10 research outputs found
Dark Energy and Dark Matter Interaction: Kernels of Volterra Type and Coincidence Problem
We study a new exactly solvable model of coupling of the Dark Energy and Dark
Matter, in the framework of which the kernel of non-gravitational interaction
is presented by the integral Volterra-type operator well-known in the classical
theory of fading memory. Exact solutions of this isotropic homogeneous
cosmological model were classified with respect to the sign of the discriminant
of the cubic characteristic polynomial associated with the key equation of the
model. Energy-density scalars of the Dark Energy and Dark Matter, the Hubble
function and acceleration parameter are presented explicitly; the scale factor
is found in quadratures. Asymptotic analysis of the exact solutions has shown
that the Big Rip, Little Rip, Pseudo Rip regimes can be realized with the
specific choice of guiding parameters of the model. We show that the
Coincidence problem can be solved if we consider the memory effect associated
with the interactions in the Dark Sector of the Universe.Comment: 15 pages, 0 figures, Invited paper for the Special Issue
"Cosmological Inflation, Dark Matter and Dark Energy" of the Journal Symmetry
(MDPI), Special Issue Editor: Kazuharu Bamb
Relativistic Neutron Stars: Rheological Type Extensions of the Equations of State
Based on the Rheological Paradigm, one has extended the equations of state
for relativistic spherically symmetric static neutron stars, taking into
consideration the derivative of the matter pressure along the so-called
director four-vector. The modified equations of state are applied to the model
of a zero-temperature neutron condensate. This model includes one new parameter
with the dimensionality of length, which describes the rheological type
screening inside the neutron star. As an illustration of the new approach, one
has considered the rheological type generalization of the non-relativistic
Lane-Emden theory and found the numerical profiles of the pressure for a number
of values of the new guiding parameter. One has found that the rheological type
self-interaction makes the neutron star more compact, since the radius of the
star, related to the first null of the pressure profile, decreases when the
modulus of the rheological type guiding parameter grows.Comment: 14 pages, 1 figure, 1 tabl
Magnetic-field enhancement of performance of superconducting nanowire single-photon detector
We present SNSPDs from NbN nanowires shaped after square-spiral that allows
an increase not only in critical currents but also an extension of spectral
detection efficiencies by just applying an external magnetic field. Using
negative electron-beam lithography with the positive resist for shaping
nanowires, made it possible to reduce the inner bend radius. Consequently, the
effect of critical-current enhancement in the magnetic field becomes stronger
than it was demonstrated earlier. Here we achieved a 13% increase of the
critical current in the magnetic field. We measured spectra of the
single-photon detection efficiency in the wavelength range from 400 to 1100 nm
in the magnetic field. At zero field, the square spiral has the spectrum
similar to that of a meander. At the field providing the maximum of the
critical current, the detection efficiency and the cut-off wavelength in the
spectrum increase by 20% and by 54%, correspondingly. The magnetic-field
dependence of dark count rate is well described by proposed analytical model
Magnetic-field enhancement of performance of superconducting nanowire single-photon detector
We present SNSPDs from NbN nanowires shaped after square-spiral that allows
an increase not only in critical currents but also an extension of spectral
detection efficiencies by just applying an external magnetic field. Using
negative electron-beam lithography with the positive resist for shaping
nanowires, made it possible to reduce the inner bend radius. Consequently, the
effect of critical-current enhancement in the magnetic field becomes stronger
than it was demonstrated earlier. Here we achieved a 13% increase of the
critical current in the magnetic field. We measured spectra of the
single-photon detection efficiency in the wavelength range from 400 to 1100 nm
in the magnetic field. At zero field, the square spiral has the spectrum
similar to that of a meander. At the field providing the maximum of the
critical current, the detection efficiency and the cut-off wavelength in the
spectrum increase by 20% and by 54%, correspondingly. The magnetic-field
dependence of dark count rate is well described by proposed analytical model
Interaction of the Cosmic Dark Fluid with Dynamic Aether: Parametric Mechanism of Axion Generation in the Early Universe
We consider an isotropic homogeneous cosmological model with five interacting
elements: first, the dynamic aether presented by a unit timelike vector field,
second, the pseudoscalar field describing an axionic component of the dark
matter, third, the cosmic dark energy, described by a rheologic fluid, fourth,
the non-axionic dark matter coupled to the dark energy, fifth, the gravity
field. We show that the early evolution of the Universe described by this model
can include two specific epochs: the first one can be characterized as a
super-inflation, the second epoch is associated with an oscillatory regime. The
dynamic aether carries out a regulatory mission; the rheologic dark fluid
provides the specific features of the spacetime evolution. The oscillations of
the scale factor and of the Hubble function are shown to switch on the
parametric (Floquet - type) mechanism of the axion number growth.Comment: 21 pages, 4 figures, accepted for publication in Symmetry, Special
Issue: Fundamental Aspects of Theoretical Physics - Memorial Issue for Prof.
Dr. Weinber
SQUID-Readout for the Superconducting Nanowire Single-Photon Detector
Superconducting single-photon detectors from thin
niobium nitride nanostrips exhibit a near-infrared cut-off of
the wavelength-independent quantum efficiency along with a moderate energy resolution in the wavelength range around the cut-off. For the wavelength range before the cut-off, we have developed the electro-thermal model that describes the initial growth, diminution and disappearance of the normal domain created by a photon in the superconducting nanostrip. The response parameters extracted from the model fit were used to optimize the
signal-to-noise ratio of the SQUID-based readout and to achieve in the temperature range from 1 to 4 K a photon counting frequency of 20 MHz and a dark count rate less than four in one hour
Ultra-thin NbN films on Si: crystalline and superconducting properties
We present results on superconducting and crystalline properties of NbN films with a thickness smaller than 10 nm. The films were deposited using reactive magnetron sputtering on heated silicon substrates. Zero resistance critical temperatures of about 9 K have been measured for films with a thickness of about 5 nm and reaches values ≈12 K for 10 nm thick films. A value of the superconducting coherence length of about 4 nm was estimated from the measurements of the second critical magnetic field. High-resolution transmission electron microscopy accompanied with electron-spectroscopy techniques was used to analyze the structure, thickness, and film-substrate interface of fabricated films. The interrelations between
fabrication conditions, superconducting and crystalline properties of NbN films on Si substrates are presented and discussed