19 research outputs found
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
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
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
Electron energy relaxation in disordered superconducting NbN films
We report onthe energy relaxation of electrons studied by means of magnetoconductance and photoresponse in a series of superconducting NbN film with thickness in the range from 3 to 33 nm. The inelastic scattering rate of electrons on phonons obeys Tntemperature dependence where the exponent is in the range ????≈3.2÷3.8and shows no systematically dependence on the degree of disorder. At 11K electron-phonon scattering times are in the range11.9 -17.5 ps.We show that in the studied NbN films the Debye temperature and the densityof phononstatesare both reduced with respect to bulk material. In the thinnest studied films reduced density of states along with the phonon trapping slowsdown the energy relaxationofelectrons by afactor of 4 as compared to the prediction of the tree dimensional phonon mode
Large-area microwire MoSi single-photon detectors at 1550 nm wavelength
© 2020 Author(s). We demonstrate saturated internal detection efficiency at 1550 nm wavelengths for meander-shaped superconducting nanowire single-photon detectors made of 3 nm thick MoSi films with widths of 1 and 3 μm and active areas up to 400 × 400 μm2. Despite hairpin turns and a large number of squares (up to 104) in the device, the dark count rate was measured to be ∼103 cps at 99% of the switching current. This value is about two orders of magnitude lower than the results reported recently for short MoSi devices with shunt resistors. We also found that 5 nm thick MoSi detectors with the same geometry were insensitive to single near-infrared photons, which may be associated with different levels of suppression of the superconducting order parameter. However, our results obtained on 3 nm thick MoSi devices are in good agreement with predictions in the frame of a kinetic-equation approach