Reciprocal Lifetime Research Articles

Phonons, Diffusons, and the Boson Peak in Two-Dimensional Lattices with Random Bonds

Within the model of stable random matrices possessing translational invariance, a two-dimensional (on a square lattice) disordered oscillatory system with random strongly fluctuating bonds is considered. By a numerical analysis of the dynamic structure factor S(q, ω), it is shown that vibrations with frequencies below the Ioffe-Regel frequency ωIR are ordinary phonons with a linear dispersion law ω(q) ∝ q and a reciprocal lifetime б ~ q3. Vibrations with frequencies above ωIR, although being delocalized, cannot be described by plane waves with a definite dispersion law ω(q). They are characterized by a diffusion structure factor with a reciprocal lifetime б ~ q2, which is typical of a diffusion process. In the literature, they are often referred to as diffusons. It is shown that, as in the three-dimensional model, the boson peak at the frequency ωb in the reduced density of vibrational states g(ω)/ω is on the order of the frequency ωIR. It is located in the transition region between phonons and diffusons and is proportional to the Young’s modulus of the lattice, ω b ≃E.

Фононы, диффузоны и бозонный пик в двумерных решетках со случайными связями

AbstractWithin the model of stable random matrices possessing translational invariance, a two-dimensional (on a square lattice) disordered oscillatory system with random strongly fluctuating bonds is considered. By a numerical analysis of the dynamic structure factor S ( q , ω), it is shown that vibrations with frequencies below the Ioffe-Regel frequency ω_IR are ordinary phonons with a linear dispersion law ω( q ) ∝ q and a reciprocal lifetime б ~ q ^3. Vibrations with frequencies above ω_IR, although being delocalized, cannot be described by plane waves with a definite dispersion law ω( q ). They are characterized by a diffusion structure factor with a reciprocal lifetime б ~ q ^2, which is typical of a diffusion process. In the literature, they are often referred to as diffusons. It is shown that, as in the three-dimensional model, the boson peak at the frequency ωb in the reduced density of vibrational states g (ω)/ω is on the order of the frequency ω_IR. It is located in the transition region between phonons and diffusons and is proportional to the Young’s modulus of the lattice, ω_ b ≃ E .

Optimization and characterization of the endogenous production of protoporphyrin IX in a yeast model.

The availability of reproducible, convenient, and inexpensive model organisms able to generate predictable levels of endogenous porphyrins, including protoporphyrin IX (PpIX), is essential in photomedicine research. Saccharomyces cerevisiae produces endogenous PpIX and was used as a model organism for this study with the aim to maximize endogenous PpIX fluorescence intensity. It was found that PpIX fluorescence was significantly enhanced by administration of 5-aminolevulinic acid (ALA) and 2,2?-bipyridyl. Fluorescence intensity and spectroscopy of PpIX produced endogenously were measured in diluted yeast solutions under various conditions. The optimal protocol was: 5 ?? ? M ALA and 1 mM 2,2?-bipyridyl administered synchronously at 32°C. After 3 h, PpIX in yeast demonstrated similar steady-state and time-resolved spectroscopy as that of PpIX in DMSO. Moreover, under hypoxic conditions, the reciprocal lifetime of PpIX delayed fluorescence measured in real time was correlated to the partial pressure of oxygen ( pO 2 ) measured concomitantly with a commercially available pO 2 probe. These data show that yeast can, in optimal conditions, reproducibly generate PpIX. This is of interest in various fields such as photodiagnosis, photodynamic therapy, and photobiomodulation. Use of this model organism focuses on essential mechanisms, without the complexity of a multicellular organism.

Effect of Surface and Interface Recombination on Carrier Lifetime in 6H-SiC Layers

Carrier lifetimes in 6H-SiC epilayers were investigated by using numerical simulations and micro-wave photoconductivity decay measurements. The measured carrier lifetimes were significantly increasing with an increased thickness up to 200 μm while it stays almost constant in layers thicker than 200 μm. From a comparison of the simulation and experimental results, we found that if the bulk lifetime in 6H-SiC is around a few microseconds, both the surface recombination and interface recombination influence the carrier lifetime in layers with thickness less than 200 μm while only the surface recombination determines the carrier lifetime in layers with thickness more than 200 μm. In samples with varying thicknesses, a bulk lifetime = 2.93 μs and carrier diffusion coefficient D= 2.87 cm2/s were derived from the linear fitting of reciprocal lifetime vs reciprocal square thickness.

An analytical model for random pseudonym change scheme in VANETs

The emerging Vehicular Ad hoc NETworks (VANETs) can improve safety, efficiency and convenience in transportation. In spite of the expected benefits, VANETs also introduce some location privacy risk by periodically broadcasting vehicular information. Pseudonym change is commonly accepted as one approach to protect location privacy in VANETs, but the effectiveness of most pseudonym change schemes has never been investigated theoretically. In this paper, focusing on the Random Pseudonym Change (RPC) scheme, we propose an analytical model to quantify its level of location privacy by calculating the size of anonymity set of a vehicle. With this model, we can analyze the probability of a vehicle changing pseudonym simultaneously with its neighbor, which can be then used to calculate the size of anonymity set of the vehicle. Further, we investigate the influence of the pseudonym lifetime distribution on the level of location privacy. Research results, which are validated via extensive simulations, show that for the RPC scheme, the uniform pseudonym lifetime distribution can provide better location privacy protection than the reciprocal pseudonym lifetime distribution. In addition, we also discuss the conditional probability of a vehicle changing pseudonym simultaneously with its neighbor given the age of pseudonym used by the neighbor at the time when the vehicle last changed pseudonym, and point out that utilizing the age of pseudonym can improve the level of location privacy.

Lifetime Measurements on Attached Epilayers and Detached Epifoils Grown on Reorganised Porous Silicon Showing a Bulk Lifetime Exceeding 100 μs

This paper discusses on-going efforts towards reliable lifetime measurements on epilayers and the subsequent decoupling of the bulk and surface recombination components, both while it is still attached to the p+ substrate on which it is grown (“attached epilayer”) and after its detachment from the substrate (“detached epifoil”). For the “attached epilayers”, microwave photoconductance decay (^-PCD) and simulation-assisted photoluminescence (sim-PL) were applied together with a variation in the epilayer thickness to evaluate the bulk lifetime (Tbulk) and total effective surface recombination velocity (Stot) of p-type epilayers. By applying linear fits to reciprocal effective lifetime versus reciprocal epilayer thickness data, Stot of all samples were reliably extracted, resulting in Stot of ∼265cm/s in the absence of porous silicon at the epilayer/substrate interface compared to ∼9220cm/s when present and ∼775cm/s when shielded by a thin back surface field. Based on these Stot values, sim-PL was used to estimate Tbulk to be ∼160 |us in the porous silicon area. For the n-type “detached epifoils”, quasi-steady state photoconductance (QSSPC) was used. However, reliable Tbulk could not be extracted, despite the lower Stot because Tbulk was too high. However, a lower limit to Tbulk was estimated to be >138 μs. Such high bulk lifetimes imply bulk diffusion lengths that are an order of magnitude longer than the epilayer thicknesses, potentially leading to high cell efficiencies.

Study of the Solid-Matrix Phosphorescence Properties of a Heterocyclic Aromatic Amine and the Heat Capacities of Glucose Glasses as the Temperature Decreases

The heat capacities were obtained from 294 to 133 K for four glucose-glass systems. Two of the glasses were prepared from crystalline glucose. One of the glasses contained the heavy-atom salt NaI and the other glass did not contain NaI. The other two glasses were similar, but they were prepared from glucose melts. Correlations were developed between the solid-matrix phosphorescence (SMP) lifetimes and intensities of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the glucose glasses and the heat capacities of the glucose sugar glasses as the temperature was lowered. Several plots of reciprocal SMP lifetime versus reciprocal temperature and reciprocal SMP lifetime versus reciprocal heat capacity were compared. Also, the reciprocal SMP intensity versus reciprocal temperature plots were compared with the corresponding reciprocal SMP intensity versus reciprocal heat capacity plots. In addition, basic photo-physical equations were used to develop relationships among the lifetime data, the intensity data, and the heat capacity data. The heat capacity data and SMP lifetime data, obtained as the temperature was lowered, were discussed in relationship to low-frequency vibrational modes and beta-relaxation phenomena in the glucose glasses. The discussion of these phenomena offered explanations for some of the loss of the excited triplet-state energy of PhIP in the glucose sugar glasses.

Nonequilibrium statistical operators for systems with finite lifetime

A family of nonequilibrium statistical operators (NSO) is introduced which differ by the system lifetime distribution over which the quasiequilibrium (relevant) distribution is averaged. This changes the form of the source in the Liouville equation, as well as the expressions for the kinetic coefficients, average fluxes, and kinetic equations obtained with use of NSO. The difference from the Zubarev form of NSO is of the order of the reciprocal lifetime of system.

Quantitation of naproxen by quenching of phosphorescence from a ternary complex of 2-bromo-6-methoxynaphthalene and α-cyclodextrin

A unique method, based on the measurement of phosphorescence lifetimes, is reported for use in analytically detecting the widely used anti-inflammatory drug naproxen. The reciprocal phosphorescence lifetime of a ternary complex containing α-cyclodextrin and 2-bromo-6-methoxynaphthalene increases linearly with naproxen concentration and has a quenching constant of 2.1 ± 0.05 × 106 M−1 s−1. Similar behavior is observed for quenching by the sodium salt of naproxen that has a quenching constant of 1.8 × 106 ± 0.05 M−1 s−1. The experimental sensitivity is sufficient to permit measurements of naproxen concentrations differing by 11 μg/ml. Phosphorescence lifetime measurements reveal that the quenching mechanism does not involve displacement of the unrelaxed triplet-state guest molecule by naproxen from the complex and may occur by energy transfer or electron transfer.

Interactions between dipole-exchange spin waves in ultrathin antiferromagnetic films

A finite-temperature perturbation formalism is employed to study the nonlinear processes for interactions between the discrete spin waves (SW) in ultrathin antiferromagnetic films, taking account of both dipole–dipole and exchange coupling between the spins. Within a microscopic Hamiltonian approach we obtain the dominant three- and four-magnon interactions at low temperatures, and their contributions to the frequency shift and damping (or reciprocal lifetime) of the discrete SW are evaluated. Numerical results applied to MnF 2 are presented.

Temperature and carrier density dependence of Auger recombination in a 3.4 µm InAs/GaSb/AlSb type-II laser device

We report on the temperature and carrier density dependence of non-radiative recombination processes in an InAs/GaSb/InAs type-II W-laser emitting at 3.4 μm. The measurements were performed with a sub-picosecond photoluminescence upconversion set-up in a temperature range between 10 K and 300 K and with initial excited carrier densities in the range between 2.96 × 1018 cm−3 and 4.44 × 1019 cm−3. The excellent growth quality of the device is indicated by a Shockley–Read–Hall coefficient of 2.2 × 108 s−1 at 10 K and 1.1 × 108 s−1 at 300 K. The cubic Auger recombination (AR) coefficient decreases in a characteristic manner with increasing initial excited carrier density. From a convergence equation, we obtained a cubic AR coefficient C03 of 1.2 × 10−28 cm6 s−1 for low carrier densities at 200 K. For low temperatures, due to degenerate carrier population of valence and conduction bands, a sublinear increase of the reciprocal lifetime versus carrier density is measured. With rising temperature the sublinear increase becomes linear and at 300 K a quadratic AR coefficient C02 of 1.73 × 10−11 cm3 s−1 was determined.

Luminescence properties of Tb3+ and Eu3+-doped alumina films prepared by sol-gel method under various conditions and sensitized luminescence

Rare earth ion (Tb3+ and Eu3+)-doped alumina films were prepared by the aqueous sol-gel method under various conditions. The influences of the OH groups (phonon relaxation) and rare earth ion concentration (cross-relaxation) on luminescence were examined. In regard to the former relaxation, at treatment temperature above 600°C, reciprocal lifetime decreased with OH concentration, and below 500°C, decreased markedly and nonlinearly. On the other hand, in regard to the latter relaxation, there was negligible effect on luminescence for these doped films. The quantitative treatment was tried to lifetime considering these influences. Tb3+ and Eu3+ co-doped alumina films showed enhanced Eu3+ luminescence by the energy transfer from Tb3+ to Eu3+. Eu3+ luminescence intensity increased with a greater Tb3+ concentration.

Displacement currents and the real part of high-frequency conductance of the resonant-tunneling diode

I have shown that weak variation of the tunnel transparency of the collector barrier with bias has substantial (and frequently crucial) effect on the high-frequency properties of the resonant-tunneling diodes (RTDs). Also it has been shown that the real part of the RTD conductance can be negative and large at the frequencies much higher than the reciprocal quasibound-state lifetime in the quantum well between the barriers of RTD, if (as opposed to common practice) the RTD collector is heavily doped and does not have thick spacer layers. The displacement currents are responsible for the effects. A simple equivalent circuit of RTD is proposed, and it fairly well describes the published experimental data.

Microscopic theory of dipole-exchange spin waves in ferromagnetic films: Linear and nonlinear processes

The linear and nonlinear processes in ferromagnetic films at low temperatures $(T\ensuremath{\ll}{T}_{c})$ are studied in a microscopic theory. Both the long-range magnetic dipole-dipole and the Heisenberg exchange interactions to nearest and next-nearest neighbors are included. The results obtained for the linearized spin-wave spectrum are compared with previous macroscopic theories. For ultrathin films (or for large wave vectors) the microscopic theory provides important corrections. The nonlinear dynamics of the spin waves are studied through a finite-temperature perturbation theory based on Feynman diagrams. We obtain explicit results for the energy shift and damping (or reciprocal lifetime) of the dipole-exchange spin waves due to all possible three-magnon and four-magnon processes involving combinations of the surface and quantized bulk spin waves at low temperatures. To investigate different dipole interaction strengths (relative to the exchange) numerical results are presented using parameters for Fe, EuO, and ${\mathrm{GdCl}}_{3}.$

Localization of phonons and low-frequency sound attenuation in layered crystals

It is shown that specific interference processes emerging in an anharmonic layered lattice in the localization regime leads to a considerable renormalization of the reciprocal phonon lifetime. This mechanism may dominate over the standard anharmonic mechanism. The coefficient of low-frequency sound attenuation in an insulator with diagonal disorder is discussed.

Energy shift and damping of dipole-exchange spin waves in ultrathin ferromagnetic films

A finite-temperature perturbation formalism is developed to study the processes of spin-wave interactions in ultrathin ferromagnetic films, including the effects of both the long-range magnetic dipole-dipole interactions and the short-range exchange interactions. By contrast with previous macroscopic methods for evaluating spin-wave interactions, the dipolar terms are treated here within a Hamiltonian method, together with exchange and anisotropy terms, to obtain a theory applicable to ultrathin films at temperatures below the Curie temperature. The contributions to the energy shift and the damping (or reciprocal lifetime) of the dipole-exchange spin waves due to three-magnon and four magnon processes are calculated. Numerical examples are presented.

Iron detection in polished and epitaxial silicon wafers using generation lifetime measurements

For iron detection in silicon, minority carrier diffusion length measurements are often used. These methods analyze only the bulk properties of the wafer and therefore failed in the case of epitaxial wafers due to the disturbing influence of the highly doped substrate. A technique is presented for the determination of the iron content in epitaxial as well as in polished wafers using generation lifetime measurements on metal-oxide-silicon capacitors. The influence of interstitial iron and iron-boron pairs on generation lifetime was investigated for samples diffused with iron in boron-doped silicon. The difference in reciprocal generation lifetime before and after a thermal anneal of 200 °C for three minutes was calibrated using deep level transient spectroscopy to detect the iron-boron pair concentration. For a typical metal-oxide-silicon capacitor fabrication process this means a detectability limit for iron-boron concentrations of about 8×1010 at/cm3. The advantage of this method is that it detects iron in a surface near region in contrast to the often used diffusion length measurements. This gives us the opportunity of controlling the iron content in epitaxial wafers as well as in wafers after an internal gettering process. In addition, the iron-boron pairing reaction was investigated in the temperature range between 21 and 50 °C. The activation energy for this reaction was determined to be 0.70 eV.

1H‐NMR spin‐spin relaxation time assessment of the reflection coefficient of the Triticale seed cell wall–plasmalemma barrier to mannitol

ABSTRACTThe 1H‐NMR spin‐spin relaxation time (T2) in Triticale seeds swelling in external osmotica, polyethylene glycol 8000 or mannitol can identify both bound and free water. At the same water content, the free water spin‐spin relaxation time increases for seeds imbibed with the mannitol solution, demonstrating inadequate water potential adjustment. The exchange rate of free/bound water molecules is apparently influenced by the driving force for water flow. The reciprocal lifetime of free water molecules, as a measure of water flow through the main cell barrier, was obtained. From a model of the seed as a resistance–capacitor network for water flow, a method was derived for calculating the reflection coefficient σ as a lifetime ratio of the free water molecules in seeds imbibed with two different osmotica (one penetrating across the main cell barrier and one not penetrating) at the same water potential. The 1H‐NMR method and the classical method based on volume rate changes yielded reflection coefficients for mannitol for the cell wall–plasmalemma barrier of 0.78 ± 0.08 and 0.68 ± 0.06, respectively.

Effective generation-recombination parameters in high-energy proton irradiated silicon diodes

The degradation of the generation and the recombination lifetime of silicon junction diodes by 10 MeV proton irradiation is compared with the introduction of the radiation-induced deep levels. It is shown that for the fluence range studied, both the reverse current and the reciprocal lifetime increase linearly with 10 MeV H+ fluence. From this study it follows that n-type Si is more prone to high-energy proton irradiation damage than p-type material. From the electrical diode characteristics, it is derived that the dominant generation center in both n- and p-type Float-Zone Si is approximately 0.12 eV from the midgap position. This strongly suggests that in both types of material, the divacancy level at Ec−0.42 eV is the dominant generation center.

Test of a model for reversible excimer kinetics: Pyrene in cyclohexanol

A model for reversible monomer–excimer kinetics that considers the time dependence of the excimer formation rate coefficient is proposed and tested for pyrene in cyclohexanol from 25 up to 85 °C. Simultaneous analysis of the monomer and excimer experimental decay curves allows the determination of all the relevant parameters for this reaction. The diffusion coefficient follows an Arrhenius plot with activation energy of 36±1 KJ mol−1, the encounter radius varies between 7.3 and 8.9 Å, and the intrinsic rate constant for excimer formation varies between 2.4×109 and 1.1×1010 M−1 s−1. The intrinsic reciprocal lifetime of the excimer follows an Arrhenius plot with activation energy of 12±1 KJ mol−1, and the rate constant for excimer dissociation, determined for high temperatures (T>55 °C) when reversibility is important, has different values depending whether geminate pair effects are considered or not in the analysis. The binding energy of pyrene excimer obtained considering pair effects (ΔH=32±2 KJ mol−1) agrees with the published values for nonviscous solvents.