125 research outputs found
Surface Josephson plasma waves in layered superconductors
We predict the existence of surface waves in layered superconductors in the
THz frequency range, below the Josephson plasma frequency . This wave
propagates along the vacuum-superconductor interface and dampens in both
transverse directions out of the surface (i.e., towards the superconductor and
towards the vacuum). This is the first prediction of propagating surface waves
in any superconductor. These predicted surface Josephson plasma waves are
important for different phenomena, including the complete suppression of the
specular reflection from a sample (Wood's anomalies) and a huge enhancement of
the wave absorption (which can be used as a THz detector).Comment: 4 pages, 2 figure
Electron Beam Instability in Left-Handed Media
We predict that two electron beams can develop an instability when passing
through a slab of left-handed media (LHM). This instability, which is inherent
only for LHM, originates from the backward Cherenkov radiation and results in a
self-modulation of the beams and radiation of electromagnetic waves. These
waves leave the sample via the rear surface of the slab (the beam injection
plane) and form two shifted bright circles centered at the beams. A simulated
spectrum of radiation has well-separated lines on top of a broad continuous
spectrum, which indicates dynamical chaos in the system. The radiation
intensity and its spectrum can be controlled either by the beams' current or by
the distance between the two beams.Comment: 4 pages, 4 figure
Resonance effects due to the excitation of surface Josephson plasma waves in layered superconductors
We analytically examine the excitation of surface Josephson plasma waves
(SJPWs) in periodically-modulated layered superconductors. We show that the
absorption of the incident electromagnetic wave can be substantially increased,
for certain incident angles, due to the resonance excitation of SJPWs. The
absorption increase is accompanied by the decrease of the specular reflection.
Moreover, we find the physical conditions guaranteeing the total absorption
(and total suppression of the specular reflection). These conditions can be
realized for Bi2212 superconductor films.Comment: 17 pages, 3 figure
Diffusion-controlled generation of a proton-motive force across a biomembrane
Respiration in bacteria involves a sequence of energetically-coupled electron
and proton transfers creating an electrochemical gradient of protons (a
proton-motive force) across the inner bacterial membrane. With a simple kinetic
model we analyze a redox loop mechanism of proton-motive force generation
mediated by a molecular shuttle diffusing inside the membrane. This model,
which includes six electron-binding and two proton-binding sites, reflects the
main features of nitrate respiration in E. coli bacteria. We describe the time
evolution of the proton translocation process. We find that the electron-proton
electrostatic coupling on the shuttle plays a significant role in the process
of energy conversion between electron and proton components. We determine the
conditions where the redox loop mechanism is able to translocate protons
against the transmembrane voltage gradient above 200 mV with a thermodynamic
efficiency of about 37%, in the physiologically important range of temperatures
from 250 to 350 K.Comment: 26 pages, 4 figures. A similar model is used in arXiv:0806.3233 for a
different biological system. Minor changes in the Acknowledgements sectio
Directed motion of domain walls in biaxial ferromagnets under the influence of periodic external magnetic fields
Directed motion of domain walls (DWs) in a classical biaxial ferromagnet
placed under the influence of periodic unbiased external magnetic fields is
investigated. Using the symmetry approach developed in this article the
necessary conditions for the directed DW motion are found. This motion turns
out to be possible if the magnetic field is applied along the most easy axis.
The symmetry approach prohibits the directed DW motion if the magnetic field is
applied along any of the hard axes. With the help of the soliton perturbation
theory and numerical simulations, the average DW velocity as a function of
different system parameters such as damping constant, amplitude, and frequency
of the external field, is computed.Comment: Added references, corrected typos, extended introductio
Transport and localization in periodic and disordered graphene superlattices
We study charge transport in one-dimensional graphene superlattices created
by applying layered periodic and disordered potentials. It is shown that the
transport and spectral properties of such structures are strongly anisotropic.
In the direction perpendicular to the layers, the eigenstates in a disordered
sample are delocalized for all energies and provide a minimal non-zero
conductivity, which cannot be destroyed by disorder, no matter how strong this
is. However, along with extended states, there exist discrete sets of angles
and energies with exponentially localized eigenfunctions (disorder-induced
resonances). It is shown that, depending on the type of the unperturbed system,
the disorder could either suppress or enhance the transmission. Most remarkable
properties of the transmission have been found in graphene systems built of
alternating p-n and n-p junctions. This transmission has anomalously narrow
angular spectrum and, surprisingly, in some range of directions it is
practically independent of the amplitude of fluctuations of the potential.
Owing to these features, such samples could be used as building blocks in
tunable electronic circuits. To better understand the physical implications of
the results presented here, most of our results have been contrasted with those
for analogous wave systems. Along with similarities, a number of quite
surprising differences have been found.Comment: 10 page
Large temperature dependence of the Casimir force at the metal-insulator transition
The dependence of the Casimir force on material properties is important for
both future applications and to gain further insight on its fundamental
aspects. Here we derive a general theory of the Casimir force for
low-conducting compounds, or poor metals. For distances in the micrometer
range, a large variety of such materials is described by universal equations
containing a few parameters: the effective plasma frequency, dissipation rate
of the free carriers, and electric permittivity in the infrared range. This
theory can also describe inhomogeneous composite materials containing small
regions with different conductivity. The Casimir force for mechanical systems
involving samples made with compounds that have a metal-insulator transition
shows an abrupt large temperature dependence of the Casimir force within the
transition region, where metallic and dielectric phases coexist.Comment: 23 pages, 9 figure
Nonuniform Self-Organized Dynamical States in Superconductors with Periodic Pinning
We consider magnetic flux moving in superconductors with periodic pinning
arrays. We show that sample heating by moving vortices produces negative
differential resistivity (NDR) of both N and S type (i.e., N- and S-shaped) in
the voltage-current characteristic (VI curve). The uniform flux flow state is
unstable in the NDR region of the VI curve. Domain structures appear during the
NDR part of the VI curve of an N type, while a filamentary instability is
observed for the NDR of an S type. The simultaneous existence of the NDR of
both types gives rise to the appearance of striking self-organized (both
stationary and non-stationary) two-dimensional dynamical structures.Comment: 4 pages, 2 figure
Molecular dynamics simulations of oxide memory resistors (memristors)
Reversible bipolar nano-switches that can be set and read electronically in a
solid-state two-terminal device are very promising for applications. We have
performed molecular-dynamics simulations that mimic systems with oxygen
vacancies interacting via realistic potentials and driven by an external bias
voltage. The competing short- and long-range interactions among charged mobile
vacancies lead to density fluctuations and short-range ordering, while
illustrating some aspects of observed experimental behavior, such as memristor
polarity inversion.Comment: 15 pages, 5 figure
Detection of Phage-Induced Mobile Genetic Elements in Strains of <i>Vibrio cholerae</i> O1 Biovar El Tor
The acquisition of new mobile genetic elements contributes to the genetic diversity of Vibrio cholerae strains. An important role in this process belongs to the genetic material obtained from phages. The aim of this work was to identify phage-induced PLE islands in strains of V. cholerae O1 serogroup and to determine the resistance of isolates with and without those mobile genetic elements to the lytic activity of the diagnostic cholera El Tor bacteriophage. Materials and methods. Whole genomes nucleotide sequences of toxigenic and non-toxigenic V. cholerae O1 strains presented in the NCBI GenBank were used for the work. Bioinformatic analysis was performed using the BLAST algorithm, MEGA X (or BioEdit v. 7.0.9.0). The test with phages was carried out according A. Gratia technique. Results and discussion. The analysis of 39 toxigenic strains imported to the territory of the Russian Federation and neighboring countries has revealed one strain of V. cholerae O1 of the classical biovar containing the PLE5 island, and 13 strains of V. cholerae O1 of the El Tor biovar containing the PLE4 island. PLE islands have not been found in non-toxigenic strains. It is shown that strains with PLE4 belong to V. cholerae O1 genovariants of the El Tor biovar and have the ctxB1 gene allele. Isolates with this mobile element caused sporadic cases of the disease in 1994–1999, as well as cholera outbreaks in the Russian Federation (in 1993–1994, in 1998 – Dagestan, and 1993 – Tatarstan) and Ukraine (1994–1995). It has been suggested that, perhaps, the presence of the PLE4 island makes a certain contribution to the resistance of V. cholerae O1 strains of the El Tor biovar to the diagnostic cholera El Tor phage (55.6 % of phage-resistant isolates were detected), but there are other mechanisms that have not yet been identified. Thus, the data on the presence of new mobile genetic elements in the genome of earlier imported toxigenic strains of V. cholerae O1, biovar El Tor have been obtained, which expands information about their genetic organization
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