1,265 research outputs found
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
Conductance of a STM contact on the surface of a thin film
The conductance of a contact, having a radius smaller than the Fermi wave
length, on the surface of a thin metal film is investigated theoretically. It
is shown that quantization of the electron energy spectrum in the film leads to
a step-like dependence of differential conductance G(V) as a function of
applied bias eV. The distance between neighboring steps in eV equals the energy
level spacing due to size quantization. We demonstrate that a study of G(V) for
both signs of the voltage maps the spectrum of energy levels above and below
Fermi surface in scanning tunneling experiments.Comment: 15 pages, 5 figure
Spectroscopy of phonons and spin torques in magnetic point contacts
Phonon spectroscopy is used to investigate the mechanism of current-induced
spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization
excitations observed in the magneto-conductance of the point contacts are
pronounced for diffusive and thermal contacts, where the electrons experience
significant scattering in the contact region. We find no magnetic excitations
in highly ballistic contacts. Our results show that impurity scattering at the
N/F interface is the origin of the new single-interface spin torque effect.Comment: 4 pages, 5 figs., accepted for publication in PR
Reduced leakage current in Josephson tunnel junctions with codeposited barriers
Josephson junctions were fabricated using two different methods of barrier
formation. The trilayers employed were Nb/Al-AlOx/Nb on sapphire, where the
first two layers were epitaxial. The oxide barrier was formed either by
exposing the Al surface to O2 or by codepositing Al in an O2 background. The
codeposition process yielded junctions that showed the theoretically predicted
subgap current and no measurable shunt conductance. In contrast, devices with
barriers formed by thermal oxidation showed a small shunt conductance in
addition to the predicted subgap current.Comment: 3 pages, 4 figure
Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)
Analysis of the point-contact spectroscopy (PCS) data on the new dramatic
high-T superconductor MgB reveals quite different behavior of two
disconnected and electronic bands, deriving from their
anisotropy, different dimensionality, and electron-phonon interaction. PCS
allows direct registration of both the superconducting gaps and
electron-phonon-interaction spectral function of the two-dimensional
and three-dimensional band, establishing correlation between the gap
value and intensity of the high-T driving force -- the boron
vibration mode. PCS data on some nonsuperconducting transition-metal diborides
are surveyed for comparison.Comment: 17 pages, 30 figs., will be published in Low Temp. Phys. V.30 (2004)
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Minimalist design of a robust real-time quantum random number generator
We present a simple and robust construction of a real-time quantum random
number generator (QRNG). Our minimalist approach ensures stable operation of
the device as well as its simple and straightforward hardware implementation as
a stand-alone module. As a source of randomness the device uses measurements of
time intervals between clicks of a single-photon detector. The obtained raw
sequence is then filtered and processed by a deterministic randomness
extractor, which is realized as a look-up table. This enables high speed
on-the-fly processing without the need of extensive computations. The overall
performance of the device is around 1 random bit per detector click, resulting
in 1.2 Mbit/s generation rate in our implementation
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