135 research outputs found
Josephson Flux Flow Oscillator: the Microscopic Tunneling Approach
We elaborate a theoretical description of large Josephson junctions which is
based on the Werthamer's microscopic tunneling theory. The model naturally
incorporates coupling of electromagnetic radiation to the tunnel currents and,
therefore, is particularly suitable for description of the self-coupling effect
in Josephson junction. In our numerical calculations we treat the arising
integro-differential equation, which describes temporal evolution of the
superconducting phase difference coupled to the electromagnetic field, by the
Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the
time integrals at each time step while taking into account all the memory
effects. To validate the obtained microscopic model of large Josephson junction
we focus our attention on the Josephson flux flow oscillator. The proposed
microscopic model of flux flow oscillator does not involve the phenomenological
damping parameter, rather, the damping is taken into account naturally in the
tunnel current amplitudes calculated at a given temperature. The theoretically
calculated current-voltage characteristics is compared to our experimental
results obtained for a set of fabricated flux flow oscillators of different
lengths. Our theoretical calculation agrees well with the obtained experimental
results, and, to our knowledge, is the first where theoretical description of
Josephson flux flow oscillator is brought beyond the perturbed sine-Gordon
equation.Comment: 13 pages, 2 figure
Is a single photon's wave front observable?
The ultimate goal and the theoretical limit of weak signal detection is the
ability to detect a single photon against a noisy background. [...] In this
paper we show, that a combination of a quantum metamaterial (QMM)-based sensor
matrix and quantum non-demolition (QND) readout of its quantum state allows, in
principle, to detect a single photon in several points, i.e., to observe its
wave front.
Actually, there are a few possible ways of doing this, with at least one
within the reach of current experimental techniques for the microwave range.
The ability to resolve the quantum-limited signal from a remote source against
a much stronger local noise would bring significant advantages to such diverse
fields of activity as, e.g., microwave astronomy and missile defence.
The key components of the proposed method are 1) the entangling interaction
of the incoming photon with the QMM sensor array, which produces the spatially
correlated quantum state of the latter, and 2) the QND readout of the
collective observable (e.g., total magnetic moment), which characterizes this
quantum state. The effects of local noise (e.g., fluctuations affecting the
elements of the matrix) will be suppressed relative to the signal from the
spatially coherent field of (even) a single photon.Comment: 13 pages, 4 figure
Excitonic effects in time-dependent density functional theory from zeros of the density response
We show that the analytic structure of the dynamical xc kernels of
semiconductors and insulators can be sensed in terms of its poles which mark
physically relevant frequencies of the system where the counter-phase motion of
discrete collective excitations occurs: if excited, the collective modes
counterbalance each other, making the system to exhibit none at all or
extremely weak density response. This property can be employed to construct
simple and practically relevant approximations of the dynamical xc kernel for
time-dependent density functional theory (TDDFT). Such kernels have simple
analytic structure, are able to reproduce dominant excitonic features of the
absorption spectra of monolayer semiconductors and bulk solids, and promise
high potential for future uses in efficient real-time calculations with TDDFT.Comment: 10 pages, 4 figure
Perturbation theory for localized solutions of sine-Gordon equation: decay of a breather and pinning by microresistor
We develop a perturbation theory that describes bound states of solitons
localized in a confined area. External forces and influence of inhomogeneities
are taken into account as perturbations to exact solutions of the sine-Gordon
equation. We have investigated two special cases of fluxon trapped by a
microresistor and decay of a breather under dissipation. Also, we have carried
out numerical simulations with dissipative sine-Gordon equation and made
comparison with the McLaughlin-Scott theory. Significant distinction between
the McLaughlin-Scott calculation for a breather decay and our numerical result
indicates that the history dependence of the breather evolution can not be
neglected even for small damping parameter
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