Nonequilibrium Population Research Articles

Phonon generation and decay in hydrogenated amorphous silicon

We report on investigations of the temporal evolution of nonequilibrium phonon populations in hydrogenated amorphous silicon held at 1.8 K. Two types of a-Si:H were examined, one grown by plasma enhanced chemical vapor deposition (PE) and one by hot-wire assisted chemical vapor deposition (HW). Phonons were created during the relaxation and recombination of optically excited charge carriers, and detected by means of pulsed anti-Stokes Raman spectroscopy. In the same setup, pulsed luminescence experiments were performed, under identical experimental conditions. The temporal shape of the Raman signals turned out to be determined both by the electronic processes responsible for the phonon generation and by the anharmonic decay of the excited phonon population itself. In the PE films we observed a slowly decaying $(\ensuremath{\gg}100$ ns) contribution to the Raman signal, which was not present in the HW layers. We propose a model to explain this slow background as resulting from laser-induced fast nonradiative recombination of mobile with localized charge carriers. The results of the pulsed luminescence experiments support our model. In addition, phonon decay times were observed to be the same in all samples: decay times of $\ensuremath{\sim}70$ ns were obtained for LA- and TO-like vibrations, whereas TA-like vibrations decayed faster $(<10$ ns) than could be resolved with the experimental setup. We propose that the extreme longevity of the LA and TO phonons is related to the microstructure of amorphous silicon.

Can surface-enhanced Raman scattering serve as a channel for strong optical pumping?

The surface-enhanced Raman scattering spectra of a number of dye and colorless molecules adsorbed on deposited coloidal silver films were systematically studied as a function of power and position using a Raman microscope. The anti-Stokes portions of the spectra of the dyes reproducibly show line intensities much greater than what is expected on the basis of the equilibrium population of the excited vibrational states, even at the lowest incident light intensities used. This behavior was observed previously and attributed to optical pumping of vibrationally excited states of the molecules by unusually intense surface-enhanced Raman transitions, [Phys. Rev. Lett. 76, 2444 (1996)] suggesting either uncommonly large Raman cross-sections or very intense local field strengths exceeding those encountered in the most powerful currently available lasers. Based on this work, however, we ascribe the apparently large anti-Stokes intensities primarily to a difference in the Stokes and anti-Stokes Raman cross-sections resulting from resonance or pre-resonance Raman processes in the adsorbate–surface complex rather than to strongly nonequilibrium populations in the molecular vibrational states. Finally, we observed no significant inhomogeneity in the Raman enhancement in the images of the deposited silver coloid samples down to spatial resolutions of ∼1 μm.

Zygotic associations and multilocus statistics in a nonequilibrium diploid population.

The usual approach to characterizing and estimating multilocus associations in a diploid population assumes that the population is in Hardy-Weinberg equilibrium. The purpose of this study is to develop a set of summary statistics that can be used to characterize and estimate the multilocus associations in a nonequilibrium population. The concept of "zygotic associations" is first expanded to facilitate the development. The summary statistics are calculated using the distribution of a random variable, the number of heterozygous loci (K) found in diploid individuals in the population. In particular, the variance of K consists of single-locus and multilocus components with the latter being the sum of zygotic associations between pairs of loci. Simulation results show that the multilocus associations in the variance of K are detectable in a sample of moderate size (> or =30) when the sum of all pairwise zygotic associations is greater than zero and when gene frequency is intermediate. The method presented here is a generalization of the well-known development for the Hardy-Weinberg equilibrium population and thus may be of more general use in elucidating the multilocus organizations in nonequilibrium and equilibrium populations.

Ultrafast vibrational energy redistribution within C–H and O–H stretching modes of liquid methanol

An ultrafast IR–Raman technique is used to study fast redistribution of vibrational energy within CH and OH stretching vibrations of liquid methanol. The instantaneous population distribution throughout the entire CH and OH stretch region (2700–3700 cm −1) is probed simultaneously. The fast redistribution produces a nonthermal population distribution within CH and OH stretching excitations. The nonequilibrium CH stretch population occurs because different CH stretching modes have different lifetimes. The nonequilibrium OH stretch population occurs when OH stretching excitations that are pumped by the laser on the red edge of the band undergo spectral diffusion toward the band center.

Estimation of parameters of deleterious mutations in partial selfing or partial outcrossing populations and in nonequilibrium populations.

The Deng-Lynch method was developed to estimate the rate and effects of deleterious genomic mutations (DGM) in natural populations under the assumption that populations are either completely outcrossing or completely selfing and that populations are at mutation-selection (M-S) balance. However, in many plant and animal populations, selfing or outcrossing is often incomplete in that a proportion of populations undergo inbreeding while the rest are outcrossing. In addition, the degrees of deviation of populations from M-S balance are often not known. Through computer simulations, we investigated the robustness and the applicability of the Deng-Lynch method under different degrees of partial selfing or partial outcrossing and for nonequilibrium populations approaching M-S balance at different stages. The investigation was implemented under constant, variable, and epistatic mutation effects. We found that, generally, the estimation by the Deng-Lynch method is fairly robust if the selfing rate (S) is <0.10 in outcrossing populations and if S > 0.8 in selfing populations. The estimation may be unbiased under partial selfing with variable and epistatic mutation effects in predominantly outcrossing populations. The estimation is fairly robust in nonequilibrium populations at different stages approaching M-S balance. The dynamics of populations approaching M-S balance under various parameters are also studied. Under mutation and selection, populations approach balance at a rapid pace. Generally, it takes 400-2000 generations to reach M-S balance even when starting from homogeneous individuals free of DGM. Our investigation here provides a basis for characterizing DGM in partial selfing or outcrossing populations and for nonequilibrium populations.

GENETIC VARIATION IN THE SPREAD OF DROSOPHILA SUBOBSCURA FROM A NONEQUILIBRIUM POPULATION

Drosophila subobscura was first identified in North America in the early 1980s, and a newer D. subobscura population in Utah appears to have been established more than 10 years later. In this study, we use nuclear microsatellite allele frequencies, mitochondrial restriction fragment length polymorphism (RFLP) allele frequencies, and computer simulations to investigate possible scenarios of how this species has spread across North America. Our method develops a 95% confidence interval for the maximum and minimum number of founders that could have colonized the new population given various scenarios for spread. Unlike many other methods, it may be applied to nonequilibrium source populations given certain conditions. We find that observed allele frequency differences between newer and older D. subobscura populations are consistent with very few inseminated females being transported east from the coast in a single step or with larger numbers of colonizers invading after several intermediate steps. They are not consistent with a large, panmictic population of D. subobscura colonizing Utah in a single step.

Scanned potential microscopy of edge states in a quantum Hall liquid

Using a low-temperature atomic force microscope as a local voltmeter, we measure the Hall voltage profile in a quantum Hall conductor in the presence of a gate-induced non-equilibrium edge state population at ν=3. We observe sharp voltage drops at the sample edges which are suppressed by re-equilibrating the edge states.

GENETIC VARIATION IN THE SPREAD OF DROSOPHILA SUBOBSCURA FROM A NONEQUILIBRIUM POPULATION

Drosophila subobscura was first identified in North America in the early 1980s, and a newer D. subobscura population in Utah appears to have been established more than 10 years later. In this study, we use nuclear microsatellite allele frequencies, mitochondrial restriction fragment length polymorphism (RFLP) allele frequencies, and computer simulations to investigate possible scenarios of how this species has spread across North America. Our method develops a 95% confidence interval for the maximum and minimum number of founders that could have colonized the new population given various scenarios for spread. Unlike many other methods, it may be applied to nonequilibrium source populations given certain conditions. We find that observed allele frequency differences between newer and older D. subobscura populations are consistent with very few inseminated females being transported east from the coast in a single step or with larger numbers of colonizers invading after several intermediate steps. They are not consistent with a large, panmictic population of D. subobscura colonizing Utah in a single step.Corresponding Editor: K. Ross

Hierarchy of vibrational state sets for the solution of the CO2 non-LTE problem

The extended non-LTE model for the CO2 infrared bands in the Earth atmosphere includes a set of 328 vibrational states of seven CO2 isotopes (Shved et al., 1998). To simplify solving the non-LTE problem of radiative transfer in the CO2 bands, a procedure of reducing the amount of the vibrational states is formulated. A hierarchy of three models, which are simplier than the extended model but provide a reliable accuracy of estimating non-equilibrium populations, is proposed. In the direction of decreasing accuracy, these simplified models include 206 and 117 states of seven CO2 isotopes, and 60 states of five CO2 isotopes, respectively. Even the simplest model allows evaluation the populations of the lower vibrational states with a reasonable precision. The errors in estimating the total non-LTE limb radiance for the 4.3 and 15 μm CO2 bands with this model do not exceed the order of 10% and 1%, respectively. Use of the simplest model in evaluating the rate of net radiative heating in all the infrared CO2 bands gives errors which do not exceed ∼ 0.01 K/day above about 80 km and increase down to the stratopause up to ∼ 0.04 K/day.

Observation of bottleneck effects on the photoluminescence from polaritons in II–VI microcavities

The photoluminescence dynamics of microcavity polaritons are investigated by ps time-resolved measurements as a function of the photon–exciton detuning in the low, non-resonant excitation regime. For negative detuning, we observe the appearance of a relaxation slow-down accompanied by a build-up of a non-equilibrium polariton population on the exciton-like part of the lower branch. Finally, the role in the PL dynamics of the dark excitonic states existing in microcavities containing several quantum wells as alternative relaxation channels is addressed.

Laser-Induced Nonequilibrium Electron Distribution in Metals on a Femtosecond Time Scale

A nonequilibrium distribution function for electrons in metals excited by ultrashort laser pulses and its relaxation dynamics is analyzed. This distribution function is based on a kinetic differential equation for the nonequilibrium population of excited electrons. The deviation of the distribution function from the equilibrium one can be monitored by femtosecond broadband pump-probe spectroscopy. Transient changes of the dielectric function Δε and the relaxation of nonequilibrium charge carriers in a thin gold film are discussed for different excitation energies ℏωpump that corresponds to intra- or interband excitation or to photoemission of the excited electrons.

Time-Resolved Optical Observation of Spin-Wave Dynamics

We have created a nonequilibrium population of antiferromagnetic spin waves in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, and characterized its dynamics, using frequency- and time-resolved nonlinear optical spectroscopy of the exciton-magnon transition. We observe a time-dependent pump-probe line shape, which results from excitation induced renormalization of the spin-wave band structure. We present a model that reproduces the basic characteristics of the data, in which we postulate the optical nonlinearity to be dominated by interactions with long-wavelength spin waves, and the dynamics to be due to spin-wave thermalization.

Spin tunnel injection properties of YBa2Cu3Oy films

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by tunnel injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The injection of spin-polarized quasiparticles generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The measurements of transport properties for the directionally oriented spin-polarized quasiparticle injection on the [001] axis and the [110] surfaces of YBCO thin film show anisotropic behaviour according to the junction orientation.

Spectral and energy characteristics of a pulsed hydrogen fluoride laser and rotational relaxation of HF molecules

The spectrotemporal and energy characteristics of the radiation of a pulsed chemical H2—F2 laser were calculated on the basis of a multilevel model taking into account the nonequilibrium population of the rotational levels, and were compared with experimental data. The theoretical model took into account the stimulated emission of radiation as a result of the ν, j−1 → ν−1, j transitions (ν = 1–6 and j=4–12) in the HF molecule. The rotational relaxation rate constants of the HF molecule were varied in the calculations. Comparison of the calculated and experimental spectral characteristics of the laser radiation indicates that the rate of the rotational transitions involving high (j=10–12) rotational sublevels of HF is insufficient, leading to an appreciable reduction in the energy of the output radiation for lines with such values of j, compared with lines characterised by j=7, 8.

Mode specific vibrational kinetics after intramolecular electron transfer studied by picosecond anti-Stokes Raman spectroscopy

Picosecond vibrational relaxation initiated by intramolecular electron back-transfer of Betaine 30 in propylene carbonate is studied by time-resolved anti-Stokes Raman scattering. We demonstrate for the first time distinct nonequilibrium populations of vibrational modes in the electronic ground state that persist up to 10 ps. The strongest Raman band at 1607 cm −1 exhibits a rise time close to the electron transfer time of 1 ps, suggesting that this mode is involved in electron transfer. Vibrations between 400 and 1400 cm −1 display slower rise times. Quasi-equilibrium populations of the Raman active modes are established after 10 ps.

New methods for determining the polarization state of vacuum ultraviolet radiation

Two new methods are proposed for determining the polarization of vacuum ultraviolet radiation which permit the determination of an arbitrary polarization mode for photons with energies of 10–100 eV. The essence of these methods is to create and detect a nonequilibrium population of the magnetic sublevels of atoms and molecules excited by the original VUV radiation and then determine the polarization of this radiation based on these measurements in accordance with known formulas.

Spin-polarized quasiparticle tunnel injection in a YBa2Cu3Oy/Au/Co junction

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by injection of spin-polarized quasiparticles (QP) using a cobalt ferromagnetic injector. The injection of spin-polarized QP generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The observed current gain depends on the thickness of Au interlayer (dAu) and is directly related to the nonequilibrium magnetization due to spin relaxation effects. For dAu=15 nm, the tunnel characteristic a YBCO/Au/Co junction exhibited a zero bias conductance peak, which may be interpreted by spin scattering processes at a ferromagnetic/d-wave superconductor junction.

Nonequilibrium Population Dynamics of "Ideal and Free" Prey and Predators.

Aggregated spatial distributions of prey and predators promote stability of the otherwise unstable Nicholson-Bailey model. Nevertheless, when both predators and prey choose patches in an ideal free way, sufficiently aggregated distributions will only arise if patch quality (e.g., reflected in prey fecundity) is very heterogeneous. This requirement profoundly limits the possibilities for simultaneous population dynamical and evolutionary stability. However, stability is not necessary for coexistence. Under cyclic or chaotic dynamics, the rate at which species change their distributions becomes important. Here we consider two endpoints of a continuum of rates. The first is "rigid" selection of patch types based on the expected long-term distribution of conditions. The second is "flexible" selection based on current conditions. We carried out systematic surveys of the population-level consequences of coevolutionarily stable patch-selection strategies for different combinations of rigid and flexible strategies of predator and prey. First, if both prey and predators have rigid strategies, the evolutionary end result is either a stable population dynamical equilibrium or diverging cycles eventually leading to extinction. Second, for the case with rigid prey and flexible predators, the persistence boundary in parameter space is shifted from the boundaries obtained for rigid predators. The mechanism underlying persistence is different in that the flexible strategies of the predators destabilize the equilibrium, while the evolutionary response of the rigid prey leads to reduced cycles. Third, if both prey and predators are flexible, simulations lead either to chaotic fluctuations or to extinction (but not to stable equilibria, nor to limit cycles), and the conditions for coexistence are much wider than those under rigid patch selection. These simulations suggest that information constraints on adaptive patch choice have a major impact on predator-prey persistence under nonequilibrium conditions. We discuss how these predictions can be tested by field observations on expansions and contractions in dietary range (or habitat range) in relation to population fluctuations.

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).

Separation of chlorocyclohexane's axial and equatorial conformer infrared spectrum by isothermal relaxation in the glass→liquid transition region

A new low-temperature method enables separation of the infrared spectrum of a mixture of conformers into the distinct spectra of its constituents, and its merits are demonstrated here for chlorocyclohexane (Cl–CH). Nonequilibrium population of Cl–CH's axial (a) and equatorial (e) conformers was generated by rapid quenching of their vapours into the glassy state. Interconversion of conformers was accomplished by isothermal relaxation in the glass→liquid transition, and was followed by Fourier-transform infrared spectroscopy. These spectra were separated into the distinct spectra of the a and e-conformer. Assignment of infrared bands of the separated spectra to the normal vibrations of the conformers is consistent with those reported in the literature with two exceptions.