Landau thermodynamic potential for BaTiO_3

In the paper, the description of the dielectric and ferroelectric properties of BaTiO_3 single crystals using Landau thermodynamic potential is addressed. Our results suggest that when using the sixth-power free energy expansion of the thermodynamic potential, remarkably different values of the fourth-power coefficient, \beta (the coefficient of P^4_i terms), are required to adequately reproduce the nonlinear dielectric behavior of the paraelectric phase and the electric field induced ferroelectric phase, respectively. In contrast, the eighth-power expansion with a common set of coefficients enables a good description for both phases at the same time. These features, together with the data available in literature, strongly attest to the necessity of the eighth-power terms in Landau thermodynamic potential of BaTiO_3. In addition, the fourth-power coefficients, \beta and \xi (the coefficient of P^2_i P^2_j terms), were evaluated from the nonlinear dielectric responses along [001], [011], and [111] orientations in the paraelectric phase. Appreciable temperature dependence was evidenced for both coefficients above T_C. Further analysis on the linear dielectric response of the single domain crystal in the tetragonal phase demonstrated that temperature dependent anharmonic coefficients are also necessary for an adequate description of the dielectric behavior in the ferroelectric phase. As a consequence, an eighth-power thermodynamic potential, with some of the anharmonic coefficients being temperature dependent, was proposed and compared with the existing potentials. In general, the potential proposed in this work exhibits a higher quality in reproducing the dielectric and ferroelectric properties of this prototypic ferroelectric substance.Comment: 7 figures, 5 table

Anharmonicity of BaTiO_3 single crystals

By analyzing the dielectric non-linearity with the Landau thermodynamic expansion, we find a simple and direct way to assess the importance of the eighth order term. Following this approach, it is demonstrated that the eighth order term is essential for the adequate description of the para/ferroelectric phase transition of BaTiO_3. The temperature dependence of the quartic coefficient \beta is accordingly reconsidered and is strongly evidenced by the change of its sign above 165 C. All these findings attest to the strong polarization anharmonicity of this material, which is unexpected for classical displacive ferroelectrics.Comment: 4 figures, to be published in Phys. Rev.

Phase transitions in open quantum systems

We consider the behaviour of open quantum systems in dependence on the coupling to one decay channel by introducing the coupling parameter $\alpha$ being proportional to the average degree of overlapping. Under critical conditions, a reorganization of the spectrum takes place which creates a bifurcation of the time scales with respect to the lifetimes of the resonance states. We derive analytically the conditions under which the reorganization process can be understood as a second-order phase transition and illustrate our results by numerical investigations. The conditions are fulfilled e.g. for a picket fence with equal coupling of the states to the continuum. Energy dependencies within the system are included. We consider also the generic case of an unfolded Gaussian Orthogonal Ensemble. In all these cases, the reorganization of the spectrum occurs at the critical value $\alpha_{crit}$ of the control parameter globally over the whole energy range of the spectrum. All states act cooperatively.Comment: 28 pages, 22 Postscript figure

Multiscale fluctuations in nuclear response

The nuclear collective response is investigated in the framework of a doorway picture in which the spreading width of the collective motion is described as a coupling to more and more complex configurations. It is shown that this coupling induces fluctuations of the observed strength. In the case of a hierarchy of overlapping decay channels, we observe Ericson fluctuations at different scales. Methods for extracting these scales and the related lifetimes are discussed. Finally, we show that the coupling of different states at one level of complexity to some common decay channels at the next level, may produce interference-like patterns in the nuclear response. This quantum effect leads to a new type of fluctuations with a typical width related to the level spacing.Comment: 34 Latex pages including 6 figures (submitted to Phys. Rev. C

Визначення конфігурації обчислювальних комплексів високопродуктивної обробки великих об’ємів даних

This paper provides a mathematical model of computing complex (a network of queuing). In what volume of processed data outgrows memory compute nodes, so all data are divided into equal blocks of fixed size that are distributed through volumes of data storage and process the task is divided into cycles so that one node computing cluster process one unit data for one cycle. Also describes the method for determining the optimal (by the criterion of productivity) complex configurations based on industry standard benchmarks computing clusters and storage systems.В данной работе приводится математическая модель вычислительного комплекса (как сети массового обслуживания). В какой объем обрабатываемых данных существенно превышает объем памяти вычислительных узлов, поэтому все данные разбиваются на равные блоки фиксированного размера, которые распределяются по томам системы хранения данных, а процесс решения задачи делится на циклы таким образом, что один узел вычислительного кластера обрабатывает один блок данных за один цикл. А также описывается метод определения оптимальной (по критерию производительности) конфигурации комплекса на основе результатов стандартных промышленных тестов производительности вычислительных кластеров и систем хранения данных.У даній роботі наводиться математична модель обчислювального комплексу (як мережі масового обслуговування). В якій обсяг оброблюваних даних істотно перевищує обсяг пам'яті обчислювальних вузлів, тому всі дані розбиваються на рівні блоки фіксованого розміру, що розподіляються по томам системи зберігання даних, а процес вирішення завдання поділяється на цикли таким чином, що один вузол обчислювального кластера обробляє один блок даних за один цикл. А також описується метод визначення оптимальної (за критерієм продуктивності) конфігурації комплексу на основі результатів стандартних промислових тестів продуктивності обчислювальних кластерів і систем зберігання даних

Сегментація мовного сигналу з використанням вейтвлет-перетворення

This article provides a voice channel segmentation algorithm using the multiresolution analysis and wavelet transform, which features both within the stationary sites significant role for the analysis of language play signal spectral characteristics that are determined by the transfer function of the vocal tract, and is changed in the process of articulation. I.e., speech is characterized by nonlinear fluctuations of different size, which are unique to the various speakers. Proposed in this paper algorithm modification for multiresolution analysis allows you to find borders labels of interphoneme transitions for the sounds of different speakers.В данной статье приводится алгоритм сегментации речевого канала с использованием кратномасштабного анализа и вейвлет-преобразования, который показывает как в пределах стационарных участков значительную роль для анализа языка играют спектральные особенности сигнала, которые определяются передаточной характеристикой речевого тракта и изменяется в процессе артикуляции. Т.е. речевой сигнал характеризуется нелинейными флуктуациями различных масштабов, которые являются уникальными для различных дикторов. Предложенные в статье модификации алгоритма кратномасштабного анализа позволяет находить метки границ межфонемних переходов звуковых сигналов различных дикторов.У даній статті наводиться алгоритм сегментації мовного каналу з використанням кратномасштабного аналізу та вейвлет-перетворення, що показує як в межах стаціонарних ділянок значну роль для аналізу мови відіграють спектральні особливості сигналу, що визначаються передатною характеристикою мовного тракту, яка змінюється в процесі артикуляції. Тобто мовний сигнал характеризується нелінійними флуктуаціями різних масштабів, які є унікальними для різних дикторів. Запропоновані у статті модифікації алгоритму кратномасштабного аналізу дозволяє знаходити мітки границь мiжфонемних переходів у звукових сигналах різних дикторів

A CMOS-based neural implantable optrode for optogenetic stimulation and electrical recording

This paper presents a novel integrated optrode for simultaneous optical stimulation and electrical recording for closed -loop optogenetic neuro-prosthetic applications. The design has been implemented in a commercially available 0.35μm CMOS process. The system includes circuits for controlling the optical stimulations; recording local field potentials (LFPs); and onboard diagnostics. The neural interface has two clusters of stimulation and recording sites. Each stimulation site has a bonding point for connecting a micro light emitting diode (μLED) to deliver light to the targeted area of brain tissue. Each recording site is designed to be post-processed with electrode materials to provide monitoring of neural activity. On-chip diagnostic sensing has been included to provide real-time diagnostics for post-implantation and during normal operation

Interfering resonances in a quantum billiard

We present a method for numerically obtaining the positions, widths and wavefunctions of resonance states in a two dimensional billiard connected to a waveguide. For a rectangular billiard, we study the dynamics of three resonance poles lying separated from the other ones. As a function of increasing coupling strength between the waveguide and the billiard two of the states become trapped while the width of the third one continues to increase for all coupling strengths. This behavior of the resonance poles is reflected in the time delay function which can be studied experimentally.Comment: 2 pages, 3 figure

Neutron, electron and X-ray scattering investigation of Cr1-xVx near Quantum Criticality

The weakness of electron-electron correlations in the itinerant antiferromagnet Cr doped with V has long been considered the reason that neither new collective electronic states or even non Fermi liquid behaviour are observed when antiferromagnetism in Cr$_{1-x}$V$_{x}$ is suppressed to zero temperature. We present the results of neutron and electron diffraction measurements of several lightly doped single crystals of Cr$_{1-x}$V$_{x}$ in which the archtypal spin density wave instability is progressively suppressed as the V content increases, freeing the nesting-prone Fermi surface for a new striped charge instability that occurs at x$_{c}$=0.037. This novel nesting driven instability relieves the entropy accumulation associated with the suppression of the spin density wave and avoids the formation of a quantum critical point by stabilising a new type of charge order at temperatures in excess of 400 K. Restructuring of the Fermi surface near quantum critical points is a feature found in materials as diverse as heavy fermions, high temperature copper oxide superconductors and now even elemental metals such as Cr.Comment: 6 pages, 6 figures. Accepted to Physical Review