5,439 research outputs found
Vortex instability in molybdenum-germanium superconducting films
We studied the high driving force regime of the current-voltage transport
response in the mixed state of amorphous molybdenum-germanium superconducting
films to the point where the flux flow becomes unstable. The observed nonlinear
response conforms with the classic Larkin-Ovchinikov (LO) picture with a
quasiparticle-energy-relaxation rate dominated by the quasiparticle
recombination process. The measured energy relaxation rate was found to have a
magnitude and temperature dependence in agreement with theory.
PACS: 74.40.Gh, 74.25.Uv, 72.15.Lh, 73.50.Gr, 73.50.Fq
Keywords: fluxon, vortices, TDGL, FFF, negative differential conductivity,
NDC, non-monotonic IV curveComment: 5 pages, 3 figure
Energy Relaxation at a Hot-Electron Vortex Instability
At high dissipation levels, vortex motion in a superconducting film has been
observed to become unstable at a certain critical vortex velocity v*. At
substrate temperatures substantially below Tc, the observed behavior can be
accounted for by a model in which the electrons reach an elevated temperature
relative to the phonons and the substrate. Here we examine the underlying
assumptions concerning energy flow and relaxation times in this model. A
calculation of the rate of energy transfer from the electron gas to the lattice
finds that at the instability, the electronic temperature reaches a very high
value close to the critical temperature. Our calculated energy relaxation times
are consistent with those deduced from the experiments. We also estimate the
phonon mean free path and assess its effect on the flow of energy in the film.Comment: 8 pages, 7 figure
Dynamics of vortex penetration, jumpwise instabilities and nonlinear surface resistance of type-II superconductors in strong rf fields
We consider nonlinear dynamics of a single vortex in a superconductor in a
strong rf magnetic field . Using the London theory, we
calculate the dissipated power , and the transient time scales
of vortex motion for the linear Bardeen-Stephen viscous drag force, which
results in unphysically high vortex velocities during vortex penetration
through the oscillating surface barrier. It is shown that penetration of a
single vortex through the ac surface barrier always involves penetration of an
antivortex and the subsequent annihilation of the vortex antivortex pairs.
Using the nonlinear Larkin-Ovchinnikov (LO) viscous drag force at higher vortex
velocities results in a jump-wise vortex penetration through the surface
barrier and a significant increase of the dissipated power. We calculate the
effect of dissipation on nonlinear vortex viscosity and the rf vortex
dynamics and show that it can also result in the LO-type behavior,
instabilities, and thermal localization of penetrating vortex channels. We
propose a thermal feedback model of , which not only results in the LO
dependence of for a steady-state motion, but also takes into account
retardation of temperature field around rapidly accelerating vortex, and a
long-range interaction with the surface. We also address the effect of pinning
on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on
the surface resistance calculated as a function or rf frequency and
field. It is shown that trapped flux can result in a temperature-independent
residual resistance at low , and a hysteretic low-field dependence of
, which can {\it decrease} as is increased, reaching a minimum
at much smaller than the thermodynamic critical field .Comment: 18 figure
Performance Studies of Prototype II for the CASTOR forward Calorimeter at the CMS Experiment
We present results of the performance of the second prototype of the CASTOR
quartz-tungsten sampling calorimeter, to be installed in the very forward
region of the CMS experiment at the LHC. The energy linearity and resolution,
as well as the spatial resolution of the prototype to electromagnetic and
hadronic showers are studied with E=20-200 GeV electrons, E=20-350 GeV pions,
and E=50,150 GeV muons from beam tests carried out at CERN/SPS in 2004. The
responses of the calorimeter using two different types of photodetectors
(avalanche photodiodes APDs, and photomultiplier tubes PMTs) are compared.Comment: 16 pages, 22 figs., submitted to EPJ-
Rearrangement of the vortex lattice due to instabilities of vortex flow
With increasing applied current we show that the moving vortex lattice
changes its structure from a triangular one to a set of parallel vortex rows in
a pinning free superconductor. This effect originates from the change of the
shape of the vortex core due to non-equilibrium effects (similar to the
mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The
moving vortex creates a deficit of quasiparticles in front of its motion and an
excess of quasiparticles behind the core of the moving vortex. This results in
the appearance of a wake (region with suppressed order parameter) behind the
vortex which attracts other vortices resulting in an effective
direction-dependent interaction between vortices. When the vortex velocity
reaches the critical value quasi-phase slip lines (lines with fast vortex
motion) appear which may coexist with slowly moving vortices between such
lines. Our results are found within the framework of the time-dependent
Ginzburg-Landau equations and are strictly valid when the coherence length
is larger or comparable with the decay length of the
non-equilibrium quasiparticle distribution function. We qualitatively explain
experiments on the instability of vortex flow at low magnetic fields when the
distance between vortices . We speculate that a
similar instability of the vortex lattice should exist for even when
.Comment: 10 pages, 11 figure
Spontaneous superconducting islands and Hall voltage in clean superconductors
We study a clean superconductor in the Hall configuration, in the framework
of a purely dissipative time-dependent Ginzburg--Landau theory. We find
situations in which the order parameter differs significantly from zero in a
set of islands that appear to form a periodic structure. When the pattern of
islands becomes irregular, it moves in or against the direction of the current
and a Hall voltage is found. Tiny differences in the initial state may reverse
the sign of the Hall voltage. When the average Hall voltage vanishes, the local
Hall voltage does not necessarily vanish. We examine the influence that several
boundary conditions at the electrodes have on these effects.Comment: 6 pages, Includes additional cases and more detailed result
Steps in the Negative-Differential-Conductivity Regime of a Superconductor
Current-voltage characteristics were measured in the mixed state of
Y1Ba2Cu3O(7-delta) superconducting films in the regime where flux flow becomes
unstable and the differential conductivity dj/dE becomes negative. Under
conditions where its negative slope is steep, the j(E) curve develops a
pronounced staircase like pattern. We attribute the steps in j(E) to the
formation of a dynamical phase consisting of the succesive nucleation of
quantized distortions in the local vortex velocity and flux distribution within
the moving flux matter.Comment: 5 pages, 3 figure
Scintillator counters with WLS fiber/MPPC readout for the side muon range detector (SMRD)of the T2K experiment
The T2K neutrino experiment at J-PARC uses a set of near detectors to measure
the properties of an unoscillated neutrino beam and neutrino interaction
cross-sections. One of the sub-detectors of the near-detector complex, the side
muon range detector (SMRD), is described in the paper. The detector is designed
to help measure the neutrino energy spectrum, to identify background and to
calibrate the other detectors. The active elements of the SMRD consist of 0.7
cm thick extruded scintillator slabs inserted into air gaps of the UA1 magnet
yokes. The readout of each scintillator slab is provided through a single WLS
fiber embedded into a serpentine shaped groove. Two Hamamatsu multi-pixel
avalanche photodiodes (MPPC's) are coupled to both ends of the WLS fiber. This
design allows us to achieve a high MIP detection efficiency of greater than
99%. A light yield of 25-50 p.e./MIP, a time resolution of about 1 ns and a
spatial resolution along the slab better than 10 cm were obtained for the SMRD
counters.Comment: 7 pages, 4 figures; talk at TIPP09, March 12-17, Tsukuba, Japan; to
be published in the conference proceeding
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Влияние освещения кризиса на финансовый рынок России
The article examines the impact of informational messages characterizing the crisis in the economy on the financial market indicators. The aim of the article is to build an index that allows assessing the crisis situation in the country based on textual analysis of informational messages. Due to the literature review, the factors determining the crisis in the economy were identified. The empirical base of the study included more than 10 million news texts from various sources accredited by Thomson Reuters. For the first time, the authors compiled a “bag of words” (dictionary) to determine the crisis situation in the country; and by means of the text analysis, they developed the author’s crisis index calculated on the basis of news reports in foreign media about Russia. They conducted the analysis of the relations between the crisis index and the stock index MOEX. According to the results of the study, it has been established that an increase in the number of news reports determining the crisis situation in the economy has a negative effect on the financial market: it leads to a drop in stock prices. Thus, not only objective economic factors, but also the information component influencing the mood of investors and the behavior of economic entities, affects the key indicators of the financial market. The proposed author’s crisis index can also be used to assess other relations, for example, the effect of the crisis on the exchange rate.В статье исследуется влияние информационных сообщений, характеризующих проявление кризиса в экономике, на показатели финансового рынка. Цель статьи — построение индекса, позволяющего оценивать кризисную ситуацию в стране на основе текстового анализа информационных сообщений. С помощью обзора литературы выявлены факторы, определяющие кризисные явления в экономике. Эмпирическая база исследования включала в себя более 10 млн новостных текстов из различных источников, аккредитованных ThomsonReuters. Авторы впервые составили «мешок слов» (словарь) для определения кризисной ситуации в стране и с помощью текстового анализа разработали авторский индекс кризиса, рассчитанный на основе новостных сообщений в зарубежных СМИ о России. Проведен анализ зависимости между индексом кризиса и биржевым индексом MOEX. По результатам исследования установлено, что увеличение количества новостных сообщений, определяющих проявление кризисной ситуации в экономике, негативно отражается на финансовом рынке: приводит к падению котировок акций. Таким образом, не только объективные экономические факторы, но и информационная составляющая, оказывающая влияние на настроения инвесторов и поведение экономических субъектов, воздействует на ключевые показатели финансового рынка. Предложенный авторский индекс кризиса может также быть использован для оценки других зависимостей, например влияния кризиса на валютный курс
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