57 research outputs found
Discreteness-induced resonances and AC voltage amplitudes in long one-dimensional Josephson junction arrays
New resonance steps are found in the experimental current-voltage
characteristics of long, discrete, one-dimensional Josephson junction arrays
with open boundaries and in an external magnetic field. The junctions are
underdamped, connected in parallel, and DC biased. Numerical simulations based
on the discrete sine-Gordon model are carried out, and show that the solutions
on the steps are periodic trains of fluxons, phase-locked by a finite amplitude
radiation. Power spectra of the voltages consist of a small number of harmonic
peaks, which may be exploited for possible oscillator applications. The steps
form a family that can be numbered by the harmonic content of the radiation,
the first member corresponding to the Eck step. Discreteness of the arrays is
shown to be essential for appearance of the higher order steps. We use a
multi-mode extension of the harmonic balance analysis, and estimate the
resonance frequencies, the AC voltage amplitudes, and the theoretical limit on
the output power on the first two steps.Comment: REVTeX, 17 pages, 7 figures, psfig; to appear in J. Applied Physic
Superconducting and Quantum-Effect Devices
Contains reports on six research projects and a list of publications.National Science Foundation Grant DMR 94-02020National Science Foundation Fellowship MIP 88-58764U.S. Air Force - Office of Scientific Research Grant F30602-96-1-0059 Rome LaboratoryDefense Advanced Research Projects Agency/Consortium for Superconducting Electronics Contract MDA 972-90-C-002
Superconducting and Quantum-Effect Electronics
Contains reports on five research projects and a list of publications.National Science Foundation Grant DMR 94-02020Rome LaboratoryU.S. Air Force - Office of Scientific Research Grant F30602-96-1-0059U.S. Air Force - Office of Scientific Research Grant F49-620-92-J-006
Row-switched states in two-dimensional underdamped Josephson junction arrays
When magnetic flux moves across layered or granular superconductor
structures, the passage of vortices can take place along channels which develop
finite voltage, while the rest of the material remains in the zero-voltage
state. We present analytical studies of an example of such mixed dynamics: the
row-switched (RS) states in underdamped two-dimensional Josephson arrays,
driven by a uniform DC current under external magnetic field but neglecting
self-fields. The governing equations are cast into a compact
differential-algebraic system which describes the dynamics of an assembly of
Josephson oscillators coupled through the mesh current. We carry out a formal
perturbation expansion, and obtain the DC and AC spatial distributions of the
junction phases and induced circulating currents. We also estimate the interval
of the driving current in which a given RS state is stable. All these
analytical predictions compare well with our numerics. We then combine these
results to deduce the parameter region (in the damping coefficient versus
magnetic field plane) where RS states can exist.Comment: latex, 48 pages, 15 figs using psfi
Superconducting and Quantum-Effect Electronics
Contains reports on seven research projects.National Science Foundation Grant DMR 96-10042U.S. Air Force - Office of Scientific Research Grant F08671-97000989Rome Air Force LaboratoryNational Science Foundation Grant DMR 94-02020U.S. Air Force - Office of Scientific Research Grant F49-620-92-J-006
Minimization of phonon-tunneling dissipation in mechanical resonators
Micro- and nanoscale mechanical resonators have recently emerged as
ubiquitous devices for use in advanced technological applications, for example
in mobile communications and inertial sensors, and as novel tools for
fundamental scientific endeavors. Their performance is in many cases limited by
the deleterious effects of mechanical damping. Here, we report a significant
advancement towards understanding and controlling support-induced losses in
generic mechanical resonators. We begin by introducing an efficient numerical
solver, based on the "phonon-tunneling" approach, capable of predicting the
design-limited damping of high-quality mechanical resonators. Further, through
careful device engineering, we isolate support-induced losses and perform the
first rigorous experimental test of the strong geometric dependence of this
loss mechanism. Our results are in excellent agreement with theory,
demonstrating the predictive power of our approach. In combination with recent
progress on complementary dissipation mechanisms, our phonon-tunneling solver
represents a major step towards accurate prediction of the mechanical quality
factor.Comment: 12 pages, 4 figure
Moving glass theory of driven lattices with disorder
We study periodic structures, such as vortex lattices, moving in a random
potential. As predicted in [T. Giamarchi, P. Le Doussal Phys. Rev. Lett. 76
3408 (1996)] the periodicity in the direction transverse to motion leads to a
new class of driven systems: the Moving Glasses. We analyse using several RG
techniques the properties at T=0 and : (i) decay of translational long
range order (ii) particles flow along static channels (iii) the channel pattern
is highly correlated (iv) barriers to transverse motion. We demonstrate the
existence of the ``transverse critical force'' at T=0. A ``static random
force'' is shown to be generated by motion. Displacements grow logarithmically
in and algebraically in . The persistence of quasi long range
translational order in at weak disorder, or large velocity leads to
predict a topologically ordered ``Moving Bragg Glass''. This state continues
the static Bragg glass and is stable at , with non linear transverse
response and linear asymptotic behavior. In , or in at intermediate
disorder, another moving glass exist (the Moving Transverse Glass) with smectic
quasi order in the transverse direction. A phase diagram in force and
disorder for static and moving structures is proposed. For correlated disorder
we predict a ``moving Bose glass'' state with anisotropic transverse Meissner
effect and transverse pinning. We discuss experimental consequences such as
anomalous Hall effect in Wigner crystal and transverse critical current in
vortex lattice.Comment: 74 pages, 27 figures, RevTe
Quasi-two-dimensional optomechanical crystals with a complete phononic bandgap
A fully planar two-dimensional optomechanical crystal formed in a silicon
microchip is used to create a structure devoid of phonons in the GHz frequency
range. A nanoscale photonic crystal cavity is placed inside the phononic
bandgap crystal in order to probe the properties of the localized acoustic
modes. By studying the trends in mechanical damping, mode density, and
optomechanical coupling strength of the acoustic resonances over an array of
structures with varying geometric properties, clear evidence of a complete
phononic bandgap is shown.Comment: 9 pages, 7 figure
Loss of Akt activity increases circulating soluble endoglin release in preeclampsia:identification of inter-dependency between Akt-1 and heme oxygenase-1
Aims - Endothelial dysfunction is a hallmark of preeclampsia. Desensitization of the phosphoinositide 3-kinase (PI3K)/Akt pathway underlies endothelial dysfunction and haeme oxygenase-1 (HO-1) is decreased in preeclampsia. To identify therapeutic targets, we sought to assess whether these two regulators act to suppress soluble endoglin (sEng), an antagonist of transforming growth factor-ß (TGF-ß) signalling, which is known to be elevated in preeclampsia. Methods and results - Vascular endothelial growth factor-A (VEGF-A), fibroblast growth factor (FGF-2), angiopoietin-1 (Ang-1), and insulin, which all activate the PI3K/Akt pathway, inhibited the release of sEng from endothelial cells. Inhibition of the PI3K/Akt pathway, by overexpression of phosphatase and tensin homolog (PTEN) or a dominant-negative isoform of Akt (Aktdn) induced sEng release from endothelial cells and prevented the inhibitory effect of VEGF-A. Conversely, overexpression of a constitutively active Akt (Aktmyr) inhibited PTEN and cytokine-induced sEng release. Systemic delivery of Aktmyr to mice significantly reduced circulating sEng, whereas Aktdn promoted sEng release. Phosphorylation of Akt was reduced in preeclamptic placenta and this correlated with the elevated level of circulating sEng. Knock-down of Akt using siRNA prevented HO-1-mediated inhibition of sEng release and reduced HO-1 expression. Furthermore, HO-1 null mice have reduced phosphorylated Akt in their organs and overexpression of Aktmyr failed to suppress the elevated levels of sEng detected in HO-1 null mice, indicating that HO-1 is required for the Akt-mediated inhibition of sEng. Conclusion - The loss of PI3K/Akt and/or HO-1 activity promotes sEng release and positive manipulation of these pathways offers a strategy to circumvent endothelial dysfunction
Comparative dynamics of two-dimensional shorted arrays and continuous stacked Josephson junctions
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