572 research outputs found
3-junction SQUID rocking ratchet
We investigate 3-junction SQUIDs which show voltage rectification if biased
with an ac current drive with zero mean value. The Josephson phase across the
SQUID experiences an effective ratchet potential, and the device acts as an
efficient rocking ratchet, as demonstrated experimentally for adiabatic and
nonadiabatic drive frequencies. For high-frequency drives the rectified voltage
is quantized due to synchronization of the phase dynamics with the external
drive. The experimental data are in excellent agreement with numerical
simulations including thermal fluctuations.Comment: 5 pages, 4 figures -- Fig.4 revise
Voltage-flux-characteristics of asymmetric dc SQUIDs
We present a detailed analysis of voltage-flux V(Phi)-characteristics for
asymmetric dc SQUIDs with various kinds of asymmetries. For finite asymmetry
alpha_I in the critical currents of the two Josephson junctions, the minima in
the V(Phi)-characteristics for bias currents of opposite polarity are shifted
along the flux axis by Delta_Phi = (alpha_I)*(beta_L) relative to each other;
beta_L is the screening parameter. This simple relation allows the
determination of alpha_I in our experiments on YBa_2Cu_3O_(7-x} dc SQUIDs and
comparison with theory. Extensive numerical simulations within a wide range of
beta_L and noise parameter Gamma reveal a systematic dependence of the transfer
function V_Phi on alpha_I and alpha_R (junction resistance asymmetry). As for
the symmetric dc SQUID, V_Phi factorizes into
g(Gamma*beta_L)*f(alpha_I,beta_L), where now f also depends on alpha_I. For
\beta_L below five we find mostly a decrease of V_Phi with increasing alpha_I,
which however can only partially account for the frequently observed
discrepancy in V_Phi between theory and experiment for high-T_c dc SQUIDs.Comment: 4 pages, 7 figures, Applied Superconductivity Conference 2000, to be
published in IEEE Trans. Appl. Supercon
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 ;-
Commensurability effects in superconducting Nb films with quasiperiodic pinning arrays
We study experimentally the critical depinning current Ic versus applied
magnetic field B in Nb thin films which contain 2D arrays of circular antidots
placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to
the transition temperature Tc we observe matching of the vortex lattice with
the QP pinning array, confirming essential features in the Ic(B) patterns as
predicted by Misko et al. [Phys. Rev. Lett, vol.95, 177007 (2005)]. We find a
significant enhancement in Ic(B) for QP pinning arrays in comparison to Ic in
samples with randomly distributed antidots or no antidots.Comment: 4 pages, 3 figure
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
Multigrid reduction-in-time convergence for advection problems: A Fourier analysis perspective
A long-standing issue in the parallel-in-time community is the poor
convergence of standard iterative parallel-in-time methods for hyperbolic
partial differential equations (PDEs), and for advection-dominated PDEs more
broadly. Here, a local Fourier analysis (LFA) convergence theory is derived for
the two-level variant of the iterative parallel-in-time method of multigrid
reduction-in-time (MGRIT). This closed-form theory allows for new insights into
the poor convergence of MGRIT for advection-dominated PDEs when using the
standard approach of rediscretizing the fine-grid problem on the coarse grid.
Specifically, we show that this poor convergence arises, at least in part, from
inadequate coarse-grid correction of certain smooth Fourier modes known as
characteristic components, which was previously identified as causing poor
convergence of classical spatial multigrid on steady-state advection-dominated
PDEs. We apply this convergence theory to show that, for certain
semi-Lagrangian discretizations of advection problems, MGRIT convergence using
rediscretized coarse-grid operators cannot be robust with respect to CFL number
or coarsening factor. A consequence of this analysis is that techniques
developed for improving convergence in the spatial multigrid context can be
re-purposed in the MGRIT context to develop more robust parallel-in-time
solvers. This strategy has been used in recent work to great effect; here, we
provide further theoretical evidence supporting the effectiveness of this
approach
Pressure balance at the magnetopause: Experimental studies
The pressure balance at the magnetopause is formed by magnetic field and
plasma in the magnetosheath, on one side, and inside the magnetosphere, on the
other side. In the approach of dipole earth's magnetic field configuration and
gas-dynamics solar wind flowing around the magnetosphere, the pressure balance
predicts that the magnetopause distance R depends on solar wind dynamic
pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the
real magnetosphere the magnetic filed is contributed by additional sources:
Chapman-Ferraro current system, field-aligned currents, tail current, and
storm-time ring current. Net contribution of those sources depends on
particular magnetospheric region and varies with solar wind conditions and
geomagnetic activity. As a result, the parameters of pressure balance,
including power index alpha, depend on both the local position at the
magnetopause and geomagnetic activity. In addition, the pressure balance can be
affected by a non-linear transfer of the solar wind energy to the
magnetosheath, especially for quasi-radial regime of the subsolar bow shock
formation proper for the interplanetary magnetic field vector aligned with the
solar wind plasma flow.Comment: 8 pages, 2 figure
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