Hydrated Electron Dynamics at a Five Femtosecond Time Scale

Hydrated electrons are studied by frequency resolved pump probe with 5 fs time resolution in the spectral range from 600 nm to 1000 nm. A recurrence detected in the pumpprobe signal at —40 fs is tentatively assigned to coupling to librational motions in the electron's solvent cage

Magnetization of ferrofluids with dipolar interactions - a Born--Mayer expansion

For ferrofluids that are described by a system of hard spheres interacting via dipolar forces we evaluate the magnetization as a function of the internal magnetic field with a Born--Mayer technique and an expansion in the dipolar coupling strength. Two different approximations are presented for the magnetization considering different contributions to a series expansion in terms of the volume fraction of the particles and the dipolar coupling strength.Comment: 19 pages, 11 figures submitted to PR

Magnetic properties of colloidal suspensions of interacting magnetic particles

We review equilibrium thermodynamic properties of systems of magnetic particles like ferrofluids in which dipolar interactions play an important role. The review is focussed on two subjects: ({\em i}) the magnetization with the initial magnetic susceptibility as a special case and ({\em ii}) the phase transition behavior. Here the condensation ("gas/liquid") transition in the subsystem of the suspended particles is treated as well as the isotropic/ferromagnetic transition to a state with spontaneously generated long--range magnetic order.Comment: Review. 62 pages, 4 figure

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: A combined experimental and theoretical study

The interweave of competing individual relaxations influenced by the presence of temperature and concentration dependent correlations is an intrinsic feature of superparamagnetic nanoparticle suspensions. This unique combination gives rise to multiple applications of such suspensions in medicine, nanotechnology and microfluidics. Here, using theory and experiment, we investigate dynamic magnetic susceptibility in a broad range of temperatures and frequencies. Our approach allows, for the first time to our knowledge, to separate clearly the effects of superparamagnetic particle polydispersity and interparticle magnetic interactions on the dynamic spectra of these systems. In this way, we not only provide a theoretical model that can predict well the dynamic response of magnetic nanoparticles systems, but also deepen the understanding of the dynamic nanoparticle self-assembly, opening new perspectives in tuning and controlling the magnetic behaviour of such systems in AC fields. © 2016 the Owner Societies

Theoretical study of the magnetization dynamics of non-dilute ferrofluids

The paper is devoted to the theoretical investigation of the magnetodipolar interparticle interaction effect on remagnetization dynamics in moderately concentrated ferrofluids. We consider a homogeneous (without particle aggregates) ferrofluid consisting of identical spherical particles and employ a rigid dipole model, where magnetic moment of a particle is fixed with respect to the particle itself. In particular, for the magnetization relaxation after the external field is instantly switched off, we show that the magnetodipolar interaction leads to the increase of the initial magnetization relaxation time. For the complex ac-susceptibility we find that the this interaction leads to an overall increase of the imaginary susceptibility part and shifts the peak on its frequency dependence towards lower frequencies. Comparing results obtained with our analytical approach (second order virial expansion) to numerical simulation data (Langevin dynamics method), we demonstrate that the employed virial expansion approximation gives a good qualitative description of the ferrofluid magnetization dynamics and provides a satisfactory quantitative agreement with numerical simulations for the dc magnetization relaxation - up to the particle volume fraction c ~ 10% and for the ac-susceptibility - up to c ~ 5 %.Comment: 12 pages, 6 figures, submitted to PR

Теоретические предложения по повышению помехоустойчивости приема многопозиционных сигналов в каналах с переменными параметрами

Introduction. At present, the noise immunity of receiving multi-position signals in channels with variable parameters is improved using various signal structures (SS). In particular, in communication systems and the DVB-T2 television standard, these are quadrature amplitude modulation (QAM) signals with transformed constellation diagrams. However, in practical calculations of communication systems, the existing SS models fail to take into account the random nature of changes in the phases of the transformed signal constellation. This, in turn, leads to a discrepancy between the analytical value of error probability and its real value due to asynchronism in the radio link. The SS model proposed in this paper and the obtained analytical ratio take into account the introduced phase distortions in channels with variable parameters.Aim. Development of theoretical proposals for improving the efficiency of receiving QAM signals in radio channels with variable parameters.Materials and methods. The considered transformed SS model and the resulting analytical relation are described on the basis of communication theory and signal theory in the subject area of noise immunity research methods. This, in turn, enables analysis of the effect of phase distortions in channels with variable parameters on the error probability of receiving QAM signal elements.Results. A transformed SS model with improved energy characteristics and an analytical relation for calculating the error probability of receiving QAM signal elements are proposed. Theoretical proposals for improving the noise immunity of receiving multi-position signals in channels with variable parameters are formulated.Conclusion. The developed theoretical proposals for improving the noise immunity of multi-position quadrature signal structures in channels with variable parameters make it possible to improve their energy characteristics, taking into account phase distortions introduced by the communication channel. The presented dependence makes it possible to evaluate the relationship between the values of the probability of a pair error of receiving QAM signal elements and the limits of the change in phase shifts introduced by a communication channel with variable parameters. Future research will address the development of scientific and practical proposals for improving the noise immunity of quadrature multi-position signals, including an algorithm and block diagram for compensating phase shifts introduced in communication channels; processing of the amplitude values of the signal, which assumes the difference in the paths in terms of frequency-polarization and determines the accuracy of eliminating phase distortions.Введение. В настоящее время для повышения помехоустойчивости приема многопозиционных сигналов в каналах с переменными параметрами применяют различные сигнальные конструкции (СК). В частности, в системах связи и телевизионном стандарте DVB-T2 это сигналы квадратурной амплитудной модуляции (КАМ) с трансформированными констелляционными диаграммами. Однако существующие модели СК при практических расчетах систем связи не учитывают случайный характер изменения фаз трансформированного сигнального созвездия. Это, в свою очередь, приводит к расхождению аналитического значения вероятности ошибки с реальным вследствие асинхронизма в радиолинии. Предлагаемая модель СК и полученное аналитическое соотношение позволяют учитывать вносимые фазовые искажения в каналах с переменными параметрами.Цель работы. Разработка теоретических предложений по повышению эффективности приема сигналов КАМ в радиоканалах с переменными параметрами.Материалы и методы. Рассматриваемая трансформированная модель СК и полученное аналитическое соотношение описаны на основе теории связи, теории сигналов в предметной области методов исследования помехоустойчивости. Это, в свою очередь, позволяет проанализировать влияние фазовых искажений в каналах с переменными параметрами на значение вероятности ошибки приема элементов сигналов КАМ.Результаты. Предложена трансформированная модель СК с улучшенными энергетическими характеристиками и аналитическое соотношение для расчета вероятности ошибки приема элементов сигналов КАМ. Сформулированы теоретические предложения по повышению помехоустойчивости приема многопозиционных сигналов в каналах с переменными параметрами.Заключение. Разработанные теоретические предложения по повышению помехоустойчивости многопозиционных квадратурных сигнальных конструкций в каналах с переменными параметрами позволяют улучшить их энергетические характеристики с учетом фазовых искажений, вносимых каналом связи. Представленная зависимость дает возможность оценить взаимосвязь между значениями вероятности парной ошибки приема элементов сигналов КАМ и пределами изменения фазовых сдвигов, вносимых каналом связи с переменными параметрами. Кроме того, указанные теоретические предложения предопределили направления дальнейших исследований, заключающиеся в разработке научно-практических предложений по повышению помехоустойчивости квадратурных многопозиционных сигналов: алгоритм и структурную схему компенсации фазовых сдвигов, вносимых в каналах связи; обработку амплитудных значений сигнала, предполагающую различие трактов по частоте-поляризации и определяющую точность устранения фазовых искажений

Pair Correlations in a Bidisperse Ferrofluid in an External Magnetic Field:Theory and Computer Simulations

The pair distribution function g(r) for a ferrofluid modeled by a bidisperse system of dipolar hard spheres is calculated. The influence of an external uniform magnetic field and polydispersity on g(r) and the related structure factor is studied. The calculation is performed by diagrammatic expansion methods within the thermodynamic perturbation theory in terms of the particle number density and the interparticle dipole–dipole interaction strength. Analytical expressions are provided for the pair distribution function to within the first order in number density and the second order in dipole–dipole interaction strength. The constructed theory is compared with the results of computer (Monte Carlo) simulations to determine the range of its validity. The scattering structure factor is determined using the Fourier transform of the pair correlation func-tion g(r) – 1. The influence of the granulometric composition and magnetic field strength on the height and position of the first peak of the structure factor that is most amenable to an experimental study is analyzed. The data obtained can serve as a basis for interpreting the experimental small[1]angle neutron scattering results and determining the regularities in the behavior of the structure factor, its dependence on the fractional com-position of a ferrofluid, interparticle correlations, and external magnetic field. © Pleiades Publishing, Inc., 2014