132 research outputs found
Current-voltage characteristics of asymmetric double-barrier Josephson junctions
We develop a theory for the current-voltage characteristics of diffusive
superconductor-normal metal-superconductor Josephson junctions with resistive
interfaces and the distance between the electrodes smaller than the
superconducting coherence length. The theory allows for a quantitative
analytical and numerical analysis in the whole range of the interface
transparencies and asymmetry. We focus on the regime of large interface
resistance compared to the resistance of the normal region, when the
electron-hole dephasing in the normal region is significant and the finite
length of the junction plays a role. In the limit of strong asymmetry we find
pronounced current structures at the combination subharmonics of
, where is the proximity minigap in the normal
region, in addition to the subharmonics of the energy gap in the
electrodes. In the limit of rather transparent interfaces, our theory recovers
a known formula for the current in a short mesoscopic connector - a convolution
of the current through a single-channel point contact with the transparency
distribution for an asymmetric double-barrier potential.Comment: 10 pages, 3 figure
Scattering theory of superconductive tunneling in quantum junctions
We present a consistent theory of superconductive tunneling in single-mode
junctions within a scattering formulation of Bogoliubov-de Gennes quantum
mechanics. Both dc Josephson effect and dc quasiparticle transport in voltage
biased junctions are considered. Elastic quasiparticle scattering by the
junction determines equilibrium Josephson current. We discuss the origin of
Andreev bound states in tunnel junctions and their role in equilibrium
Josephson transport. In contrast, quasiparticle tunneling in voltage biased
junctions is determined by inelastic scattering. We derive a general expression
for inelastic scattering amplitudes and calculate the quasiparticle current at
all voltages with emphasis on a discussion of the properties of subgap tunnel
current and the nature of subharmonic gap structure.Comment: 47 pages, 9 figures, [preprint,eqsecnum,aps]{revtex
Non-Makovian decoherence of a two-level system weakly coupled to a bosonic bath
Bloch-Redfield equation is a common tool for studying evolution of qubit
systems weakly coupled to environment. We investigate the accuracy of the Born
approximation underlying this equation. We find that the high order terms in
the perturbative expansion contain accumulating divergences that make
straightforward Born approximation inappropriate. We develop diagrammatic
technique to formulate, and solve the improved self-consistent Born
approximation. This more accurate treatment reveals an exponential time
dependent prefactor in the non-Markovian contribution dominating the qubit
long-time relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time,
the associated dephasing is not affected and is described by the Born-Markov
approximation.Comment: To appear in EuroPhys. Let
Dissipative charge transport in diffusive superconducting double-barrier junctions
We solve the coherent multiple Andreev reflection (MAR) problem and calculate
current-voltage characteristics (IVCs) for Josephson SINIS junctions, where S
are local-equilibrium superconducting reservoirs, I denotes tunnel barriers,
and N is a short diffusive normal wire, the length of which is much smaller
than the coherence length, and the resistance is much smaller than the
resistance of the tunnel barriers. The charge transport regime in such
junctions qualitatively depends on a characteristic value \gamma = \Delta
\tau_d of relative phase shifts between the electrons and retro-reflected holes
accumulated during the dwell time \tau_d. In the limit of small electron-hole
dephasing \gamma << 1, our solution recovers a known formula for a short
mesoscopic connector extended to the MAR regime. At large dephasing, the
subharmonic gap structure in the IVC scales with 1/ \gamma, which thus plays
the role of an effective tunneling parameter. In this limit, the even gap
subharmonics are resonantly enhanced, and the IVC exhibits portions with
negative differential resistance.Comment: 8 pages, 3 figures, typos corrected, to be published in Phys. Rev.
On diffusive stability of Eigen's quasispecies model
Eigen's quasispecies system with explicit space and global regulation is
considered. Limit behavior and stability of the system in a functional space
under perturbations of a diffusion matrix with nonnegative spectrum are
investigated. It is proven that if the diffusion matrix has only positive
eigenvalues then the solutions of the distributed system converge to the
equilibrium solution of the corresponding local dynamical system. These results
imply that the error threshold does not change if the spatial interactions
under the principle of global regulation are taken into account.Comment: 16 pages, 1 figure, several typos are fixe
Практикум з мовної комунікації (англійська мова)
Навчально-методичний посібник з курсу "Практикум з мовної комунікації (англійська мова)" призначено для формування та удосконалення знань і навичок, необхідних для високого практичного володіння англійською мовою, точного та всебічного розуміння оригінальних англійських текстів, а також подальшого поповнення словникового запасу студентів. У навчально-методичному посібнику наведені вправи з лексики, граматики та говоріння, пропонуються тексти з проходження співбесід та відповідні завдання для розвитку навичок письмового та усного мовлення
Multiple Andreev reflections and enhanced shot noise in diffusive SNS junctions
We study the dc conductance and current fluctuations in diffusive voltage
biased SNS junctions with a tunnel barrier inside the mesoscopic normal region.
We find that at subgap voltages, eV<2Delta/n, the current associated with the
chain of n Andreev reflections is mapped onto the quasiparticle flow through a
structure of n+1 voltage biased barriers connected by diffusive conductors. As
a result, the current-voltage characteristic of a long SNINS structure obeys
Ohm's law, in spite of the complex multiparticle transport process. At the same
time, nonequilibrium heating of subgap electrons produces giant shot noise with
pronounced subharmonic gap structure which corresponds to stepwise growth of
the effective transferred charge. At eV\to 0, the shot noise approaches the
magnitude of the Johnson-Nyquist noise with the effective temperature
T^*=Delta/3, and the effective charge increases as (e/3)(1 + 2Delta/eV), with
the universal ``one third suppression'' factor. We analyse the role of
inelastic scattering and present a criterion of strong nonequilibrium.Comment: 4 pages, 2 figure
On the theory of Josephson effect in a diffusive tunnel junction
Specific features of the equilibrium current-carrying state of a Josephson
tunnel junction between diffusive superconductors are studied theoretically in
the 1D geometry. It is found that the Josephson current induces localized
states of electron excitations in the vicinity of the tunnel barrier, which are
a continuous analog of Andreev levels in a ballistic junction. The depth of the
corresponding ``potential well'' is much greater than the separation between an
Andreev level and the continuous energy spectrum boundary for the same
transmissivity of the barrier. In contrast to a ballistic junction in which the
Josephson current is transported completely by localized excitations, the
contribution to current in a diffusive junction comes from whole spectral
region near the energy gap boundary, where the density of states differs
considerably from its unperturbed value. The correction to the Josephson
current in the second order of the barrier transmissivity, which contains the
second harmonic of the phase jump, is calculated and it is found that the true
expansion parameter of the perturbation theory for a diffusive junction is not
the tunneling probability itself, but a much larger parameter .Comment: 8 pages, 5 Postscript figures, submitted to Low Temp. Phy
- …