720 research outputs found
Dynamics of a rolling robot
Equations describing the rolling of a spherical ball on a horizontal surface are obtained, the motion being activated by an internal rotor driven by a battery mechanism. The rotor is modeled as a point mass mounted inside a spherical shell and caused to move in a prescribed circular orbit relative to the shell. The system is described in terms of four independent dimensionless parameters. The equations governing the angular momentum of the ball relative to the point of contact with the plane constitute a six-dimensional, nonholonomic, nonautonomous dynamical system with cubic nonlinearity. This system is decoupled from a subsidiary system that describes the trajectories of the center of the ball. Numerical integration of these equations for prescribed values of the parameters and initial conditions reveals a tendency toward chaotic behavior as the radius of the circular orbit of the point mass increases (other parameters being held constant). It is further shown that there is a range of values of the initial angular velocity of the shell for which chaotic trajectories are realized while contact between the shell and the plane is maintained. The predicted behavior has been observed in our experiments
Proximity effect model of ultra-narrow NbN strips
We show that narrow superconducting strips in superconducting (S) and normal
(N) states are universally described by the model presenting them as lateral
NSN proximity systems in which the superconducting central band is sandwiched
between damaged edge-bands with suppressed superconductivity.The width of the
superconducting band was experimentally determined from the value of magnetic
field at which the band transits from the Meissner state to the static vortex
state. Systematic experimental study of 4.9 nm thick NbN strips with widths in
the interval from 50 nm to 20 m, which are all smaller than the Pearl's
length, demonstrates gradual evolution of the temperature dependence of the
critical current with the change of the strip width
Enhancement of superconductivity in NbN nanowires by negative electron-beam lithography with positive resist
We performed comparative experimental investigation of superconducting NbN
nanowires which were prepared by means of positive-and negative electron-beam
lithography with the same positive tone Poly-methyl-methacrylate (PMMA) resist.
We show that nanowires with a thickness 4.9 nm and widths less than 100 nm
demonstrate at 4.2 K higher critical temperature and higher density of critical
and retrapping currents when they are prepared by negative lithography. Also
the ratio of the experimental critical-current to the depairing critical
current is larger for nanowires prepared by negative lithography. We associate
the observed enhancement of superconducting properties with the difference in
the degree of damage that nanowire edges sustain in the lithographic process. A
whole range of advantages which is offered by the negative lithography with
positive PMMA resist ensures high potential of this technology for improving
performance metrics of superconducting nanowire singe-photon detectors
Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips
Current crowding in the bends of superconducting nano-structures not only
restricts measurable critical current in such structures but also redistributes
local probabilities for dark and light counts to appear. Using structures from
strips in the form of a square spiral which contain bends with the very same
curvature with respect to the directions of bias current and external magnetic
field, we have shown that dark counts as well as light counts at small photon
energies originate from areas around the bends. The minimum in the rate of dark
counts reproduces the asymmetry of the maximum critical current density as
function of the magnetic field. Contrary, the minimum in the rate of light
counts demonstrate opposite asymmetry. The rate of light counts become
symmetric at large currents and fields. Comparing locally computed absorption
probabilities for photons and the simulated threshold detection current we
found the approximate locations of areas near bends which deliver asymmetric
light counts. Any asymmetry is absent in Archimedean spiral structures without
bends
Clade-Specific Distribution of Antibiotic Resistance Mutations in the Population of Mycobacterium tuberculosis - Prospects for Drug Resistance Reversion
Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), is a leading cause of death in humans worldwide. The emergence of antibiotic-resistant strains of Mtb is a threat to tuberculosis control. A general belief is that drug resistance is acquired by Mtb during antibiotic treatment by accumulation of spontaneous mutations. Also, it is known that the drug resistance mutations (DRM) have an associated fitness cost, reducing the transmissibility and virulence of resistant strains. In this work we show that many canonical DRM are clade specific; i.e. they occur only in specific genetic lineages of Mtb and depend on a specific genetic context necessary for the reduction of the fitness cost and sustainability of the drug resistance phenotype. Dependence of the drug resistance on occurrence of genetic variants of multiple genes and specific activities of the encoded proteins allows combating the drug resistance by impairing the global genetic context. A new drug, FS-1, reverses antibiotic resistance by compromising this genetic context and aggravating the fitness cost of DRM
Characteristics of superconducting tungsten silicide WxSi1-x for single photon detection
Superconducting properties of three series of amorphous WxSi1-x films with
different thickness and stoichiometry were investigated by dc transport
measurements in a magnetic field up to 9 T. These amorphous WxSi1-x films were
deposited by magnetron co-sputtering of the elemental source targets onto
silicon substrates at room temperature and patterned in form of bridges by
optical lithography and reactive ion etching. Analysis of the data on
magnetoconductivity allowed us to extract the critical temperature,
superconducting coherence length, magnetic penetration depth, and diffusion
coefficient of electrons in the normal state as a function of film thickness
for each stoichiometry. Two basic time constants were derived from transport
and time-resolving measurements. A dynamic process of the formation of a
hot-spot was analyzed in the framework of a diffusion-based vortex-entry model.
We used the two stage diffusion approach and defined a hotspot size by assuming
that the quasi-particles and normal-state electrons have the equal diffusion
constant. Our findings are consistent with the most recent results on a
hot-spot relaxation time in the WxSi1-x superconducting nanowire single-photon
detector. In the 5 nm thick W0.85Si0.15 film the hot-spot has a diameter of 105
nm at the peak of the number of non-equilibrium quasi-particles
Energy of dendritic avalanches in thin-film superconductors
A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment
Energy of dendritic avalanches in thin-film superconductors
A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment
Effect of the wire width on the intrinsic detection efficiency of superconducting-nanowire single-photon detectors
Thorough spectral study of the intrinsic single-photon detection efficiency
in superconducting TaN and NbN nanowires with different widths shows that the
experimental cut-off in the efficiency at near-infrared wavelengths is most
likely caused by the local deficiency of Cooper pairs available for current
transport. For both materials the reciprocal cut-off wavelength scales with the
wire width whereas the scaling factor quantitatively agrees with the hot-spot
detection models. Comparison of the experimental data with vortex-assisted
detection scenarios shows that these models predict a stronger dependence of
the cut-off wavelength on the wire width.Comment: 16 pages, 6 figure
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