Tunnelling contribution in non-radiative processes

A semiclassical formalism, already introduced to treat non-radiative processes in multidimensional spaces, has been extended to compute total non-radiative transition rates. A simple approximate analysis results in simple formulae which discriminate between quantum-tunnelling contributions and classical ones. Applications to diffusion processes of vacancies in solids are considered.     

Understanding and control of the erbium non-radiative de-excitation processes in silicon

Erbium doping of silicon has recently emerged as a promising method to tailor the optical properties of Si towards the achievement of a light emission at 1.54 μm. In this paper we will review our recent work on this subject. In particular a detailed investigation of the non-radiative processes, competing with the radiative emission of Er in Si will be presented. Among these processes, an Auger de-excitation with the energy released to free carriers will be demonstrated to be extremely efficient, with an Auger coefficient C_A4.4×10⁻¹³ cm³/s. Moreover, at temperatures above 100 K a phonon-assisted back-transfer decay process, characterized by an activation energy of 0.15 eV is seen to set in. This understanding of the physical properties competing with the radiative light emission allowed us to control them and obtain efficient room temperature luminescence. Two examples will be reported. It will be shown that by exciting Er within the depletion region of reverse biased p⁺–n⁺ Si diodes in the breakdown regime it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region of the device. In this region Auger de-excitation with free carriers sets in and quenches rapidly the luminescence. This allows to modulate the light signal at frequencies as high as a few MHz. Furthermore, the introduction of Er within Si nanocrystals is demonstrated to be a promising way to eliminate back-transfer processes by a widening of the bandgap while maintaining the full advantage of the efficient electron-hole mediated excitation present in Si. These data are presented and future perspective discussed.     

Inclusion of neighbouring paths in the evaluation of non-radiative processes by semiclassical approximations

The inclusion of neighbouring-path contributions to the classical one and a suitable normalization criterion allow the attainment of simple formulas for the evaluation of tunnelling transitions in the semiclassical approximation. This method can be usefully applied to study non-radiative processes in color centers.     

InAs QDs on InP: polarization insensitive SOA and non-radiative Auger processes

An efficient mechanical and electronic axial approximation of the strained 8 × 8 Hamiltonian is proposed for zinc-blende nanostructures with a cylindrical shape on (100) substrates. Vertically stacked InAs/InP columnar quantum dots (CQDs) for polarization insensitive semiconductor optical amplifier (SOA) in telecommunications applications are studied theoretically. Non-radiative Auger processes in InAs/InP quantum dots (QDs) are also investigated. It is shown that a multiband approach is necessary in both cases.     

Semiclassical treatment of induced schwinger processes at a finite temperature

The induced pair production in an external field at a finite temperature is considered. An one-loop correction to the Green’s function of a meson is calculated semiclassically within the framework of a saddle-point analysis of the Schwinger proper time integrals. This correction appears to be exponentially small in terms of an inverse temperature dependence. The low-temperature limit is shown to be in full agreement with previously obtained zero-temperature results. The corrections in the low-temperature limits are estimated up to the leading exponential and preexponential terms. A comparison is made to earlier calculations of the vacuum decay. The text was submitted by the authors in English.     

Free electron laser study of the suppression of non-radiative scattering processes in semiconductors

A brief review is given of pump–probe studies of far infrared inter-sub-level relaxation between conduction band states in doped `quasi' quantum dots (created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well) and of mid-infrared (MIR) interband recombination in narrow gap semiconductors, using the free electron laser at FOM-Rijnhuizen (FELIX). In the former case, the longitudinal optic (LO) phonon scattering rate is shown to be suppressed by a factor of about 100 when the Landau level separation is off-resonance with the optical phonon energy; in the latter case, Auger recombination is shown to be substantially suppressed in the lead salts due to their `mirror' energy band structure.     

Mutual influence of Auger and non-radiative recombination processes under silicon femtosecond laser irradiation

The results of theoretical study of the contribution of recombination processes in additional heating of the surface of monocrystalline silicon during multipulse femtosecond laser processing are presented to discussion. The numerical evaluations are made in regimes of the laser radiation below the ablation threshold, when the microgeometry of the surface is formed due to the processes of self-organization. The influence of Auger recombination processes on the photoexcitation of the semiconductor during the pulse and relaxation after the pulse is studied in detail. It is shown that the additional heating of the surface due to non-radiative recombination is extremely small at pulse repetition rate 10 Hz–1 MHz. Mutual influence of recombination processes of both types is shown.     

Intramolecular non-radiative transitions

Non-radiative electronic transitions are discussed. The probability W_i of the electron staying in the initial state i is obtained as a function of the vibrational relaxation time, in both large and small molecules. The result at low temperature is

where ε is the vibrational relaxation rate, ?ωm' is the energy difference of the levels which are involved in the transitions. An expression which depends on the phonon temperature is also obtained.     

Charge-state effects of deep centres in semiconductors on non-radiative capture of carriers by multiphonon processes

A set of simplified analytical expressions for carrier capture coefficients, including quantatively the charge-dependent effect, have been obtained for easy physical examination and comparison with experiments. The temperature-related charge-state-dependent factorF(T) thus calculated could be used to present more accurately the nature and magnitude of the charge state of a trap centre. The ranges of values ofF(T) valid for attractive, repulsive and neutral centres are also obtained. In addition, we show that the thermal ionization energy for theB centre in GaAs is a function of temperature. The importance of the data of capture cross-section at low temperatures in determining the charge state and characteristic of a deep centre is also manifested. Both the absolute magnitude and the temperature-dependent behaviour of the calculated capture cross-section are well-supported by the very good fits to the experimental electron cross-sections forA andB centres in GaAs reported by Lang [7] and Wang et al. [22] and for Cu centre in Ge reported by Zhdanova and Kalashnikov [23].     

Nitrocarburizing treatments using flowing afterglow processes

Nitrocarburizing of pure iron samples is achieved at 853 K and is easily controlled by introducing C₃H₈ in the afterglow of a flowing microwave Ar-N₂-H₂ plasma. The carbon uptake in the solid is actually possible with methane but strongly limited. The use of propane enhances the carbon flux and the ?/α configuration is synthesized for the first time by this kind of process. For this stack, diffusion paths in the ternary system determined from chemical analyses by secondary neutral mass spectrometry reproduce satisfactorily X-ray diffraction results which only reveal, as optical micrographs, ? and α phases. Propane offers an accurate control of the nitrocarburizing conditions. As an example, a modulation of N and C contents in iron could be achieved to create new carbonitride multilayers.     

Semiclassical description of multipair transfer processes in heavy ion collisions with superfluid systems

Pair transfer processes involving a superfluid system are studied in terms of phase space distributions constructed in a product representation which blends both ordinary- and gauge-space degrees of freedom. The time evolution of these distributions is followed by solving a collection of classical equations of motion, the quantal fluctuations being accounted for by the sampling of all possible initial orientations of the (undetermined) intrinsic system in gauge space. The excitation of the pairing rotational degree of freedom — corresponding to a variation in the number of particles — is induced by a gauge-deformed ion-ion potential, as befits the superfluid character of the target. Different orientations leading to the same final mass transfer may produce (in the case of large pairing deformation) interference effects in the final population of the members of the pairing rotational band. We also discuss how this approach can be used to describe the effect of pair transfer modes on sub-barrier fusion processes.     

Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments

Ultrasound (US) cleaning technique was applied to remove fouling of ultrafiltration (UF) and microfiltration (MF) membranes which were used to treat peptone and milk aqueous solutions, respectively. Membrane operations were performed by cross-flow filtration with 60 kPa operating pressure in an US field. The US employed had 28, 45 and 100 kHz frequency with 23 W/cm(2) output power. For each polymeric membrane made of polysulfone UF and cellulose MF, cleaning experiments were carried out with and without US after fouling. The fouled UF and MF membranes showed volume flux decline, but the membrane property was recovered by US irradiation. It was found in 28 kHz frequency that water cleaning was effective for recovery of declined condition due to fouling. Also, US-enhanced permeability of membranes was discussed in both membrane systems. We observed that US decreased the fouling condition in both membrane systems when US was irradiated before fouling. It was found that 28 kHz frequency US could enhance formation of the fouled layer in both filtration systems of peptone and milk solution.     

Semiclassical Symmetries

Essential properties of semiclassical approximation for quantum mechanics are viewed as axioms of an abstract semiclassical mechanics. Its symmetry properties are discussed. Semiclassical systems being invariant under Lie groups are considered. An infinitesimal analog of group relation is written. Sufficient conditions for reconstructing semiclassical group transformations (integrability of representation of Lie algebra) are discussed. The obtained results may be used for mathematical proof of Poincare invariance of semiclassical Hamiltonian field theory and for investigation of quantum anomalies.     

Semiclassical delta self-energy

We present a semiclassical approach of the self-energy. We show that the in-medium corrections of the width issued from the Pauli blocking and the coupling to the 2N-1h continuum are in good agreement with the previous approaches and particularly with the quantum -h model even for light nuclei. We separate out the different sources of the imaginary part of the self-energy. The predominant corrections come from two antagonistic origins: the Pauli blocking and the contribution to the two-nucleon emission channel, the latter being model dependent. We further show that the non-diagonal spin matrix elements of the self-energy, generated by its tensor component, are mostly due to the Pauli blocking.     

Semiclassical quantum mechanics

We consider the 0 limit of the quantum dynamics generated by the HamiltonianH()=–(²/2m)+V. We prove that the evolution of certain Gaussian states is determined asymptotically as 0 by classical mechanics. For suitable potentialsV inn3 dimensions, our estimates are uniform in time and our results hold for scattering theory.Supported in part by the National Science Foundation under Grant PHY 78-08066     

Semiclassical Ehrenfest paths

Trajectories are a central concept in our understanding of classical phenomena and also in rationalizing quantum mechanical effects. In this work we provide a way to determine semiclassical paths, approximations to quantum averages in phase space, directly from classical trajectories. We avoid the need of intermediate steps, like particular solutions to the Schroedinger equation or numerical integration in phase space by considering the system to be initially in a coherent state and by assuming that its early dynamics is governed by the Heller semiclassical approximation. Our result is valid for short propagation times only, but gives non-trivial information on the quantum-classical transition.     

Semiclassical Almost Isometry

Let (M , ω , J) be a compact and connected polarized Hodge manifold, an isodrastic leaf of half-weighted Bohr–Sommerfeld Lagrangian submanifolds. We study the relation between the Weinstein symplectic structure of and the asymptotics of the the pull-back of the Fubini–Study form under the projectivization of the so-called BPU maps on .