8,763 research outputs found
Theory of Hysteresis Loop in Ferromagnets
We consider three mechanisms of hysteresis phenomena in alternating magnetic
field: the domain wall motion in a random medium, the nucleation and the
retardation of magnetization due to slow (critical) fluctuations. We construct
quantitative theory for all these processes. The hysteresis is characterized by
two dynamic threshold fields, by coercive field and by the so-called reversal
field. Their ratios to the static threshold field is shown to be function of
two dimensionless variables constituted from the frequency and amplitude of the
ac field as well as from some characteristics of the magnet. The area and the
shape of the hysteresis loop are found. We consider different limiting cases in
which power dependencies are valid. Numerical simulations show the domain wall
formation and propagation and confirm the main theoretical predictions. Theory
is compared with available experimental data.Comment: RevTex, 13 pages, 8 figures (PostScript), acknowledgements adde
Concurrence Vectors in Arbitrary Multipartite Quantum Systems
For a given pure state of multipartite system, the concurrence vector is
defined by employing the defining representation of generators of the
corresponding rotation groups. The norm of concurrence vector is considered as
a measure of entanglement. For multipartite pure state, the concurrence vector
is regarded as the direct sum of concurrence subvectors in the sense that each
subvector is associated with a pair of particles. It is proposed to use the
norm of each subvector as the contribution of the corresponding pair in
entanglement of the system.Comment: 9 pages, v3, section 3 is revise
Electron Amplification in Diamond
We report on recent progress toward development of secondary emission ''amplifiers'' for photocathodes. Secondary emission gain of over 300 has been achieved in transmission mode and emission mode for a variety of diamond samples. Techniques of sample preparation, including hydrogenation to achieve negative electron affinity (NEA), have been adapted to this application
Phase diagram and magnetic properties of LaCaMnO compound for
In this article a detailed study of LaCaMnO () phase diagram using powder x-ray diffraction and magnetization
measurements is presented. Unfortunately, in the related literature no properly
characterized samples have been used, with consequence the smearing of the real
physics in this complicated system. As the present results reveal, there are
two families of samples. The first family concerns samples prepared in
atmosphere ( Atm) which are all ferromagnetic with Curie
temperature rising with . The second family concerns samples, where a post
annealing in nearly zero oxygen partial pressure is applied. These samples show
a canted antiferromagnetic structure for below , while
for an unconventional ferromagnetic insulated phase is
present below . The most important difference between nonstoichiometric
and stoichiometric samples concerning the magnetic behavior, is the anisotropy
in the exchange interactions, in the stoichiometric samples putting forward the
idea that a new orbital ordered phase is responsible for the ferromagnetic
insulating regime in the LaCaMnO compound
Magnetism and field-induced effect in a spin-orbit entangled Jeff = 1/2 honeycomb lattice
The interplay between spin-orbit coupling, frustration-induced anisotropic
magnetic interaction, and spin correlations can lead to novel states with
exotic excitations in rare-earth-based quantum magnets. Herein, we present the
crystal structure, magnetization, electron spin resonance (ESR), specific heat,
and nuclear magnetic resonance (NMR) experiments on the polycrystalline samples
of Ba9Yb2Si6O24 in which Yb3+ ions form a perfect honeycomb lattice without
detectable anti-site disorder. Magnetization data reveal antiferromagnetically
coupled spin-orbit entangled Jeff = 1/2 degrees of freedom of Yb3+ ions in the
Kramers doublet state where the Curie-Weiss temperature is - 2.97 K, as
obtained from the low-temperature magnetic susceptibility data. The ESR
measurements reveal that the first excited Kramers doublet is 32.3(7) meV above
the ground state. The specific heat results suggest the presence of an
antiferromagnetic phase transition at 2.26 K. The long-range antiferromagnetic
order is completely suppressed upon the application of magnetic field and a
field-induced disordered state is observed in an applied magnetic field of 2.5
T, which is also confirmed by NMR measurements. Furthermore, the NMR
spin-lattice relaxation rate reveals the presence of a field-induced gap that
is attributed to the Zeeman splitting of Kramers doublet state in this quantum
material. Our experiments suggest the presence of a phase transition and
short-range spin correlations appearing well above the antiferromagnetic phase
transition temperature and a field-induced disordered state in this spin-orbit
entangled Jeff =1/2 rare-earth magnet on a honeycomb lattice
Stress-free Spatial Anisotropy in Phase-Ordering
We find spatial anisotropy in the asymptotic correlations of two-dimensional
Ising models under non-equilibrium phase-ordering. Anisotropy is seen for
critical and off-critical quenches and both conserved and non-conserved
dynamics. We argue that spatial anisotropy is generic for scalar systems
(including Potts models) with an anisotropic surface tension. Correlation
functions will not be universal in these systems since anisotropy will depend
on, e.g., temperature, microscopic interactions and dynamics, disorder, and
frustration.Comment: 4 pages, 4 figures include
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