419 research outputs found
Fluctuation spectroscopy of granularity in superconducting structures
We suggest to use `fluctuation spectroscopy' as a method to detect
granularity in a disordered metal close to a superconducting transition. We
show that with lowering temperature the resistance of a system of
relatively large grains initially grows due to the fluctuation suppression of
the one-electron tunneling but decreases with further lowering due to the
coherent charge transfer of the fluctuation Cooper pairs. Under certain
conditions, such a maximum in turns out to be sensitive to weak magnetic
fields due to a novel Maki -- Thompson type mechanism.Comment: A final version, as published; the introduction and summary are
considerably revise
Josephson current noise above Tc in superconducting tunnel junctions
Tunnel junction between two superconductors is considered in the vicinity of
the critical temperature. Superconductive fluctuations above Tc give rise to
the noise of the ac Josephson current although the current itself is zero in
average. As a result of fluctuations, current noise spectrum is peaked at the
Josephson frequency, which may be considered as precursor of superconductivity
in the normal state. Temperature dependence and shape of the Josephson current
noise resonance line is calculated for various junction configurations.Comment: 8 pages, 2 figure
Spherical functions on the de Sitter group
Matrix elements and spherical functions of irreducible representations of the
de Sitter group are studied on the various homogeneous spaces of this group. It
is shown that a universal covering of the de Sitter group gives rise to
quaternion Euler angles. An explicit form of Casimir and Laplace-Beltrami
operators on the homogeneous spaces is given. Different expressions of the
matrix elements and spherical functions are given in terms of multiple
hypergeometric functions both for finite-dimensional and unitary
representations of the principal series of the de Sitter group.Comment: 40 page
UCN anomalous losses and the UCN capture cross-section on material defects
Experimental data shows anomalously large Ultra Cold Neutrons (UCN)
reflection losses and that the process of UCN reflection is not completely
coherent. UCN anomalous losses under reflection cannot be explained in the
context of neutron optics calculations. UCN losses by means of incoherent
scattering on material defects are considered and cross-section values
calculated. The UCN capture cross-section on material defects is enhanced by a
factor of 10^4 due to localization of UCN around defects. This phenomenon can
explain anomalous losses of UCN.Comment: 13 pages, 4 figure
Effect of boundary conditions on the character of ambipolar diffusion in electrolytes
PREInternational audienceWe discuss the details of ambipolar relaxation of the electric field in liquid asymmetric electrolytes to its stationary value. It is demonstrated that the account for finite boundary conditions modifies the existing concepts of this diffusion process. In particular, we succeeded to suggest a qualitatively correct explanation of the observed distribution of the electric fields over the bulk of the cuvette and its nonmonotonic behavior in measurements on the finite-size cuvette. We analyze the conditions of such an anomaly at the intermediate stages of the relaxation proces
New experimental limits on neutron - mirror neutron oscillations in the presence of mirror magnetic field
Present probes do not exclude that the neutron () oscillation into mirror
neutron (), a sterile state exactly degenerate in mass with the neutron,
can be a very fast process, in fact faster than the neutron decay itself. This
process is sensitive to the magnetic field. Namely, if the mirror magnetic
field exists at the Earth, oscillation probability can be
suppressed or resonantly amplified by the applied magnetic field ,
depending on its strength and on the angle between and
. We present the results of ultra-cold neutron storage measurements
aiming to check the anomalies observed in previous experiments which could be a
signal for oscillation in the presence of mirror magnetic field ~G. Analyzing the experimental data on neutron loses, we obtain a new lower
limit on oscillation time s (95 % C.L.) for any
between 0.08 and 0.17 G, and s (95 % C.L.)
for any in the interval () G
Macroscopic quantum tunneling in "small" Josephson junctions in magnetic field
We study the phenomenon of macroscopic quantum tunneling (MQT) in small
Josephson junctions (JJ) with an externally applied magnetic field. The latter
results in the appearance of the Fraunhofer type modulation of the current
density along the barrier. The problem of MQT for a point-like JJ is reduced to
the motion of the quantum particle in the washboard potential. In the case of a
finite size JJ under consideration, this problem corresponds to a MQT in
potential which itself, besides the phase, depends on space variables. Finally,
the general expression for the crossover temperature T_0 between thermally
activated and macroscopic quantum tunneling regimes and the escaping time
tau_esc have been calculated
Strong vortex-antivortex fluctuations in the type II superconducting film
The small size vortex-antivortex pairs proliferation in type II
superconducting film is considered for the wide interval of temperatures below
Tc. The corresponding contribution to free energy is calculated. It is shown
that these fluctuations give the main contribution to the heat capacity of the
film both at low temperatures and in the vicinity of transition
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