81 research outputs found
Spatially-resolved probing of a non-equilibrium superconductor
Spatially resolved relaxation of non-equilibrium quasiparticles in a
superconductor at ultra-low temperatures was experimentally studied. It was
found that the quasiparticle injection through a tunnel junction results in
modification of the shape of I-V characteristic of a remote `detector'
junction. The effect depends on temperature, injection current and proximity to
the injector. The phenomena can be understood in terms of creation of
quasiparticle charge and energy disequilibrium characterized by two different
length scales m and
m. The findings are in good agreement with existing phenomenological
models, while more elaborated microscopic theory is mandatory for detailed
quantitative comparison with experiment. The results are of fundamental
importance for understanding electron transport phenomena in various
nanoelectronic circuits.Comment: 7 pages, 5 figure
Loss of Andreev Backscattering in Superconducting Quantum Point Contacts
We study effects of magnetic field on the energy spectrum in a
superconducting quantum point contact. The supercurrent induced by the magnetic
field leads to intermode transitions between the electron waves that pass and
do not pass through the constriction. The latter experience normal reflections
which couple the states with opposite momenta inside the quantum channel and
create a minigap in the energy spectrum that depends on the magnetic field
IMAGE PROCESSING AT ELLIPSOIDAL PHOTOMETRY
Ellipsoidal photometry using Charge-Coupled Device (CCD photometry), as a new kind of optical diagnostics of scattering media in reflected and/or transmitted light, requires the development of specific principles of data analysis. The object of this work was substantiation the principles of ellipsoidal CCD photometry at implementation of a new data processing method of spatial distribution of scattered optical flux. Procedure of photometric analysis include the steps of determining the image significance, as well as the size, configuration and illuminance in its respective areas with regard to the criteria to optimize the shape and sensitivity of the zone. Zone analysis schemes of photometric images for media with a radially symmetric and directed scattering are developed. Recommended to use the method of comparison with etalon for studies the technical surfaces and turbid media to determine their roughness and optical characteristics, respectively. During the analysis of biological media, there is a possible of prognosis the spatial distribution of the brightness of the image by means of statistical modeling of optical radiation in the system «biological medium + ellipsoidal reflector». That confirmed by comparing results of the numerical (direct Monte Carlo simulation) and the real experiment for samples different thickness of muscle porcine tissue in vitro. Parameters of the optical radiation, which used as input data for the simulation, correspond to a laser wavelength of 632.8 nm with a Gaussian distribution profile of power of 2 mW. As a CCD detector was used monochrome camera DMK-21Au04.AS, and ellipsoidal reflector with an eccentricity of 0.66 and working aperture of 33.75 mm. Obtained results of zone distribution of illumination across the field in real experimental photometric images during biometrics showed correlation with the total transmission, absorption and diffuse scattering coefficients. This interdependence may be important part of improvement inverse methods of determination the optical parameters of biological media
Circuit theory of multiple Andreev reflections in diffusive SNS junctions: the incoherent case
The incoherent regime of Multiple Andreev Reflections (MAR) is studied in
long diffusive SNS junctions at applied voltages larger than the Thouless
energy. Incoherent MAR is treated as a transport problem in energy space by
means of a circuit theory for an equivalent electrical network. The current
through NS interfaces is explained in terms of diffusion flows of electrons and
holes through tunnel and Andreev resistors. These resistors in diffusive
junctions play roles analogous to the normal and Andreev reflection
coefficients in OTBK theory for ballistic junctions. The theory is applied to
the subharmonic gap structure (SGS); simple analytical results are obtained for
the distribution function and current spectral density for the limiting cases
of resistive and transparent NS interfaces. In the general case, the exact
solution is found in terms of chain-fractions, and the current is calculated
numerically. SGS shows qualitatively different behavior for even and odd
subharmonic numbers, and the maximum slopes of the differential resistance
correspond to the gap subharmonics. The influence of inelastic scattering on
the subgap anomalies of the differential resistance is analyzed.Comment: 14 pages, 9 figures, title and text revised, to appear in PR
Proximity Effect and Multiple Andreev Reflections in Chaotic Josephson junctions
We study the dc-current transport in a voltage biased superconductor-chaotic
dot-superconductor junction with an induced proximity effect(PE) in the dot. It
is found that for a Thouless energy of the dot smaller than the
superconducting energy gap , the PE is manifested as peaks in the
differential conductance at voltages of order away from the even
subharmonic gap structures . These peaks are
insensitive to temperatures but are suppressed by a weak
magnetic field. The current for suppressed PE is independent of and
magnetic field and is shown to be given by the Octavio-Tinkham-Blonder-Klapwijk
theory.Comment: 4 pages, 3 figure
Nonequilibrium electron cooling by NIS tunnel junctions
We discuss the theoretical framework to describe quasiparticle electric and
heat currents in NIS tunnel junctions in the dirty limit. The approach is based
on quasiclassical Keldysh-Usadel equations. We apply this theory to diffusive
NIS'S tunnel junctions. Here N and S are respectively normal metal and
superconductor reservoirs, I is an insulator layer and S' is a nonequilibrium
superconducting lead. We calculate the quasiparticle electric and heat currents
in such structures and consider the effect of inelastic relaxation in the S'
lead. We find that in the absence of strong relaxation the electric current and
the cooling power for voltages are suppressed. The value of this
suppression scales with the diffusive transparency parameter. We ascribe this
suppression to the effect of backtunneling of nonequilibrium quasiparticles
into the normal metal.Comment: 12 pages, 6 figures, proceedings, to be published in JLT
Acoustic Cyclotron Resonance and Giant High Frequency Magnetoacoustic Oscillations in Metals with Locally Flattened Fermi Surface
We consider the effect of local flattening on the Fermi surface (FS) of a
metal upon geometric oscillations of the velocity and attenuation of ultrasonic
waves in the neighborhood of the acoustic cyclotron resonance. It is shown that
such peculiarities of the local geometry of the FS can lead to a significant
enhancement of both cyclotron resonance and geometric oscillations.
Characteristic features of the coupling of ultrasound to shortwave cyclotron
waves arising due to the local flattening of the FS are analyzed.
PACS numbers 71.18.+y; 72.15.Gd; 72.15.-vComment: 8 pages, 3 figures, text revise
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