290 research outputs found
Observation of superluminal geometrical resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing
only few stacked intrinsic Josephson junctions. Careful alignment of magnetic
field prevents penetration of Abrikosov vortices and facilitates observation of
a large variety of high quality geometrical resonances, including superluminal
with velocities larger than the slowest velocity of electromagnetic waves. A
small number of junctions limits the number of resonant modes and allows
accurate identification of modes and velocities. It is shown that superluminal
geometrical resonances can be excited by subluminal fluxon motion and that
flux-flow itself becomes superluminal at high magnetic fields. We argue that
observation of high-quality superluminal geometrical resonances is crucial for
realization of the coherent flux-flow oscillator in the THz frequency range
Fiske Steps and Abrikosov Vortices in Josephson Tunnel Junctions
We present a theoretical and experimental study of the Fiske resonances in
the current-voltage characteristics of "small" Josephson junctions with
randomly distributed misaligned Abrikosov vortices. We obtained that in the
presence of Abrikosov vortices the resonant interaction of electromagnetic
waves, excited inside a junction, with the ac Josephson current manifests
itself by Fiske steps in a current-voltage characteristics even in the absence
of external magnetic field. We found that the voltage positions of the Fiske
steps are determined by a junction size, but the Fiske step magnitudes depend
both on the density of trapped Abrikosov vortices and on their misalignment
parameter. We measured the magnetic field dependence of both the amplitude of
the first Fiske step and the Josephson critical current of low-dissipative
small based Josephson tunnel junctions with artificially introduced
Abrikosov vortices. A strong decay of the Josephson critical current and a weak
non-monotonic decrease of the first Fiske step amplitude on the Abrikosov
vortex density were observed. The experimentally observed dependencies are well
described by the developed theory.Comment: 21 pages, 7 figures, submitted to Physical Review
Laser Scanning Microscopy of HTS Films and Devices
The work describes the capabilities of Laser Scanning Microscopy (LSM) as a
spatially resolved method of testing high_Tc materials and devices. The earlier
results obtained by the authors are briefly reviewed. Some novel applications
of the LSM are illustrated, including imaging the HTS responses in rf mode,
probing the superconducting properties of HTS single crystals, development of
twobeam laser scanning microscopy. The existence of the phase slip lines
mechanism of resistivity in HTS materials is proven by LSM imaging.Comment: 17 pages, 21 figures, Submitted to Fizika Nizkikh Temperatur (Low
Temperature Physics
Анализ состояния вопроса утилизации низкопотенциальных энергетических ресурсов на объектах малой энергетики
Studying the issues of recovery of low-potential energy at smallscale energy facilities allowed to show the promising character of the organic Rankine cycle (ORC) technology as a technology for recovery or conversion of low-potential energy.The most promising developments in the field of the use and recovery of waste heat are described regarding application of ORC, which is widely used in geothermal sources, hot water boilers, gas turbine plants. Due to the constantly growing diversity of working fluids, ORC can be used within a wide temperature range from 100°C to over 350°C. Also, developments are underway in the design of ORC generators to increase reliability of its individual system units, such as turbines and expanders. Based on the above factors, it can be concluded that with a deeper study of the problems of adopting ORC technologies, they can become a very promising direction in development of heat power engineering.It has been determined that the main factor hindering the widespread adoption of the ORC technology is associated with high cost of heat exchange equipment due to increased heat exchange surfaces. It is shown that design of mini power plants and energy centres based on the use of low-potential energy requires improvement of mathematical modelling methods to reliably determine operating modes and characteristics of each of the units. Methods for modelling evaporation and condensation systems, including turbines and expanders using organic low-boiling working fluids, should be considered among the methods that are highly sought after. The methods for selecting a working fluid for ORC devices also have a significant impact on characteristics of the installation determining the range of cycle operating temperatures and pressures. The solution of the above problems can lead to a reduction in the cost of heat exchange equipment, and, consequently, to a decrease in costs for design of ORC generators. В статье рассмотрены вопросы утилизации низкопотенциальных энергетических ресурсов на объектах малой энергетики. Показана перспектива использования технологии органического цикла Ренкина (ОЦР) в качестве технологии утилизации или преобразования низкопотенциальной энергии.Приведены разработки в области реализации и утилизации бросового тепла. Наиболее перспективные из них представлены в применении ОЦР, который широко используется на геотермальных источниках, в водогрейных котельных, газотурбинных установках. За счёт постоянного растущего сортамента рабочих тел ОЦР может применяться в широком температурном диапазоне, начиная от 100°С и заканчивая свыше 350°С. Также ведутся разработки в области проектирования ОЦР-генераторов с целью повышения надёжности отдельных узлов системы, таких как турбины и детандеры. Исходя из вышеперечисленных факторов, можно сделать вывод, что при более глубоком исследовании проблем внедрения ОЦР-технологий они могут стать весьма перспективным направлением в развитии теплоэнергетики.Определено, что основным фактором, препятствующим широкому внедрению ОЦР-технологии, является высокая стоимость теплообменного оборудования из-за повышенных теплообменных поверхностей. Показано, что проектирование миниэлектростанций и энергокомплексов на основе использования низкопотенциальной энергии требует совершенствования методов математического моделирования для достоверного определения режимов работы и характеристик каждого из агрегатов. К востребованным следует отнести методы моделирования испарительных и конденсационных систем, в том числе турбин и детандеров, работающих на органических низкокипящих рабочих телах. Методики выбора рабочего тела для ОЦР-устройств также оказывают существенное влияние на характеристики установки, которые определяют диапазон эксплуатационных температур и давлений цикла. Решение вышеуказанных задач способно привести к удешевлению теплообменного оборудования, а, следовательно, снижению издержек на проектирование ОЦР-генераторов.
Fractional ac Josephson effect in unconventional superconductors
For certain orientations of Josephson junctions between two p_x-wave or two
d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic
relation between the Josephson current I and the phase difference phi: I ~
sin(phi/2). Consequently, the ac Josephson current has the fractional frequency
eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current
is proportional to the first power (not square) of the electron tunneling
amplitude. Thus, the Josephson current between unconventional superconductors
is carried by single electrons, rather than by Cooper pairs. The fractional ac
Josephson effect can be observed experimentally by measuring frequency spectrum
of microwave radiation from the junction.Comment: 8 pages, 3 figures, RevTEX 4; v2. - minor typos corrected in proof
Current-voltage characteristic of narrow superconducting wires: bifurcation phenomena
The current-voltage characteristics of long and narrow superconducting
channels are investigated using the time-dependent Ginzburg-Landau equations
for complex order parameter. We found out that the steps in the current voltage
characteristic can be associated with bifurcations of either steady or
oscillatory solution. We revealed typical instabilities which induced the
singularities in current-voltage characteristics, and analytically estimated
period of oscillations and average voltage in the vicinity of the critical
currents. Our results show that these bifurcations can substantially complicate
dynamics of the order parameter and eventually lead to appearance of such
phenomena as multistability and chaos. The discussed bifurcation phenomena
sheds a light on some recent experimental findings
Quantum superposition of three macroscopic states and superconducting qutrit detector
Superconducting quantum coherent circuits have opened up a novel area of
fundamental low-temperature science since they could potentially be the element
base for future quantum computers. Here we report a quasi-three-level coherent
system, the so-called superconducting qutrit, which has some advantages over a
two-level information cell (qubit), and is based on the qutrit readout circuit
intended to measure individually the states of each qubit in a quantum
computer. The designed and implemented radio-frequency superconducting qutrit
detector (rf SQUTRID) with atomic-size ScS-type contact utilizes the
coherent-state superposition in the three-well potential with energy splitting
Delta E_01/k_B=1.5 K at the 30th quantized energy level with good isolation
from the electromagnetic environment. The reason why large values of Delta E_01
(and thus using atomic-size Nb-Nb contact) are required is to ensure an
adiabatic limit for the quantum dynamics of magnetic flux in the rf SQUTRID.Comment: 9 pages, 5 figures, in v.3: text extended, inset in figure 1 (the
device design) adde
Halocline water modification and along slope advection at the Laptev Sea continental margin
A general pattern in water mass distribution and potential shelf–basin exchange is revealed at the Laptev Sea continental slope based on hydrochemical and stable oxygen isotope data from the summers 2005–2009. Despite considerable interannual variations, a frontal system can be inferred between shelf, continental slope and central Eurasian Basin waters in the upper 100 m of the water column along the continental slope. Net sea-ice melt is consistently found at the continental slope. However, the sea-ice meltwater signal is independent from the local retreat of the ice cover and appears to be advected from upwind locations.
In addition to the along-slope frontal system at the continental shelf break, a strong gradient is identified on the Laptev Sea shelf between 122° E and 126° E with an eastward increase of riverine and sea-ice related brine water contents. These waters cross the shelf break at ~ 140° E and feed the low-salinity halocline water (LSHW, salinity S < 33) in the upper 50 m of the water column. High silicate concentrations in Laptev Sea bottom waters may lead to speculation about a link to the local silicate maximum found within the salinity range of ~ 33 to 34.5, typical for the Lower Halocline Water (LHW) at the continental slope. However brine signatures and nutrient ratios from the central Laptev Sea differ from those observed at the continental slope. Thus a significant contribution of Laptev Sea bottom waters to the LHW at the continental slope can be excluded. The silicate maximum within the LHW at the continental slope may be formed locally or at the outer Laptev Sea shelf. Similar to the advection of the sea-ice melt signal along the Laptev Sea continental slope, the nutrient signal at 50–70 m water depth within the LHW might also be fed by advection parallel to the slope. Thus, our analyses suggest that advective processes from upstream locations play a significant role in the halocline formation in the northern Laptev Sea
Exchange of Laptev Sea and Arctic Ocean halocline waters in response to atmospheric forcing
Investigation of new modification strategies for PVA membranes to improve their dehydration properties by pervaporation
International audienceNovel supported membranes based on polyvinyl alcohol (PVA) were developed using two strategies: first, by the modification of the PVA network, via so-called bulk modification, with the formation of the selective layer accomplished through the introduction of fullerenol and/or poly(allylamine hydrochloride), and second, by the functionalization of the surface with successive depositions of multilayered films of polyelectrolytes, such as poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) on the PVA surface. The membrane surface modifications were characterized by scanning electron microscopy and contact angle measurements. The modified PVA membranes were examined for their dehydration transport properties by the perva-poration of isopropyl alcohol-water (80/20% w/w), which was chosen as a model mixture. Compared with the pristine PVA membrane, the main improvement was a marked increase in permeance. It was found that the surface modifications mainly gave rise to a higher global flux but with a strong reduction in selectivity. Only the combination of both bulk and surface modifications with PEL could significantly increase the flux with a high water content in the permeate (over 98%). Lastly, it should be noted that this study developed a green procedure to prepare innovative membrane layers for dehydration, making use of only water as a working medium
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