184 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
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
Three-particle states and brightening of intervalley excitons in a doped MoS monolayer
Optical spectra of two-dimensional transition-metal dichalcogenides (TMDC)
are influenced by complex multi-particle excitonic states. Their theoretical
analysis requires solving the many-body problem, which in most cases, is
prohibitively complicated. In this work, we calculate the optical spectra by
exact diagonalization of the three-particle Hamiltonian within the Tamm-Dancoff
approximation where the doping effects are accounted for via the Pauli blocking
mechanism, modelled by a discretized mesh in the momentum space. The
single-particle basis is extracted from the {\it ab initio} calculations.
Obtained three-particle eigenstates and the corresponding transition dipole
matrix elements are used to calculate the linear absorption spectra as a
function of the doping level. Results for negatively doped MoS monolayer
(ML) are in an excellent quantitative agreement with the available experimental
data, validating our approach. The results predict additional spectral features
due to the intervalley exciton that is optically dark in an undoped ML but is
brightened by the doping. Our approach can be applied to a plethora of other
atomically thin semiconductors, where the doping induced brightening of the
many-particle states is also anticipated
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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