727 research outputs found
Plasmonic nanostructures with local temporal response: a platform for time-varying photonics
This work is devoted to the development of an approach for implementation and designing time-varying media. A mechanism based on the use of plasmonic nanostructures with a reduced plasmon lifetime is proposed. It is shown that such nanostructures can be used to enhance the strength and speed of modulation of the refractive index ofnonlinear media. This is achieved through decreasing of the spectral dispersion of the real permittivity. Plasmonic materials with peculiar optical properties, such as flatdispersion in the near-infrared range, were synthesized. For this purpose, we prepared TiON thin films and performed thermal post-treatment for fine-tuning permittivity of TiON. It has been shown that the proposed materials allow one to achieve an ultrashort plasmon lifetime on the order of 0.1 fs, which is an order of magnitude shorter than in the case of traditional plasmonic materials
Experimental determination and analysis of the geometrical distortions in the pick-up tube of a television system for measuring track co-ordinates in a spark chamber
Subexponential estimations in Shirshov's height theorem (in English)
In 1993 E. I. Zelmanov asked the following question in Dniester Notebook:
"Suppose that F_{2, m} is a 2-generated associative ring with the identity
x^m=0. Is it true, that the nilpotency degree of F_{2, m} has exponential
growth?" We show that the nilpotency degree of l-generated associative algebra
with the identity x^d=0 is smaller than Psi(d,d,l), where Psi(n,d,l)=2^{18} l
(nd)^{3 log_3 (nd)+13}d^2. We give the definitive answer to E. I. Zelmanov by
this result. It is the consequence of one fact, which is based on combinatorics
of words. Let l, n and d>n be positive integers. Then all the words over
alphabet of cardinality l which length is greater than Psi(n,d,l) are either
n-divided or contain d-th power of subword, where a word W is n-divided, if it
can be represented in the following form W=W_0 W_1...W_n such that W_1 >'
W_2>'...>'W_n. The symbol >' means lexicographical order here. A. I. Shirshov
proved that the set of non n-divided words over alphabet of cardinality l has
bounded height h over the set Y consisting of all the words of degree <n.
Original Shirshov's estimation was just recursive, in 1982 double exponent was
obtained by A.G.Kolotov and in 1993 A.Ya.Belov obtained exponential estimation.
We show, that h<Phi(n,l), where Phi(n,l) = 2^{87} n^{12 log_3 n + 48} l. Our
proof uses Latyshev idea of Dilworth theorem application.Comment: 21 pages, Russian version of the article is located at the link
arXiv:1101.4909; Sbornik: Mathematics, 203:4 (2012), 534 -- 55
Coulomb interaction and magnetic catalysis in the quantum Hall effect in graphene
The dynamics of symmetry breaking responsible for lifting the degeneracy of
the Landau levels in the integer quantum Hall effect in graphene is studied in
a low-energy model with the Coulomb interaction. The gap equation for Dirac
quasiparticles is analyzed for both the lowest and higher Landau levels, taking
into account the Landau levels mixing. It is shown that the characteristic
feature of the long-range Coulomb interaction is the decrease of the gap
parameters with increasing the Landau level index ("running" gaps). The
renormalization (running) of the Fermi velocity as a function of is also
studied. The solutions of the gap equation reproduce correctly the
experimentally observed integer quantum Hall plateaus in graphene in strong
magnetic fields.Comment: 22 pages, 5 figures; Final version published in the Proceedings of
the 2010 Nobel Symposium on Graphene and Quantum Matte
Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field
We show that the superconducting transition temperature T_c(H) of a very thin
highly disordered film with strong spin-orbital scattering can be increased by
parallel magnetic field H. This effect is due to polarization of magnetic
impurity spins which reduces the full exchange scattering rate of electrons;
the largest effect is predicted for spin-1/2 impurities. Moreover, for some
range of magnetic impurity concentrations the phenomenon of {\it
superconductivity induced by magnetic field} is predicted: superconducting
transition temperature T_c(H) is found to be nonzero in the range of magnetic
fields .Comment: 4 pages, 2 figure
Neutron lifetime measurements using gravitationally trapped ultracold neutrons
Our experiment using gravitationally trapped ultracold neutrons (UCN) to
measure the neutron lifetime is reviewed. Ultracold neutrons were trapped in a
material bottle covered with perfluoropolyether. The neutron lifetime was
deduced from comparison of UCN losses in the traps with different
surface-to-volume ratios. The precise value of the neutron lifetime is of
fundamental importance to particle physics and cosmology. In this experiment,
the UCN storage time is brought closer to the neutron lifetime than in any
experiments before:the probability of UCN losses from the trap was only 1% of
that for neutron beta decay. The neutron lifetime
obtained,878.5+/-0.7stat+/-0.3sys s, is the most accurate experimental
measurement to date.Comment: 38 pages, 19 figures,changed conten
Oscillations of Induced Magnetization in Superconductor-Ferromagnet Heterostructures
We study a change in the spin magnetization of a superconductor-ferromagnet
(SF) heterostructure, when temperature is lowered below the superconducting
transition temperature. It is assumed that the SF interface is smooth on the
atomic scale and the mean free path is not too short. Solving the Eilenberger
equation we show that the spin magnetic moment induced in the superconductor is
an oscillating sign-changing function of the product of the exchange field
and the thickness of the ferromagnet. Therefore the total spin magnetic
moment of the system in the superconducting state can be not only smaller
(screening) but also greater (anti-screening) than that in the normal state, in
contrast with the case of highly disordered (diffusive) systems, where only
screening is possible. This surprising effect is due to peculiar periodic
properties of localized Andreev states in the system. It is most pronounced in
systems with ideal ballistic transport (no bulk disorder in the samples, smooth
ideally transparent interface), however these ideal conditions are not crucial
for the very existence of the effect. We show that oscillations exist (although
suppressed) even for arbitrary low interface transparency and in the presence
of bulk disorder, provided that ( -- mean free path). At
low interface transparency we solve the problem for arbitrary strength of
disorder and obtain oscillating magnetization in ballistic regime () and nonoscillating magnetization in diffusive one () as
limiting cases of one formula.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.
Hall Transport in Granular Metals and Effects of Coulomb Interactions
We present a theory of Hall effect in granular systems at large tunneling
conductance . Hall transport is essentially determined by the
intragrain electron dynamics, which, as we find using the Kubo formula and
diagrammatic technique, can be described by nonzero diffusion modes inside the
grains. We show that in the absence of Coulomb interaction the Hall resistivity
depends neither on the tunneling conductance nor on the intragrain
disorder and is given by the classical formula , where
differs from the carrier density inside the grains by a numerical
coefficient determined by the shape of the grains and type of granular lattice.
Further, we study the effects of Coulomb interactions by calculating
first-order in corrections and find that (i) in a wide range of
temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the
Hall resistivity and conductivity \sig_{xy} acquire logarithmic
in corrections, which are of local origin and absent in homogeneously
disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to
\sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of
homogeneously disordered metals.Comment: 29 pages, 16 figure
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