Spatial-temporal dynamics of the terahertz field generated by femtosecond filament

We present the study on spatial distribution of the maximum of terahertz field amplitude in time domain when generated by a femtosecond filament. It is shown that as a result of the propagation of the terahertz field forms a spherical wave front, on the edge of which the maximum of amplitude has a temporary delay in contrary to its central part

Concentration dependence of terahertz generation in jets of water and ethanol mixtures

In this work, we considered mixtures of ethanol and water in the form of jets as samples for THz generation based on laser-induced filamentation. The dependence of the output energy of terahertz radiation on the concentration of ethanol in water was experimentally studied. It is shown that the energy grows linearly, which can be explained by an increase in the ionization energy due to the linear replacement of low-efficient charge carriers (water) with highly-efficient (ethanol). The dependence of the THz generation on the optical angle of incidence on the mixture jets was also demonstrated. The results of this study can be further used to create universal source of terahertz radiation

High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)

We have measured the pressure sensitivity of Tc in fluorinated HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying pressures up to 30 GPa. We obtained that Tc increases with increasing pressure, reaching different maximum values, depending on the F doping level, and decreases for a further increase of pressure. A new high Tc record (166 K +/- 1 K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample near the optimum doping level. Our results show that all our samples are at the optimal doping, and that fluorine incorporation decreases the crystallographic $a$-parameter concomitantly increasing the maximum attainable Tc. This effect reveals that the compression of the $a$ axes is one of the keys that controls the Tc of high temperature superconductors.Comment: 4 pages, 4 figures, submitted to Phys. Rev.

Synthesis of Single Phase Hg-1223 High Tc Superconducting Films With Multistep Electrolytic Process

We report the multistep electrolytic process for the synthesis of high Tc single phase HgBa2Ca2Cu3O8+ (Hg-1223) superconducting films. The process includes : i) deposition of BaCaCu precursor alloy, ii) oxidation of BaCaCu films, iii) electrolytic intercalation of Hg in precursor BaCaCuO films and iv) electrochemical oxidation and annealing of Hg-intercalated BaCaCuO films to convert into Hg1Ba2Ca2Cu3O8+ (Hg-1223). Films were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrolytic intercalation of Hg in BaCaCuO precursor is proved to be a novel alternative to high temperature-high pressure mercuration process. The films are single phase Hg-1223 with Tc = 121.5 K and Jc = 4.3 x 104 A/cm2.Comment: 17 Pages, 10 Figures. Submitted to Superconductor Science and Technolog

ОПТИМИЗАЦИЯ ПАРАМЕТРОВ РАДИОЧАСТОТНОГО И ЦИФРОВОГО ТРАКТА СВЧ ШИРОКОПОЛОСНОГО АНАЛИЗАТОРА СПЕКТРА

The paper deals with improving the performance of the RF path and the digital path of microwave multi-function measuring devices into range up to 8 GHz. Development of the two-loop PLL system was performed using a VCO and DDS and allowed us to realize the required parameters of measuring devices. Additionally, the development and research of IQ imbalance correction method of quadrature components of the signal in a digital IF path was carried out, which enabled to reduce the signal distortions and incorporate the requirements for multi-function measuring devicesРассмотрены вопросы улучшения рабочих характеристик радиочастотного цифрового тракта СВЧ многофункциональных измерительных приборов в диапазоне до 8 ГГц. Выполнена разработка двухпетлевой системы фазовой автоподстройки частоты с использованием интегральных синтезаторов на основе генераторов, управляемых напряжением и синтезаторов прямого цифрового синтеза на элементной базе высокой степени интеграции, которая позволила реализовать требуемые параметры. Проведены исследования метода коррекции дисбаланса квадратурных составляющих сигнала в цифровом тракте промежуточной частоты, учитывающие требования к многофункциональному режиму работы измерительного прибора

Анализ шумовых характеристик диодного генератора в многоконтурной системе

In modern microwave electronics, it is extremely important to evaluate the possibility of efficient generation of highly stable oscillation modes and the creation of low-noise generators with a millimeter wavelength range. The purpose of the research was to determine the type of wave for the most effective modes of oscillation and to ensure the operation of various modes in the mode of mutual synchronization, which becomes possible because the real design of generators can include a number of reactive elements that form a number of resonant frequencies in the microwave range. This paper shows how it is possible to provide conditions for generation at a particular frequency and simultaneously obtain high stability of vibrations. The main research methods were: analysis and generalization of literature data and results of our own research on the peculiarities of oscillation generation in a multi-circuit system of a Gann diode generator and conditions under which high frequency stability is established; experimental studies using microwave spectrum analyzers, microwave devices. In this paper, the possible generation frequencies are calculated, the operation modes of the diode generator are studied, and their phase noise is experimentally measured. It is shown that when the generation frequency approaches the cut-off frequency of the waveguide with the help of structural elements or by changing the supply voltage, a mode of mutual synchronization of vibrations at high harmonics occurs using the H102 wave type in a very narrow range of frequency (phase) noise. A generator with a multicircuit resonant system was experimentally studied and a small amount of phase noise was obtained for both types of H101 and H102 waves. At the same time, the best parameters and noise characteristics occurred at the lowest types of vibration types. These results have an experimental novelty, which allows us to hope for the development of constructive solutions that provide the creation of highly stable low-noise generators of millimeter wavelength range with the minimum mass dimensional parameters.В современной СВЧ электронике крайне актуальна оценка возможности эффективной генерации высокостабильных мод колебаний и создания малошумящих генераторов миллиметрового диапазона длин волн. Цель исследований заключалась в определении типа волны для наиболее эффективных мод колебаний и обеспечении работы различных мод в режиме взаимной синхронизации, которая становится возможной, поскольку в реальные конструкции генераторов могут входить ряд реактивных элементов, которые в диапазоне СВЧ формируют ряд резонансных частот. В данной работе показывается, как можно обеспечить условия генерации на той или иной частоте и одновременно получить высокую стабильность колебаний. Основными методами исследований являлись: анализ и обобщение литературных данных и результатов собственных исследований особенностей генерирования колебаний в многоконтурной системе генератора на диоде Ганна и условий, при которых устанавливается высокая стабильность частоты; экспериментальные исследования с использованием СВЧ анализаторов спектра, микроволновых устройств. В работе рассчитаны возможные частоты генерации, проведены исследования режимов работы диодного генератора и экспериментально измерены их фазовые шумы. Показано, что при приближении частоты генерации к частоте отсечки волновода с помощью подстроечных элементов конструкции или изменением напряжения питания возникает режим взаимной синхронизации колебаний на высоких гармониках с использованием типа волны Н102 в очень узком диапазоне частотных (фазовых) шумов. Экспериментально исследован генератор с многоконтурной резонансной системой и получена небольшая величина фазовых шумов для обоих типов волн Н101 и Н102. При этом наилучшие параметры и шумовые характеристики имели место на наиболее низких типах видов колебаний. Данные результаты обладают экспериментальной новизной, что позволяет надеяться на разработку конструктивных решений, обеспечивающих создание высокостабильных малошумящих генераторов миллиметрового диапазона длин волн с минимальными массогабаритными параметрами

Band-structure trend in hole-doped cuprates and correlation with Tcmax

By calculation and analysis of the bare conduction bands in a large number of hole-doped high-temperature superconductors, we have identified the energy of the so-called axial-orbital as the essential, material-dependent parameter. It is uniquely related to the range of the intra-layer hopping. It controls the Cu 4s-character, influences the perpendicular hopping, and correlates with the observed Tc at optimal doping. We explain its dependence on chemical composition and structure, and present a generic tight-binding model.Comment: 5 pages, Latex, 5 eps figure

The role of dynamical polarization of the ligand to metal charge transfer excitations in {\em ab initio} determination of effective exchange parameters

The role of the bridging ligand on the effective Heisenberg coupling parameters is analyzed in detail. This analysis strongly suggests that the ligand-to-metal charge transfer excitations are responsible for a large part of the final value of the magnetic coupling constant. This permits to suggest a new variant of the Difference Dedicated Configuration Interaction (DDCI) method, presently one of the most accurate and reliable for the evaluation of magnetic effective interactions. This new method treats the bridging ligand orbitals mediating the interaction at the same level than the magnetic orbitals and preserves the high quality of the DDCI results while being much less computationally demanding. The numerical accuracy of the new approach is illustrated on various systems with one or two magnetic electrons per magnetic center. The fact that accurate results can be obtained using a rather reduced configuration interaction space opens the possibility to study more complex systems with many magnetic centers and/or many electrons per center.Comment: 7 pages, 4 figure

Phonon dispersion in 1-layer cuprate HgBa2CuO4+d

We investigate the low energy acoustical and optical modes in $\mathrm{HgBa_2CuO_{4+\delta}}$ using inelastic x-ray scattering (IXS). The experimental phonon dispersion and the dynamical structure factor are compared with an atomic shell model, and the set of the atomic potentials obtained are discussed. Our results are also compared with those obtained by Raman spectroscopy and with density-of-state data measured by inelastic neutron scattering