CHARACTERISTICS OF THE MAGNETIC DEPOPULATION OF SUBBANDS IN VERY NARROW SYSTEMS

We present a model for electrons confined in narrow conducting channels by a parabolic well under moderate to high magnetic fields which takes into account a cutoff in the filling of the subbands. Such a cutoff gives rise to energy-separated subbands and a two-dimensional (2D) like subband depopulation, resulting in a relation between sublevel index n and inverse magnetic field B-1 such that in the high-field regime it changes over to the well-known 2D form as expected, and in the moderate field regime it shows pronounced deviation from linearity. This agrees well with the experimental results. The linear region of the n-B-1 experimental plot is believed to arise from the two dimensionality of the system. Calculations show that no resolvable 1D sublevel exists in the 0.5-mu-m-wide wire at very small magnetic fields (including zero field), which agrees qualitatively with the experimental results found in other wires that the Hall resistance, R(H), approaches its classical value B/n(e)e in this region and R(H) = 0 at B = 0, where n(e) is the electron concentration. In this model the linear and nonlinear regions in the experimental n-B-1 plot are used to extract the characteristic frequency omega-0, and the effective 2D electron concentration N(e)2D, respectively.

Sources and spread of thermophilic Campylobacter spp. during partial depopulation of broiler chicken flocks

The practice of partial depopulation or ‘thinning’, i.e. early removal of a proportion of birds from a commercial broiler flock, is a reported risk factor for Campylobacter colonization of residual birds because of the difficulty in maintaining biosecurity during the process. Therefore, the effect of this practice was studied in detail for 51 target flocks, each at a different growing farm belonging to one of seven major poultry companies throughout the United Kingdom. On 21 of these farms, the target flock was already colonized by Campylobacter and at slaughter all cecal samples examined were positive, with a mean of log10 8 cfu / g. A further 27 flocks became positive within 2 – 6 days of the start of thinning and had similarly high levels of cecal carriage at slaughter. Just prior to the thinning process, Campylobacter could be isolated frequently from the farm driveways, transport vehicles, equipment and personnel. Strains from seven such farms on which flocks became colonized after thinning were examined by PFGE typing. The study demonstrated an association between strains occurring at specific sampling sites and those isolated subsequently from the thinned flocks. There were also indications that particular strains had spread from one farm to another, when the farms were jointly company-owned and served by the same bird-catching teams and / or vehicles. The results highlighted the need for better hygiene control in relation to catching equipment and personnel, and more effective cleaning and disinfection of vehicles, and bird-transport crates.

Assessing the Free Land Programs for Reversing Rural Depopulation

A number of small towns in the Great Plains have recently started to offer free land and other incentives to entice new residents in the hope of reversing persistent depopulation. Based on in-depth interviews, this study assesses the initial performance of the free land programs in six small towns in central Kansas and analyzes the factors that have affected the migration decisions of the new residents. The initial results of these programs have been impressive. Not only have they attracted multiple new residents and increased enrollments in local schools, but they have also elevated long-time residents' pride in their community and created a positive synergy. The new residents' migration decisions were influenced by a number of push and pull factors. The free land and other incentives are not enough to trigger migration, but they have effectively changed some migrants' destination choice to a small town in central Kansas. Without the free land, most new residents, particularly those from out of state, would not have moved there. Contrary to our expectations, the relative locations of small towns with respect to larger cities do not appear to have affected new residents' destination choice.

GIS analysis for the study of rural depopulation: the Kingdom of Valencia (Tomas Lopez 1789)

One of the more aspects that have shaped the landscape is the human impact. The human impact has the clearest indicator of the density of settlements in a particular geographic region. In this paper we study all settlements shown on the map of the Kingdom of Valencia, Spain Geographic Atlas (AGE) of Tomas Lopez (1788), and their correspondence with the current ones. To meet this goal we have developed a specific methodology, the systematic study of all existing settlements in historical cartography. This will determine which have disappeared and which have been renamed. The material used has been the historical cartography of Tomas Lopez, part of the AGE (1789), the Kingdom of Valencia (1789), sheets numbers (78, 79, 80 and 81); Current mapping of the provinces of Alicante, Valencia, Castellon, Teruel, Tattagona and Cuenca; As main software ArcGis V.9.3. The steps followed in the methodology are as follows: 1. Check the scale of the maps. Analyze the possible use of a spherical earth model. 2. Geo-reference of maps with latitude and longitude framework. Move the historical longitude origin to the origin longitude of modern cartography. 3 Digitize of all population settlements or cities. 4 Identify historic settlements or cities corresponding with current ones. 5. If the maps have the same orientation and scale, replace the coordinate transformation of historical settlements with a new one, by a translation in latitude and longitude equal to the calculated mean value of all ancient map points corresponding to the new. 6. Calculation of absolute accuracy of the two maps, i.e. the linear distance between the points of both maps. 7 draw in the GIS, the settlements without correspondence, in the current coordinates, and with a circle of mean error of the sheet, in order to locate their current location. If there are actual settlements exist within this circle, they are candidates to be the searched settlements. We analyzed more than 2000 settlements represented in the Atlas of Tomas Lopez of the Kingdom of Valencia (1789), of which almost 14.5% have no correspondence with the existing settlements. The rural landscape evolution of the Valencia, oldest kingdom of Valencia, one can say that can be severely affected by the anthropization suffered in the period from 1789 to the present, since 70% of existing settlements actually have appeared after Tomas Lopez¿s cartography, dated on 1789

Policing the rural crisis

There is a growing sense of crisis in rural ways of life, which manifests itself in economic decline, depopulation, depleted environments, and a crisis of rural identities. Crime is one potent marker of crisis, the more so as it spoils the image of healthy, cohesive community. The social reaction it elicits, the policing of this ‘other rural’, is also a guide to the dimensions of crisis. The social sciences have witnessed a renewed international interest in the study of ‘other rurals’: the neglected, invisible or excluded aspects of country life. This book brings a fresh approach to the study of crime that challenges the urban-centric assumptions of much western criminology and sociology. It explores rural crime and social reactions to it, in relation to processes and patterns of community formation and change in rural Australia, including the social, economic, cultural and political forces shaping the history, structure and everyday life of rural communities. Policing the Rural Crisis is based on five years of extensive original empirical research in rural and regional Australia. It draws on ideas and debates in contemporary social theory across several disciplines, making the analysis relevant to the study of crime and social change elsewhere.

Exciplexes in fluorescence quenching of aromatic hydrocarbons by tertiary aliphatic amines

Strong quenching of the fluorescence of aromatic hydrocarbons by tertiary aliphatic amines has been observed in solution at room temperature. Accompanying the fluorescence quenching of aromatic hydrocarbons, an anomalous emission is observed. This new emission is very broad, structureless and red-shifted from the original hydrocarbon fluorescence.

Kinetic studies indicate that this anomalous emission is due to an exciplex formed by an aromatic hydrocarbon molecule in its lowest excited singlet state with an amine molecule. The fluorescence quenching of the aromatic hydrocarbons is due to the depopulation of excited hydrocarbon molecules by the formation of exciplexes, with subsequent de-excitation of exciplexes by either radiative or non-radiative processes.

Analysis of rate constants shows the electron-transfer nature of the exciplex. Through the study of the effects on the frequencies of exciplex emissions of substituents on the hydrocarbons, it is concluded that partial electron transfer from the amine molecule to the aromatic hydrocarbon molecule in its lowest excited singlet state occurs in the formation of exciplex. Solvent effects on the exciplex emission frequencies further demonstrate the polar nature of the exciplex.

A model based on this electron-transfer nature of exciplex is proposed and proves satisfactory in interpreting the exciplex emission phenomenon in the fluorescence quenching of aromatic hydrocarbons by tertiary aliphatic amines.

I. Phosphorescence and the true lifetime of triplet states in fluid solutions. II. Why is condensed oxygen blue?

I. PHOSPHORESCENCE AND THE TRUE LIFETIME OF TRIPLET STATES IN FLUID SOLUTIONS

Phosphorescence has been observed in a highly purified fluid solution of naphthalene in 3-methylpentane (3-MP). The phosphorescence lifetime of C₁₀H₈ in 3-MP at -45 °C was found to be 0.49 ± 0.07 sec, while that of C₁₀D₈ under identical conditions is 0.64 ± 0.07 sec. At this temperature 3-MP has the same viscosity (0.65 centipoise) as that of benzene at room temperature. It is believed that even these long lifetimes are dominated by impurity quenching mechanisms. Therefore it seems that the radiationless decay times of the lowest triplet states of simple aromatic hydrocarbons in liquid solutions are sensibly the same as those in the solid phase. A slight dependence of the phosphorescence lifetime on solvent viscosity was observed in the temperature region, -60° to -18°C. This has been attributed to the diffusion-controlled quenching of the triplet state by residual impurity, perhaps oxygen. Bimolecular depopulation of the triplet state was found to be of major importance over a large part of the triplet decay.

The lifetime of triplet C₁₀H₈ at room temperature was also measured in highly purified benzene by means of both phosphorescence and triplet-triplet absorption. The lifetime was estimated to be at least ten times shorter than that in 3-MP. This is believed to be due not only to residual impurities in the solvent but also to small amounts of impurities produced through unavoidable irradiation by the excitation source. In agreement with this idea, lifetime shortening caused by intense flashes of light is readily observed. This latter result suggests that experiments employing flash lamp techniques are not suitable for these kinds of studies.

The theory of radiationless transitions, based on Robinson's theory, is briefly outlined. A simple theoretical model which is derived from Fano's autoionization gives identical result.

Il. WHY IS CONDENSED OXYGEN BLUE?

The blue color of oxygen is mostly derived from double transitions. This paper presents a theoretical calculation of the intensity of the double transition (a ¹Δ_g) (a ¹Δ_g)←(X ³Σ_g^-) (X ³Σ_g^-), using a model based on a pair of oxygen molecules at a fixed separation of 3.81 Å. The intensity enhancement is assumed to be derived from the mixing (a ¹Δ_g) (a ¹Δ_g) _~~~ (X ³Σ_g^-) (X ³Σ_u^-) and (a ¹Δ_g) (¹Δ_u) _~~~ (X ³Σ_g^-) (X ³Σ_g^-). Matrix elements for these interactions are calculated using a π-electron approximation for the pair system. Good molecular wavefunctions are used for all but the perturbing (B ³Σ_u^-) state, which is approximated in terms of ground state orbitals. The largest contribution to the matrix elements arises from large intramolecular terms multiplied by intermolecular overlap integrals. The strength of interaction depends not only on the intermolecular separation of the two oxygen molecules, but also as expected on the relative orientation. Matrix elements are calculated for different orientations, and the angular dependence is fit to an analytical expression. The theory therefore not only predicts an intensity dependence on density but also one on phase at constant density. Agreement between theory and available experimental results is satisfactory considering the nature of the approximation, and indicates the essential validity of the overall approach to this interesting intensity enhancement problem.

Anomalous temperature dependence of photoluminescence in GaInNAs/GaAs multiple quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells grown on a GaAs substrate by molecular beam epitaxy are measured in a range of temperatures and excitation power densities. The energy position of the dominant PL peak shows an anomalous S-shape temperature dependence instead of the Varshni relation. By careful inspection, especially for the PL under lower excitation power density, two near bandedge peaks are well identified. These are assigned to carriers localized in nitrogen-induced bound states and interband excitonic recombinations, respectively. It is suggested that the temperature-induced switch of such two luminescence peaks in relative intensity causes a significant mechanism responsible for the S-shape shift observed in GaInNAs. A quantitative model based on the thermal depopulation of carriers is used to explain the temperature dependence of the PL peak related to N-induced bound states.

EFFECT OF IMAGE FORCES ON ELECTRONS CONFINED IN LOW-DIMENSIONAL STRUCTURES UNDER A MAGNETIC-FIELD

We consider the effect of image forces, arising due to a difference in dielectric permeabilities of the well layer and barrier layers, on the energy spectrum of an electron confined in a rectangular potential well under a magnetic field. Depending on the value and the sign of the dielectric mismatch, image forces can localize electrons near the interfaces of the well or in well centre and change the direct intersubband gaps into indirect ones. These effects can be controlled by variation of the magnetic field, offering possibilities for exact tuning of electronic devices.

SIMULATION OF LATERAL CONFINEMENT IN VERY NARROW CHANNELS

A theoretical study is presented of the lateral confinement potential (CP) in the very narrow mesa channels fabricated in the conventional two-dimensional (2D) electron gas in GaAs-AlxGa1-xAs heterostructures. The ID electronic structures are calculated in the framework of the confinement potential: V(x) = m* omega0(2)x2/2 for Absolute value of x

THEORETICAL CALCULATION FOR SHUBNIKOV-DEHAAS OSCILLATIONS IN DEEP MESA STRUCTURES

We have used the rectangular confinement potential to describe Shubnikov-deHaas oscillations produced by one-dimensional electrons confined in deep mesa structures. The edge distortion of the confinement potential caused by electrostatic image forces is taken into account. The model contains no fitting parameters and relates well with experimental data. The comparison with earlier reported parabolic model is presented,

Ultrafast vibrational and thermal relaxation of dye molecules in solutions

Intra- and intermolecular relaxations of dye molecules are studied after the excitation to the high-lying excited states by a femtosecond laser pulse, using femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD). The biexponential decays indicate a rapid intramolecular vibrational redistribution (IVR) depopulation followed by a slower process, which was contributed by the energy transfer to the solvents and the solvation of the excited solutes. The time constants of IVR in both oxazine 750 and rhodamine 700 are at the 290-360 fs range, which are insensitive to the characters of solvents. The solvation of the excited solutes and the cooling of the hot solute molecules by collisional energy transfer to the surrounding takes place in the several picoseconds that strongly depend on the properties of solvents. The difference of Lewis basicity and states density of solvents is a possible reason to explain this solvent dependence. The more basic the solvent is, which means the more interaction between the solute and the neighboring solvent shell, the more rapid the intermolecular vibrational excess energy transfer from the solute to the surroundings and the solvation of the solutes are. The higher the states density of the solvent is, the more favorable the energy transfer between the solute and solvent molecules is.