1,288 research outputs found
Antiferromagnetic state in bilayer graphene
Motivated by the recent experiment of Velasco Jr. {\em et al.} [J. Velasco
Jr. {\em et al.}, Nat. Nanotechnology 7, {\bf 156} (2012)], we develop a
mean-field theory of the interaction-induced antiferromagnetic (AF) state in
bilayer graphene at charge neutrality point at arbitrary perpendicular magnetic
field B. We demonstrate that the AF state can persist at all . At higher
, the state continuously crosses over to the AF phase of the quantum
Hall ferromagnet, recently argued to be realized in the insulating
state. The mean-field quasiparticle gap is finite at B=0 and grows with
increasing B, becoming quasi-linear in the quantum Hall regime, in accord with
the reported behavior of the transport gap. By adjusting the two free
parameters of the model, we obtain a simultaneous quantitative agreement
between the experimental and theoretical values of the key parameters of the
gap dependence -- its zero-field value and slope at higher fields. Our findings
suggest that the insulating state observed in bilayer graphene in Ref. 1 is
antiferromagnetic (canted, once the Zeeman effect is taken into account) at all
magnetic fields.Comment: 5 pages, 3 figs; v3: published versio
Effect of perturbed flow on the transition from the supersonic laminar boundary layer to the turbulent
Results of experimental studies on the effect of various factors on the transition of a supersonic boundary layer are discussed. It is shown that in supersonic wind tunnels, a significant effect on the transition of the boundary layer on a model is exerted by the scale of acoustic perturbations, which is proportional to the boundary layer displacement thickness of the working section. Experimental data obtained over a wide range of variation of flow parameters in aerodynamically similar test installations with different dimensions of the working section are generalized by means of a correlation parameter based on the displacement thickness
An Analytical Analysis of a Wind Power Generation System Including Synchronous Generator with Permanent Magnets, Active Rectifier and Voltage Source Inverter
Non
Anomalous Hall effect in granular ferromagnetic metals and effects of weak localization
We theoretically investigate the anomalous Hall effect in a system of
dense-packed ferromagnetic grains in the metallic regime. Using the formalism
recently developed for the conventional Hall effect in granular metals, we
calculate the residual anomalous Hall conductivity and
resistivity and weak localization corrections to them for both
skew-scattering and side-jump mechanisms. We find that, unlike for
homogeneously disordered metals, the scaling relation between and
the longitudinal resistivity does not hold. The weak localization
corrections, however, are found to be in agreement with those for homogeneous
metals. We discuss recent experimental data on the anomalous Hall effect in
polycrystalline iron films in view of the obtained results.Comment: published version, 10 pages, 6 figure
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
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
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
Interaction-enhanced magnetically ordered insulating state at the edge of a two-dimensional topological insulator
We develop a theory of the correlated magnetically ordered insulating state
at the edge of a two-dimensional topological insulator. We demonstrate that the
gapped spin-polarized state, induced by the application of the magnetic field
, is naturally facilitated by electron interactions, which drive the
critical easy-plane ferromagnetic correlations in the helical liquid. As the
key manifestation, the gap \De in the spectrum of collective excitations,
which carry both spin and charge, is enhanced and exhibits a scaling dependence
\De \propto B^{1/(2-K)}, controlled by the Luttinger liquid parameter .
This scaling dependence could be probed through the activation behavior G \sim
(e^2/h) \exp(- \De/T) of the longitudinal conductance of a Hall-bar device at
lower temperatures, providing a straightforward way to extract the parameter
experimentally. Our findings thus suggest that the signatures of the
interaction-driven quantum criticality of the helical liquid could be revealed
already in a standard Hall-bar measurement.Comment: 5 pages, 3 figs; v2: published versio
Surface impedance of superconductors with magnetic impurities
Motivated by the problem of the residual surface resistance of the
superconducting radio-frequency (SRF) cavities, we develop a microscopic theory
of the surface impedance of s-wave superconductors with magnetic impurities. We
analytically calculate the current response function and surface impedance for
a sample with spatially uniform distribution of impurities, treating magnetic
impurities in the framework of the Shiba theory. The obtained general
expressions hold in a wide range of parameter values, such as temperature,
frequency, mean free path, and exchange coupling strength. This generality, on
the one hand, allows for direct numerical implementation of our results to
describe experimental systems (SRF cavities, superconducting qubits) under
various practically relevant conditions. On the other hand, explicit analytical
expressions can be obtained in a number of limiting cases, which makes possible
further theoretical investigation of certain regimes. As a feature of key
relevance to SRF cavities, we show that in the regime of "gapless
superconductivity" the surface resistance exhibits saturation at zero
temperature. Our theory thus explicitly demonstrates that magnetic impurities,
presumably contained in the oxide surface layer of the SRF cavities, provide a
microscopic mechanism for the residual resistance.Comment: 9 pages, 3 figs; v2: published versio
Anomaly crust fields from MAGSAT satellite measurements:their processing and interpretation
The space distribution of the magnetic anomaly field for the Pacific Ocean is obtained from data of the satellite
MAGSAT. A number of long-wavelength magnetic anomalies of the region are identified. A spectrum analysis of a number of profiles of the anomaly field is performed disclosing typical scales of such anomalies. The wave transform of the anomaly magnetic profiles reveals and explicitly exposes the structure of the considered profile. A schematic complex cross-section is constructed, which demonstrates that the satellite data may be used in the study of the magnetic anomaly
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