative theory. In addition, we discuss how one can take into account finite dipole size corrections. It is of fundamental value to investigate the limits of validity of the perturbative dispersion force theory.
The resonance interaction that takes place in planar nanochannels between pairs of excited-state atoms is explored. We consider interactions in channels of silica, zinc oxide, and gold. The nanosized channels induce a dramatically different interaction from that in free space. Illustrative calculations for two lithium and cesium atoms demonstrate that there is a short-range repulsion followed by long-range attraction. The binding energy is strongest near the surfaces. The size of the enlarged molecule is biggest at the center of the cavity and increases with channel width. Since the interaction is generic, we predict that enlarged molecules are formed in porous structures, and that the molecule size depends on the size of the nanochannels.
We demonstrate a physical mechanism that enhances a splitting of diatomic Li-2 at cellulose surfaces. The origin of this splitting is a possible surface-induced diatomic-excited-state resonance repulsion. The atomic Li is then free to form either physical or chemical bonds with the cellulose surface and even diffuse into the cellulose layer structure. This allows for an enhanced storage capacity of atomic Li in nanoporous cellulose.
There is an attractive Casimir-Lifshitz force between two silica surfaces in a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin (5-50 angstrom) metallic nanocoating to one of the surfaces results in repulsive Casimir-Lifshitz forces above a critical separation. The onset of such quantum levitation comes at decreasing separations as the film thickness decreases. Remarkably, the effect of retardation can turn attraction into repulsion. From that we explain how an ultrathin metallic coating may prevent nanoelectromechanical systems from crashing together.
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We demonstrate that Casimir-Polder energies between noble gas atoms (dissolved in water) and oil-water interfaces are highly surface specific. Both repulsion (e.g., hexane) and attraction (e.g., glycerine and cyclodecane) is found with different oils. For several intermediate oils (e.g., hexadecane, decane, and cyclohexane) both attraction and repulsion can be found in the same system. Near these oil-water interfaces the interaction is repulsive in the nonretarded limit and turns attractive at larger distances as retardation becomes important. These highly surface specific interactions may have a role to play in biological systems where the surface may be more or less accessible to dissolved atoms.
Unwanted stiction in micro- and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable non-monotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.
We present the theory for retarded resonance interaction between two identical atoms at arbitrary positions near a metal surface. The dipole-dipole resonance interaction force that binds isotropically excited atom pairs together in free space may turn repulsive close to an ideal (totally reflecting) metal surface. On the other hand, close to an infinitely permeable surface it may turn more attractive. We illustrate numerically how the dipole-dipole resonance interaction between two oxygen atoms near a metal surface may provide a repulsive energy of the same order of magnitude as the ground-state binding energy of an oxygen molecule. As a complement we also present results from density-functional theory.
We present the theory for a retarded resonance interaction between two identical atoms near a dielectric surface. In free space the resonance interaction between isotropically excited atom pairs is attractive at all atom-atom separations. We illustrate numerically how this interaction between oxygen, sulphur, hydrogen, or nitrogen atom pairs may turn repulsive near water droplets. The results provide evidence of a mechanism causing excited state atom pair breakage to occur in the atmosphere near water droplets.
We have used density functional theory to calculate the anisotropic dielectric functions forultrathin gold sheets (composed of 1, 3, 6, and 15 atomic layers). Such films are important components innano-electromechanical systems. When using correct dielectric functions rather than bulk gold dielectricfunctions we predict an enhanced attractive Casimir-Lifshitz force (at most around 20%) between twoatomically thin gold sheets. For thicker sheets the dielectric properties and the corresponding Casimirforces approach those of gold half-spaces. The magnitude of the corrections that we predict should, withinthe today’s level of accuracy in Casimir force measurements, be clearly detectable.
In a recent paper [Phys. Rev. A 59, R3149 (1999)] Lamoreaux reported calculations of the Casimir force. The experimentally found permittivity was used in the calculations. Large deviations were found between numerically evaluated forces and forces derived from a series expanded plasma model. We would like to comment on a few results presented in this work. First, we claim that important features of the imaginary component of the permittivity of copper, presented in Fig. 1(a) are due to the interpolation procedure and are not caused by physical phenomena. These features influence the calculated permittivity for imaginary frequencies, which is the quantity used to calculate the Casimir attraction. Second, we discuss the extrapolation procedure used for low frequencies. The results depend substantially on how this extrapolation is performed.
We first derive the zero-and-finite-temperature dispersion-forces in terms of changes in the energy of the electromagnetic normal modes of the system. We then use this to evaluate the entropy of the electromagnetic normal modes for the "Casimir system" consisting of two metal plates. We demonstrate that this entropy obeys the Nernst heat theorem. (C) 2004 Elsevier B.V. All rights reserved.
The van der Waals (vdW) interaction between thin metallic films varies with separation as the separation to a fractional power. This is in contrast to the usual integer-power separation dependence between objects such as atoms, dielectric films, or thick metallic films. We have calculated the free energy of attraction between sheets of gold, silver, copper, beryllium, and tungsten numerically using experimentally found dielectric functions. The results are compared with the corresponding analytical results obtained using simple model dielectric functions. We have investigated how thin the metallic films must be in order for the fractional vdW interaction to be present. To our knowledge, fractional vdW interaction has not yet been confirmed experimentally.
We consider the effect of atomic hydrogen exposure to a system of two undoped sheets of graphene grown near a silica surface (the first adsorbed to the surface and the second freestanding near the surface). In the absence of atomic hydrogen, the van der Waals force between the sheets is attractive at all separations, causing the sheets to come closer together. However, with the addition of atomic hydrogen between the sheets, the long-range van der Waals interaction turns repulsive at a critical concentration. The underlying triple layer structure (SiO(2)-atomic hydrogen gas-air) gives rise to a long-range repulsion that at large-enough separations dominates over the more rapidly decaying attraction between the two-dimensional undoped graphene sheets (and between the outer graphene sheet and SiO(2)). This may be an avenue to tune the separation between two graphene sheets with the gas concentration. The doping of the graphene layers increases the attractive part of the interaction and hence reduces the net repulsive interaction.
We recently investigated the van der Waals force between thin metal films. Under certain conditions this force decrease with separation to a fractional power. In the present work we use optical data of metals and the zero-temperature Lifshitz formalism to demonstrate a retardation effect. The retarded attraction between thin metal films may be larger than the nonretarded attraction. This property is related to a comparatively weak retardation dependence of the energy that originates from the transverse magnetic modes. At separations where the transverse electric modes give a significant contribution, the net effect can actually be an increased attraction. This effect vanishes with increasing film thickness and with increasing dissipation.
We present calculations of the free energy of attraction between two quantum wells in which the wells are treated as strictly two-dimensional metallic sheets. The van der Waals force exhibits fractional separation dependence in this system. This is in contrast to the usual integer separation dependence. We have performed numerical calculations at different temperatures and with different carrier densities. Except at very low temperatures thermal effects will be a dominating source of attraction. We have determined temperature criteria that must be fulfilled for the fractional separation dependence to be observable. Thermal corrections will be important already at temperatures less than 1 K. We further make some comments on a recent measurement of the Casimir force.
The vacuum stresses between a metal half-space and a metal sphere were recently measured at room temperature, in the 0.6-6 mu m range, with an estimated accuracy of 5%. In the interpretation it was assumed that the accuracy was not good enough for observing any thermal effects. We claim that thermal effects are important in this separation range and back up this claim with numerical calculations of the Casimir force at zero temperature and at 300 K, based on tabulated optical data of gold, copper, and aluminum. The effects of dissipation and temperature are investigated and we demonstrate the importance of considering these two corrections together.
The van der Waals energy of a ground-state atom (or molecule) placed between two metal films is calculated at finite temperature. The attraction between thin metal films and a polarizable object can have half-integer separation dependence. This is in contrast to the usual integer separation dependence, shown for instance in the attraction between an atom and a solid surface. We examine how film thickness, retardation, and temperature influence the interaction. To illustrate the effect of finite thickness of the metal film we calculated the van der Waals energy of ground-state hydrogen and helium atoms, and hydrogen molecules, between thin silver films. We finally, briefly, discuss the possibility to measure this effect.
We consider the interaction between a ZnO nanorod and a SiO2 nanorod in bromobenzene. Using optical data for the interacting objects and ambient we calculate the force (from short-range attractive van der Waals force to intermediate-range repulsive Casimir-Lifshitz force to long-range entropically driven attraction). The nonretarded van der Waals interaction is attractive at all separations. We demonstrate a retardation-driven repulsion at intermediate separations. At short separations (in the nonretarded limit) and at large separations (in the classical limit) the interaction is attractive. These effects can be understood from an analysis of multiple crossings of the dielectric functions of the three media as functions of imaginary frequencies.
Casimir forces between surfaces immersed in bromobenzene have recently been measured by Munday et al. [Nature (London) 454, 07610 (2009)]. Attractive Casimir forces were found between gold surfaces. The forces were repulsive between gold and silica surfaces. We show the repulsion is due to retardation effects. The van der Waals interaction is attractive at all separations. The retardation-driven repulsion sets in at around 3 nm. To our knowledge, retardation effects have never been found at such a small distance before. Retardation effects are usually associated with large distances.
We report here an experimental and theoretical study on the magnetoresistance properties of heavily phosphorous doped germanium on the metallic side of the metal-nonmetal transition. An anomalous regime, formed by negative values of the magnetoresistance, was observed by performing low-temperature measurements and explained within the generalized Drude model, due to the many-body effects. It reveals a key mechanism behind the magnetoresistance properties at low temperatures and, therefore, constitutes a path to its manipulation in such materials of great interest in fundamental physics and technological applications. Published under license by AIP Publishing.
The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. [S0163-1829(99)11747-8].
Precise measurement of the optical Hall effect in InN using magneto-optical generalized ellipsometry at IR and THz wavelengths, allows us to decouple the surface accumulation and bulk electron densities in InN films by non-contact optical means and further to precisely measure the effective mass and mobilities for polarizations parallel and perpendicular to the optical axis. Studies of InN films with different thicknesses, free electron densities and surface orientations enable an intricate picture of InN free electron properties to emerge. Striking findings on the scaling factors of the bulk electron densities with film thickness further supported by transmission electron microscopy point to an additional thickness dependent doping mechanism unrelated to dislocations. Surface electron accumulation is observed to occur not only at polar but also at non-polar and semi-polar wurtzite InN, and zinc blende InN surfaces. The persistent surface electron density shows a complex behavior with bulk density and surface orientation. This behavior might be exploited for tuning the surface charge in InN.
The free electron behavior in InN is studied on the basis of decoupled bulk and surface accumulation electron densities in InN films measured by contactless optical Hall effect. It is shown that the variation in the bulk electron density with film thickness does not follow the models of free electrons generated by dislocation-associated nitrogen vacancies. This finding, further supported by transmission electron microscopy results, indicates the existence of a different thickness-dependent doping mechanism. Furthermore, we observe a noticeable dependence of the surface electron density on the bulk density, which can be exploited for tuning the surface charge in future InN based devices.
The longitudinal-optical (LO)-phonon coupling is experimentally examined by the optical decay of various charged and neutral exciton species in single quantum dots, and the related Huang-Rhys parameters are extracted. A positive trion exhibits significantly weaker LO-phonon replicas in the photoluminescence spectrum than the neutral and negatively charged species. Model computations show that the strength of the replicas is determined by the Coulomb interactions between electrons and holes, which modify the localization of the envelope wave functions and the net charge distribution.
The critical impurity concentration Nc of the metal-nonmetal (MNM) transition for the cubic GaN, InN and AlN systems, is calculated using the following two different criteria: vanishing of the donor binding energy and the crossing point between the energies in the metallic and insulating phases. A dielectric function model with a Lorentz-Lorenz correction is used for the insulating phase. The InN presents an order of magnitude increase in Nc as compared to the other two systems. The electrical resistivity of the Si-donor system GaN is investigated theoretically and experimentally from room temperature down to 10 K. It presents a metallic character above a certain high impurity concentration identified as Nc. The samples were grown by plasma assisted molecular beam epitaxy (MBE) on GaAs (0 0 1) substrate. The model calculation is carried out from a recently proposed generalized Drude approach (GDA) presenting a very good estimation for the metallic region. The band-gap shift (BGS) of Si-doped GaN has also been investigated above the MNM transition where this shift is observed. Theoretical and experimental results have a rough agreement in a range of impurity concentration of interest. © 2001 Elsevier Science B.V. All rights reserved.
The resistivity of GaAs implanted with carbon acceptors for concentrations spanning the insulating to the metallic regimes were investigated experimentally and theoretically between room temperature and 1.7 K. The resistivities obtained experimentally were compared with resistivity values calculated from a generalized Drude approach. The value of the critical impurity concentration was found to be about 1018cm-3. Good agreement was obtained between the experimental results and calculations.
The electrical resistivity of 4H-SiC doped with nitrogen is analyzed in the temperature range 10- 700 K for nitrogen concentrations between 3.5× 1015 and 5× 1019 cm-3. For the highest doped samples, a good agreement is found between the experimental resistivity and the values calculated from a generalized Drude approach at similar dopant concentration and temperature. From these results, the critical concentration (Nc) of nitrogen impurities which corresponds to the metal-nonmetal transition in 4H-SiC is deduced. We find Nc ~ 1019 cm-3. © 2006 The American Physical Society.
We have performed longitudinal magnetoresistance measurements on heavily n-doped silicon for donor concentrations exceeding the critical value for the metal-nonmetal transition. The results are compared to those from a many-body theory where the donor electrons are assumed to reside at the bottom of the many-valley conduction band of the host. Good qualitative agreement between theory and experiment is obtained.
Films of pure and Sn-doped semiconducting In_{z}0_{3} were prepared by reactive e-beam evaporation. The spectral absorption coefficient was evaluated by spectrophotometry in the (2—6)-eV range. The extracted band gap increases with electron density (n_{e} ) approximately as n_{e 2/3} for n_{e} ≤10^{-21} cm^{-3} . This result is interpreted within an effective-mass model for n-doped semiconductors well above the Mott critical density. Because of the high degree of doping, the impurities are ionized and the associated electrons occupy the bottom of the conduction band in the form of an electron gas. The model accounts for a Burstein-Moss shift as well as electron-electron and electron-impurity scattering treated in the random-phase approximation. Experiments and theory were reconciled by assuming a parabolic valence band with an effective mass -0.6m. Earlier work on doped oxide semiconductors are assessed in the light of the present results.
The Si-doped GaN/Al0.07Ga0.93N multiple quantum wells (MQW) were investigated, using photoluminescence (PL) and time-resolved (PL) measurements. The influence of Si doping on the emission energy and recombination dynamics of the MWQs were also investigated, with different dopant position in the wells. It was observed that the redshifted emission of the MQWs was attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. It was also observed that the PL decay time of the sample was ∼760 ps, at low temperature.
The^{ }frequency-dependent polarizabilities and the C_{6} dipole-dipole dispersion coefficients for the^{ }first members of the polyacenes namely benzene, naphthalene, anthracene, and^{ }naphthacene as well as the fullerene C_{60} have been calculated^{ }at the time-dependent Hartree-Fock level and the time-dependent density-functional theory^{ }level with the hybrid B3LYP exchange-correlation functional. The dynamic polarizabilities^{ }at imaginary frequencies are obtained with use of the complex^{ }linear polarization propagator method and the C_{6} coefficients are subsequently^{ }determined from the Casimir-Polder relation. We report the first ab^{ }initio calculations of the C_{6} coefficients for the molecules under^{ }consideration, and our recommended value for the dispersion coefficient of^{ }the fullerene is 101.0 a.u.
The^{ }frequency-dependent polarizabilities of closed-shell sodium clusters containing up to 20^{ }atoms have been calculated using the linear complex polarization propagator^{ }approach in conjunction with Hartree-Fock and Kohn-Sham density functional theories.^{ }In combination with polarizabilities for C_{60} from a previous work^{ }[J. Chem. Phys. 123, 124312 (2005)], the C_{6} dipole-dipole dispersion^{ }coefficients for the metal-cluster-to-cluster and cluster-to-buckminster-fullerene interactions are obtained via^{ }the Casimir-Polder relation [Phys. Rev. 73, 360 (1948)]. The B3PW91^{ }results for the polarizability of the sodium dimer and tetramer^{ }are benchmarked against coupled cluster calculations. The error bars of^{ }the reported theoretical results for the C_{6} coefficients are estimated^{ }to be 5%, and the results are well within the^{ }error bars of the experiment.
We report on calculations of the dipole-dipole dispersion coefficients for pairs of n -alkane molecules. The results are based on first-principles calculations of the molecular polarizabilities with a purely imaginary frequency argument and which were reported by us in a previous work [P. Norman, A. Jiemchooroj, and Bo E. Sernelius, J. Chem. Phys. 118, 9167 (2003)]. The results for the static polarizabilities and dispersion coefficients are compared to simple algebraic expressions in terms of the number of CC and CH bonds in the two weakly interacting species. The bond additivity procedure is shown to perform well in the present case, and bond polarizabilities of 4.256 and 3.964 a.u . are proposed for the CH and the CC bond, respectively.
The frequency dependent polarizabilities of closed-shell alkali metal clusters containing up to ten lithium, potassium, and rubidium atoms have been calculated using the linear complex polarization propagator approach in conjunction with Hartree – Fock and Kohn – Sham density functional theory. In combination with polarizabilities for C_{60} from a previous work [J. Chem. Phys. 123, 124312 (2005)], the C_6 dipole-dipole dispersion coefficients for the metal cluster-to-cluster and cluster-to-buckminster fullerene interactions are obtained via the Casimir – Polder relation. The B3PW91 results for the polarizabilities and dispersion interactions of the alkali metal dimers and tetramers are benchmarked against couple cluster calculations, and the whole series of calculations are compared against the corresponding work on sodium clusters [J. Chem. Phys. 125, 124306 (2006)]. The error bars of the reported theoretical results for the C_6 coefficients are estimated to be 8%.
The LO‐phonon coupling is experimentally examined from the optical decay of various charged and neutral exciton species in single quantum dots. A positive trion exhibits significantly weaker LO‐phonon replicas in the photoluminescence spectrum than the neutral and negatively charged species.
We^{ }report on a detailed analysis of the transport properties and^{ }superconducting critical temperatures of boron-doped diamond films grown along the^{ }{100} direction. The system presents a metal-insulator transition (MIT) for^{ }a boron concentration (n_{B}) on the order of n_{c}~4.5×10^{20} cm^{−3}, in^{ }excellent agreement with numerical calculations. The temperature dependence of the^{ }conductivity and Hall effect can be well described by variable^{ }range hopping for n_{B}<n_{c} with a characteristic hopping temperature T_{0}^{ }strongly reduced due to the proximity of the MIT. All^{ }metallic samples (i.e., for n_{B}>n_{c}) present a superconducting transition at^{ }low temperature. The zero-temperature conductivity _{0} deduced from fits to^{ }the data above the critical temperature (T_{c}) using a classical^{ }quantum interference formula scales as _{0}(n_{B}/n_{c}−1)^{} with ~1. Large T_{c}^{ }values (0.4 K) have been obtained for boron concentration down to^{ }n_{B}/n_{c}~1.1 and T_{c} surprisingly mimics a (n_{B}/n_{c}−1)^{1/2} law. Those high^{ }T_{c} values can be explained by a slow decrease of^{ }the electron-phonon coupling parameter and a corresponding drop of^{ }the Coulomb pseudopotential µ^{*} as n_{B}n_{c}.
A comparison study of theoretical approaches to the description of the Casimir interaction in layered systems including graphene is performed. It is shown that at zero temperature, the approach using the polarization tensor leads to the same results as the approach using the longitudinal density-density correlation function of graphene. An explicit expression for the zero-temperature transverse density-density correlation function of graphene is provided. We further show that the computational results for the Casimir free energy of graphene-graphene and graphene-Au plate interactions at room temperature, obtained using the temperature-dependent polarization tensor, deviate significantly from those using the longitudinal density-density correlation function defined at zero temperature. We derive both the longitudinal and transverse density-density correlation functions of graphene at nonzero temperature. The Casimir free energy in layered structures including graphene, computed using the temperature-dependent correlation functions, is exactly equal to that found using the polarization tensor.
Thin films of polycrystalline tungsten trioxide were manufactured using DC magnetron sputtering. Films of different thickness were deposited onto glass substrates coated with indium tin oxide (ITO). The crystallinity was confirmed by X-ray diffraction, and the grain size was found to be 30 nm. Li ions and electrons were intercalated into the sample using a three-electrode setup. The samples were submitted to optical characterization by spectrophotometry, in the visible and infrared ranges. The optical spectra were recorded at different intercalation states, and the absorption of the films was obtained. At low intercalation levels, a pronounced absorption peak was observed to be centered at a wavelength of 1.8 mum. Upon intercalation, the inserted electrons enter the conduction band, but due to a strong electron-phonon interaction, they are believed to form localized polarons. Calculations of optical absorption by large polaron theory were carried out and the position of the observed peak was in good agreement with the theory. A crossover from dielectric (low reflectance and clear phonon absorption bands) to metallic (high infrared reflectance) occurred at a Li intercalation level of around 0.05-0.15 Li/W. This may be due to overlap of the polaron states. (C) 2003 Elsevier B.V All rights reserved.
The interaction between surface patches of proteins with different surface properties has a vital role to play driving conformational changes in proteins in different salt solutions. We demonstrate the existence of ion-specific attractive double-layer forces between neutral hydrophobic and hydrophilic surfaces in the presence of certain salt solutions. This is performed by solving a generalized Poisson-Boltzmann equation for two unequal surfaces. In the calculations, we utilize parametrized ion-surface potentials and dielectric-constant profiles deduced from recent non-primitive-model molecular dynamics simulations that partially account for molecular structure and hydration effects.
We employ ultrabroadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (10(15)-10(19) cm(-3)) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of similar to 200 fs at low densities, reaching similar to 20 fs for densities of similar to 10(19) cm(-3), where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial subpicosecond temporal development of the Drude response and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.
we demonstrate experimentally the relativistic Doppler frequency up-conversion of the THz pulses from the counter-propagating ionized plasma front in silicon. The observed frequency up-conversion can be well modeled by the 1D FDTD simulations if significant short scattering time (well below 10 fs) in the plasma is assumed. To further elucidate the scattering rate in the electro-hole plasma, we performed pump probe experiment employing ultra-broadband (150 THz) THz-Mid-Infrared pulse. The results show the scattering time decreases from similar to 200 fs down to similar to 20 fs when the carrier density increases up to 10(19)-cm(-3), and then saturates for higher densities. Such scattering time dependence on plasma carrier density can be very well fitted by the Drude model for thermalized electron-holes, and the saturation behavior is attributed to electron-hole phase-space restriction as the plasma becomes degenerate. The resultant much shorter scattering time measured with non-thermalized plasma is in good accordance with the Doppler experiment, which demonstrates Doppler geometry an effective method for probing non-equilibrium plasma dynamics.
We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 degrees C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 x 10(18) cm(-3) in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Angstrom) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed.
Defect^{ }related contributions to the reduction of the internal quantum efficiency^{ }of InGaN-based multiple quantum well light emitting diodes under high^{ }forward bias conditions are discussed. Screening of localization potentials for^{ }electrons is an important process to reduce the localization at^{ }high injection. The possible role of threading dislocations in inducing^{ }a parasitic tunneling current in the device is discussed. Phonon-assisted^{ }transport of holes via tunneling at defect sites along dislocations^{ }is suggested to be involved, leading to a nonradiative parasitic^{ }process enhanced by a local temperature rise at high injection.
The electrical resistivity of the Si-donor cubic GaN is investigated theoretically at low temperature. The critical impurity concentration, Nc, for the metal-nonmetal transition is estimated in three different ways: from using the generalized Drude approach (GDA) for the resistivity, from the vanishing of the chemical potential calculated using the dielectric function model with a Lorentz-Lorenz correction, from finding the crossing point between the energy in the insulating and metallic states. The bandgap narrowing (BGN) has been determined theoretically and experimentally above the MNM transition. The experimental data have been obtained with photoluminescence measurements. Theoretical and experimental results are in rough agreement in the range of impurity concentration of interest. © 2002 Elsevier Science Ltd. All rights reserved.
The band-gap shift of the heavily single and double-donor doped systems Si:Bi and Si:P,Bi, prepared by ion implantation, was investigated theoretically and experimentally at room temperature. The calculations were carried out within a framework of the random-phase approximation and the temperature and different many-body effects were taken into account. The experimental data were obtained with photoconductivity measurements. Theoretical and experimental results fall closely together in a wide range of donor concentration.