81 research outputs found
Uncertainty Principle for Control of Ensembles of Oscillators Driven by Common Noise
We discuss control techniques for noisy self-sustained oscillators with a
focus on reliability, stability of the response to noisy driving, and
oscillation coherence understood in the sense of constancy of oscillation
frequency. For any kind of linear feedback control--single and multiple delay
feedback, linear frequency filter, etc.--the phase diffusion constant,
quantifying coherence, and the Lyapunov exponent, quantifying reliability, can
be efficiently controlled but their ratio remains constant. Thus, an
"uncertainty principle" can be formulated: the loss of reliability occurs when
coherence is enhanced and, vice versa, coherence is weakened when reliability
is enhanced. Treatment of this principle for ensembles of oscillators
synchronized by common noise or global coupling reveals a substantial
difference between the cases of slightly non-identical oscillators and
identical ones with intrinsic noise.Comment: 10 pages, 5 figure
Memory cell based on a Josephson junction
The Josephson junction has a doubly degenerate ground state with
the Josephson phases . We demonstrate the use of such a
Josephson junction as a memory cell (classical bit), where writing is done by
applying a magnetic field and reading by applying a bias current. In the
"store" state, the junction does not require any bias or magnetic field, but
just needs to stay cooled for permanent storage of the logical bit.
Straightforward integration with Rapid Single Flux Quantum logic is possible.Comment: to be published in AP
Fluxon-semifluxon interaction in an annular long Josephson 0-pi-junction
We investigate theoretically the interaction between integer and half-integer
Josephson vortices (fluxons and semifluxons) in an annular Josephson junction.
Semifluxons usually appear at the 0--boundary where there is a
-discontinuity of the Josephson phase. We study the simplest, but the most
interesting case of one -discontinuity in a loop, which can be created
only artificially. We show that measuring the current-voltage characteristic
after injection of an integer fluxon, one can determine the polarity of a
semifluxon. Depending on the relative polarity of fluxon and semifluxon the
static configuration may be stable or unstable, but in the dynamic state both
configurations are stable. We also calculate the depinning current of
fluxons pinned by an arbitrary fractional vortex.Comment: 8pages, 6 figures, submitted to PR
0-pi Josephson tunnel junctions with ferromagnetic barrier
We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb
superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions.
Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi
junction, with equal lengths and critical currents of 0 and pi parts. The
ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a
spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7%
of the magnetic flux quantum Phi_0. The dependence of the critical current on
the applied magnetic field shows a clear minimum in the vicinity of zero field.Comment: submitted to PR
Dynamics of semifluxons in Nb long Josephson 0-pi junctions
We propose, implement and test experimentally long Josephson 0-pi junctions
fabricated using conventional Nb-AlOx-Nb technology. We show that using a pair
of current injectors, one can create an arbitrary discontinuity of the
Josephson phase and in particular a pi-discontinuity, just like in
d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson
vortices which spontaneously appear. Moreover, using such junctions, we can
investigate the \emph{dynamics} of the fractional vortices -- a domain which is
not yet available for natural 0-pi-junctions due to their inherently high
damping. We observe half-integer zero-field steps which appear on the
current-voltage characteristics due to hopping of semifluxons.Comment: Fractional vortices in conventional superconductors ;-
Non-ideal artificial phase discontinuity in long Josephson 0-kappa-junctions
We investigate the creation of an arbitrary -discontinuity of the
Josephson phase in a long Nb-AlO_x-Nb Josephson junction (LJJ) using a pair of
tiny current injectors, and study the formation of fractional vortices formed
at this discontinuity. The current I_inj, flowing from one injector to the
other, creates a phase discontinuity kappa ~ I_inj. The calibration of
injectors is discussed in detail. The small but finite size of injectors leads
to some deviations of the properties of such a 0-kappa-LJJ from the properties
of a LJJ with an ideal kappa-discontinuity. These experimentally observed
deviations in the dependence of the critical current on I_inj$ and magnetic
field can be well reproduced by numerical simulation assuming a finite injector
size. The physical origin of these deviations is discussed.Comment: Submitted to Phys. Rev. B (12 figures). v 2: refs updated, long eqs
fixed v 3: major changes, fractional vortex dynamics exclude
Experimental evidence of a {\phi} Josephson junction
We demonstrate experimentally the existence of Josephson junctions having a
doubly degenerate ground state with an average Josephson phase \psi=\pm{\phi}.
The value of {\phi} can be chosen by design in the interval 0<{\phi}<\pi. The
junctions used in our experiments are fabricated as 0-{\pi} Josephson junctions
of moderate normalized length with asymmetric 0 and {\pi} regions. We show that
(a) these {\phi} Josephson junctions have two critical currents, corresponding
to the escape of the phase {\psi} from -{\phi} and +{\phi} states; (b) the
phase {\psi} can be set to a particular state by tuning an external magnetic
field or (c) by using a proper bias current sweep sequence. The experimental
observations are in agreement with previous theoretical predictions
Simulation of non-stationary processes in industrial centrifugal cascades of uranium enrichment
The mathematical model of non-stationary dividing processes of uranium enrichment in industrial centrifugal cascades which can be used in a computer simulator to prepare experts in dividing production and application as an expert system in the automated control system of technological circuit has been developed and realized
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