188 research outputs found
Critical currents in superconductors with quasiperiodic pinning arrays: One-dimensional chains and two-dimensional Penrose lattices
We study the critical depinning current J_c, as a function of the applied
magnetic flux Phi, for quasiperiodic (QP) pinning arrays, including
one-dimensional (1D) chains and two-dimensional (2D) arrays of pinning centers
placed on the nodes of a five-fold Penrose lattice. In 1D QP chains of pinning
sites, the peaks in J_c(Phi) are shown to be determined by a sequence of
harmonics of long and short periods of the chain. This sequence includes as a
subset the sequence of successive Fibonacci numbers. We also analyze the
evolution of J_c(Phi) while a continuous transition occurs from a periodic
lattice of pinning centers to a QP one; the continuous transition is achieved
by varying the ratio gamma = a_S/a_L of lengths of the short a_S and the long
a_L segments, starting from gamma = 1 for a periodic sequence. We find that the
peaks related to the Fibonacci sequence are most pronounced when gamma is equal
to the "golden mean". The critical current J_c(Phi) in QP lattice has a
remarkable self-similarity. This effect is demonstrated both in real space and
in reciprocal k-space. In 2D QP pinning arrays (e.g., Penrose lattices), the
pinning of vortices is related to matching conditions between the vortex
lattice and the QP lattice of pinning centers. Although more subtle to analyze
than in 1D pinning chains, the structure in J_c(Phi) is determined by the
presence of two different kinds of elements forming the 2D QP lattice. Indeed,
we predict analytically and numerically the main features of J_c(Phi) for
Penrose lattices. Comparing the J_c's for QP (Penrose), periodic (triangular)
and random arrays of pinning sites, we have found that the QP lattice provides
an unusually broad critical current J_c(Phi), that could be useful for
practical applications demanding high J_c's over a wide range of fields.Comment: 18 pages, 15 figures (figures 7, 9, 10, 13, 15 in separate "png"
files
Electronic properties of the armchair graphene nanoribbon
We investigate the electronic band structure of an undoped graphene armchair
nanoribbon. We demonstrate that such nanoribbon always has a gap in its
electronic spectrum. Indeed, even in the situations where simple
single-electron calculations predict a metallic dispersion, the system is
unstable with respect to the deformation of the carbon-carbon bonds dangling at
the edges of the armchair nanoribbon. The edge bonds' deformation couples
electron and hole states with equal momentum. This coupling opens a gap at the
Fermi level. In a realistic sample, however, it is unlikely that this
instability could be observed in its pure form. Namely, since chemical
properties of the dangling carbon atoms are different from chemical properties
of the atoms inside the sample (for example, the atoms at the edge have only
two neighbours, besides additional non-carbon atoms might be attached to
passivate unpaired covalent carbon bonds), it is very probable that the bonds
at the edge are deformed due to chemical interactions. This chemically-induced
modification of the nanoribbon's edges can be viewed as an effective field
biasing our predicted instability in a particular direction. Yet by disordering
this field (e.g., through random substitution of the radicals attached to the
edges) we may tune the system back to the critical regime and vary the
electronic properties of the system. For example, we show that electrical
transport through a nanoribbon is strongly affected by such disorder.Comment: 12 pages, 4 figur
Photoelectric, nonlinear optical, and photorefractive properties of polyvinylcarbazole composites with graphene
Polyvinylcarbazole (PVK) composites containing graphene in an amount of somewhat less than 0.15 wt % exhibit third-order dielectric susceptibility due to the presence of graphene, as well as photoelectric and photorefractive sensitivity at 1064 nm. The photorefractive (PR) effect is known to occur in a polymer composite that possesses both photoelectric sensitivity and nonlinear optical properties. The photoelectric, nonlinear optical, and PR properties of PVK composites with graphene have been considered in this paper
Quantum electrodynamics and photon-assisted tunnelling in long Josephson junctions
We describe the interaction between an electromagnetic field and a long
Josephson junction (JJ) driven by a dc current. We calculate the amplitudes of
emission and absorption of light via the creation and annihilation of quantized
Josephson plasma waves (JPWs). Both, the energies of JPW quanta and the
amplitudes of light absorption and emission, strongly depend on the junction's
length and can be tuned by an applied dc current. Moreover, photon-assisted
macroscopic quantum tunnelling in long Josephson junctions show resonances when
the frequency of the outside radiation coincides with the current-driven
eigenfrequencies of the quantized JPWs.Comment: 9 pages, 4 figure
The nonlinear effects in 2DEG conductivity investigation by an acoustic method
The parameters of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs
heterostructure were determined by an acoustical (contactless) method in the
delocalized electrons region (2.5T). Nonlinear effects in Surface
Acoustic Wave (SAW) absorption by 2DEG are determined by the electron heating
in the electric field of SAW, which may be described in terms of electron
temperature . The energy relaxation time is determined
by the scattering at piezoelectric potential of acoustic phonons with strong
screening. At different SAW frequencies the heating depends on the relationship
between and 1 and is determined either by the
instantaneously changing wave field (), or by the
average wave power ().Comment: RevTeX, 5 pages, 3 PS-figures, submitted to Physica Status
Sol.(Technical corrections in PS-figs
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
Band structure and broadband compensation of absorption by amplification in layered optical metamaterials
The frequency dependence of the gain required to compensate for absorption is determined for a layered structure consisting of alternating absorbing and amplifying layers. It is shown that the fulfillment of the same conditions is required for the existence of a band structure consisting of alternating bands allowed and forbidden for optical radiation propagation in the frequency-wave vector parametric region. Conditions are found under which the gain required for compensation is smaller than thresholds for absolute (parasitic lasing) and convective (waveguide amplification of radiation) instabilities
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