1,489 research outputs found
Induced ferroelectric phases in TbMn_2O_5
The magnetostructural transitions and magnetoelectric effects reported in
TbMn2O5 are described theoretically and shown to correspond to two essentially
different mechanisms for the induced ferroelectricity. The incommensurate and
commensurate phases observed between 38 and 24 K exhibit a hybrid pseudoproper
ferroelectric nature resulting from an effective bilinear coupling of the
polarization with the antiferromagnetic order parameter. This explains the high
sensitivity of the dielectric properties of the material under applied magnetic
field. Below 24 K the incommensurate phase shows a standard improper
ferroelectric character induced by the coupling of two distinct magnetic order
parameters. The complex dielectric behavior observed in the material reflects
the crossover from one to the other transition regime. The temperature
dependences of the pertinent physical quantities are worked out, and previous
theoretical models are discussed
First-order multi-k phase transitions and magnetoelectric effects in multiferroic Co3TeO6
A theoretical description of the sequence of magnetic phases in Co3TeO6 is
presented. The strongly first-order character of the transition to the
commensurate multiferroic ground state, induced by coupled order parameters
corresponding to different wavevectors, is related to a large magnetoelastic
effect with an exchange energy critically sensitive to the interatomic spacing.
The monoclinic magnetic symmetry C2' of the multiferroic phase permits
spontaneous polarization and magnetization as well as the linear
magnetoelectric effect. The existence of weakly ferromagnetic domains is
verified experimentally by second harmonic generation measurements
Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine Scale Structure
We have used VLBA fringe visibility data obtained at 15 GHz to examine the
compact structure in 250 extragalactic radio sources. For 171 sources in our
sample, more than half of the total flux density seen by the VLBA remains
unresolved on the longest baselines. There are 163 sources in our list with a
median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest
baselines. For about 60% of the sources, we have at least one observation in
which the core component appears unresolved (generally smaller than 0.05 mas)
in one direction, usually transverse to the direction into which the jet
extends. BL Lacs are on average more compact than quasars, while active
galaxies are on average less compact. Also, in an active galaxy the
sub-milliarcsecond core component tends to be less dominant. IDV sources
typically have a more compact, more core-dominated structure on
sub-milliarcsecond scales than non-IDV sources, and sources with a greater
amplitude of intra-day variations tend to have a greater unresolved VLBA flux
density. The objects known to be GeV gamma-ray loud appear to have a more
compact VLBA structure than the other sources in our sample. This suggests that
the mechanisms for the production of gamma-ray emission and for the generation
of compact radio synchrotron emitting features are related. The brightness
temperature estimates and lower limits for the cores in our sample typically
range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in
excess of the equipartition brightness temperature, or the inverse Compton
limit for stationary synchrotron sources. The largest component speeds are
observed in radio sources with high observed brightness temperatures, as would
be expected from relativistic beaming (abridged).Comment: 31 pages, 13 figures, 4 tables, accepted for publication in the
Astronomical Journal; minor changes to the text are mad
Quasisymmetric graphs and Zygmund functions
A quasisymmetric graph is a curve whose projection onto a line is a
quasisymmetric map. We show that this class of curves is related to solutions
of the reduced Beltrami equation and to a generalization of the Zygmund class
. This relation makes it possible to use the tools of harmonic
analysis to construct nontrivial examples of quasisymmetric graphs and of
quasiconformal maps.Comment: 21 pages, no figure
РЕЗОНАНС ВОЛН НА ПАРОМНОЙ ПЕРЕПРАВЕ
The authors describe the researches on resonance wave oscillations in the port of Kholmsk and negative influence of the sea elements on unloading of the trains arriving to Sakhalin from the continent port of Vanino by train ferry. The authors used autonomous and cable recorders of sea-floor hydrostatic pressure that reflected the variations of sea surface. It was proved that during tsunami the zero mode of resonance oscillations with 8 min period increased largely in the port bay. During the storms not only the zero mode but also the peer longitudinal seiche of the bay increased. This seiche has nodal line in the center and antinode in the apex of the bay.Экспериментальные исследования резонансных колебаний волн в порту Холмск. Негативное влияние морской стихии на разгрузку железнодорожных составов с судов паромной переправы, связывающей о.Сахалин с материком. Использовались автономные и кабельные регистраторы придонного гидростатического давления, отражающие вариации морской поверхности. Показано, что при цунами в портовой бухте резко усиливается нулевая мода резонансных колебаний с периодом 8 мин. При штормовых ситуациях помимо нулевой моды резко усиливается одноузловая продольная сейша бухты, имеющая узловую линию в центральной и пучность в кутовой частях
Magnetoelectric effect due to local noncentrosymmetry
Magnetoelectrics often possess ions located in noncentrosymmetric
surroundings. Based on this fact we suggest a microscopic model of
magnetoelectric interaction and show that the spin-orbit coupling leads to
spin-dependent electric dipole moments of the electron orbitals of these ions,
which results in non-vanishing polarization for certain spin configurations.
The approach accounts for the macroscopic symmetry of the unit cell and is
valid both for commensurate and complex incommensurate magnetic structures. The
model is illustrated by the examples of MnWO4, MnPS3 and LiNiPO4. Application
to other magnetoelectrics is discussed.Comment: 11 pages, 2 figures, 2 table
Discrete Symmetry and Stability in Hamiltonian Dynamics
In this tutorial we address the existence and stability of periodic and
quasiperiodic orbits in N degree of freedom Hamiltonian systems and their
connection with discrete symmetries. Of primary importance in our study are the
nonlinear normal modes (NNMs), i.e periodic solutions which represent
continuations of the system's linear normal modes in the nonlinear regime. We
examine the existence of such solutions and discuss different methods for
constructing them and studying their stability under fixed and periodic
boundary conditions. In the periodic case, we employ group theoretical concepts
to identify a special type of NNMs called one-dimensional "bushes". We describe
how to use linear combinations such NNMs to construct s(>1)-dimensional bushes
of quasiperiodic orbits, for a wide variety of Hamiltonian systems and exploit
the symmetries of the linearized equations to simplify the study of their
destabilization. Applying this theory to the Fermi Pasta Ulam (FPU) chain, we
review a number of interesting results, which have appeared in the recent
literature. We then turn to an analytical and numerical construction of
quasiperiodic orbits, which does not depend on the symmetries or boundary
conditions. We demonstrate that the well-known "paradox" of FPU recurrences may
be explained in terms of the exponential localization of the energies Eq of
NNM's excited at the low part of the frequency spectrum, i.e. q=1,2,3,....
Thus, we show that the stability of these low-dimensional manifolds called
q-tori is related to the persistence or FPU recurrences at low energies.
Finally, we discuss a novel approach to the stability of orbits of conservative
systems, the GALIk, k=2,...,2N, by means of which one can determine accurately
and efficiently the destabilization of q-tori, leading to the breakdown of
recurrences and the equipartition of energy, at high values of the total energy
E.Comment: 50 pages, 13 figure
Star clusters dynamics in a laboratory: electrons in an ultracold plasma
Electrons in a spherical ultracold quasineutral plasma at temperature in the
Kelvin range can be created by laser excitation of an ultra-cold laser cooled
atomic cloud. The dynamical behavior of the electrons is similar to the one
described by conventional models of stars clusters dynamics. The single mass
component, the spherical symmetry and no stars evolution are here accurate
assumptions. The analog of binary stars formations in the cluster case is
three-body recombination in Rydberg atoms in the plasma case with the same
Heggie's law: soft binaries get softer and hard binaries get harder. We
demonstrate that the evolution of such an ultracold plasma is dominated by
Fokker-Planck kinetics equations formally identical to the ones controlling the
evolution of a stars cluster. The Virial theorem leads to a link between the
plasma temperature and the ions and electrons numbers. The Fokker-Planck
equation is approximate using gaseous and fluid models. We found that the
electrons are in a Kramers-Michie-King's type quasi-equilibrium distribution as
stars in clusters. Knowing the electron distribution and using forced fast
electron extraction we are able to determine the plasma temperature knowing the
trapping potential depth.Comment: Submitted to MNRA
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