298 research outputs found
Nanosecond quantum state detection in a current biased dc SQUID
This article presents our procedure to measure the quantum state of a dc
SQUID within a few nanoseconds, using an adiabatic dc flux pulse. Detection of
the ground state is governed by standard macroscopic quantum theory (MQT), with
a small correction due to residual noise in the bias current. In the two level
limit, where the SQUID constitutes a phase qubit, an observed contrast of 0.54
indicates a significant loss in contrast compared to the MQT prediction. It is
attributed to spurious depolarization (loss of excited state occupancy) during
the leading edge of the adiabatic flux measurement pulse. We give a simple
phenomenological relaxation model which is able to predict the observed
contrast of multilevel Rabi oscillations for various microwave amplitudes.Comment: 10 pages, 8 figure
Gamma-Ray Emission from Molecular Clouds Generated by Penetrating Cosmic Rays
We analyze the processes governing cosmic-ray (CR) penetration into molecular
clouds and the resulting generation of gamma-ray emission. The density of CRs
inside a cloud is depleted at lower energies due to the self-excited MHD
turbulence. The depletion depends on the effective gas column density ("size")
of the cloud. We consider two different environments where the depletion effect
is expected to be observed. For the Central Molecular Zone, the expected range
of CR energy depletion is GeV, leading to the depletion of
gamma-ray flux below GeV. This effect can be important for
the interpretation of the GeV gamma-ray excess in the Galactic Center, which
has been revealed from the standard model of CR propagation (assuming the CR
spectrum inside a cloud to be equal to the interstellar spectrum). Furthermore,
recent observations of some local molecular clouds suggest the depletion of the
gamma-ray emission, indicating possible self-modulation of the penetrating
low-energy CRs.Comment: 10 pages, 5 figures, accepted for publication in Ap
Secondary cosmic-ray nuclei in the model of Galactic halo with nonlinear Landau damping
We employ our recent model of the cosmic-ray (CR) halo by Chernyshov et al.
(2022) to compute the Galactic spectra of stable and unstable secondary nuclei.
In this model, confinement of the Galactic CRs is entirely determined by the
self-generated Alfvenic turbulence whose spectrum is controlled by nonlinear
Landau damping. We analyze the physical parameters affecting propagation
characteristics of CRs, and estimate the best set of free parameters providing
accurate description of available observational data. We also show that
agreement with observations at lower energies may be further improved by taking
into account the effect of ion-neutral damping which operates near the Galactic
disk.Comment: 8 pages, 2 figures. Accepted to Ap
Multilevel intellectual approach to HTTP-requests legitimacy validation
In the paper a multilevel intellectual approach to HTTP-requests legitimacy validation is proposed. The approach is devised for HTTP-flood DDoS-attacks detection and prevention in telecommunication networks with a web-server as the target attack object. The analysis of HTTP-requests attributes and their signatures is provided. On the basis of the analysis the attributes are separated into several levels that allow us to design a flow analyzer in a form of the multilevel block. Due to a multilevel structure of the flow analyzer a minimization of resources, spent for a request handling, is achieved
Evidence of two-dimensional macroscopic quantum tunneling of a current-biased DC-SQUID
The escape probability out of the superconducting state of a hysteretic
DC-SQUID has been measured at different values of the applied magnetic flux. At
low temperature, the escape current and the width of the probability
distribution are temperature independent but they depend on flux. Experimental
results do not fit the usual one-dimensional (1D) Macroscopic Quantum Tunneling
(MQT) law but are perfectly accounted for by the two-dimensional (2D) MQT
behaviour as we propose here. Near zero flux, our data confirms the recent MQT
observation in a DC-SQUID \cite{Li02}.Comment: 4 pages, 4 figures Accepted to PR
Penetration of cosmic rays into dense molecular clouds: role of diffuse envelope
A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can
excite MHD waves, thus generating magnetic disturbances. We propose a generic
model of CR penetration into molecular clouds through their diffuse envelopes,
and identify the leading physical processes controlling their transport on the
way from a highly ionized interstellar medium to a dense interior of the cloud.
The model allows us to describe a transition between a free streaming of CRs
and their diffusive propagation, determined by the scattering on the
self-generated disturbances. A self consistent set of equations, governing the
diffusive transport regime in an envelope and the MHD turbulence generated by
the modulated CR flux, is essentially characterized by two dimensionless
numbers. We demonstrate a remarkable mutual complementarity of different
mechanisms leading to the onset of the diffusive regime, which results in a
universal energy spectrum of the modulated CRs. In conclusion, we briefly
discuss implications of our results for several fundamental astrophysical
problems, such as the spatial distribution of CRs in the Galaxy as well as the
ionization, heating, and chemistry in dense molecular clouds.Comment: The manuscript is accepted for publication in the Astrophysical
Journa
Nucleation and Growth of the Superconducting Phase in the Presence of a Current
We study the localized stationary solutions of the one-dimensional
time-dependent Ginzburg-Landau equations in the presence of a current. These
threshold perturbations separate undercritical perturbations which return to
the normal phase from overcritical perturbations which lead to the
superconducting phase. Careful numerical work in the small-current limit shows
that the amplitude of these solutions is exponentially small in the current; we
provide an approximate analysis which captures this behavior. As the current is
increased toward the stall current J*, the width of these solutions diverges
resulting in widely separated normal-superconducting interfaces. We map out
numerically the dependence of J* on u (a parameter characterizing the material)
and use asymptotic analysis to derive the behaviors for large u (J* ~ u^-1/4)
and small u (J -> J_c, the critical deparing current), which agree with the
numerical work in these regimes. For currents other than J* the interface
moves, and in this case we study the interface velocity as a function of u and
J. We find that the velocities are bounded both as J -> 0 and as J -> J_c,
contrary to previous claims.Comment: 13 pages, 10 figures, Revte
Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation
Freezing and melting of large three-dimensional complex plasmas under
microgravity conditions is investigated. The neutral gas pressure is used as a
control parameter to trigger the phase changes: Complex plasma freezes (melts)
by decreasing (increasing) the pressure. Evolution of complex plasma structural
properties upon pressure variation is studied. Theoretical estimates allow us
to identify main factors responsible for the observed behavior.Comment: Phys. Rev. Lett. (in press); 4 pages, 4 figure
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