1,157 research outputs found
Landau Theory of Domain Wall Magnetoelectricity
We calculate the exact analytical solution to the domain wall properties in a
multiferroic system with two order parameters that are coupled
bi-quadratically. This is then adapted to the case of a magnetoelectric
multiferroic material such as BiFeO3, with a view to examine critically whether
the domain walls can account for the enhancement of magnetization reported for
thin films fo this material, in view of the correlation between increasing
magnetization and increasing volume fraction of domain walls as films become
thinner. The present analysis can be generalized to describe a class of
magnetoelectric devices based upon domain walls rather than bulk properties.Comment: 9 pages, 4 figure
The Current Status of the Distribution Range of the Western Pine Beetle, Dendroctonus brevicomis (Curculionidae: Scolytinae) in Northern Mexico
Abstract The distribution range of the western pine beetle Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) is supported only by scattered records in the northern parts of Mexico, suggesting that its populations may be marginal and rare in this region. In this study, we review the geographical distribution of D. brevicomis in northern Mexico and perform a geometric morphometric analysis of seminal rod shape to evaluate its reliability for identifying this species with respect to other members of the Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae) complex. Our results provide 30 new records, with 26 distributed in the Sierra Madre Occidental and 4 in the Sierra Madre Oriental. These records extend the known distribution range of D. brevicomis to Durango and Tamaulipas states in northern Mexico. Furthermore, we find high geographic variation in size and shape of the seminal rod, with conspicous differences among individuals from different geographical regions, namely west and east of the Great Basin and between mountain systems in Mexico.
Key words: seminal rod shape, geometric morphometry, Dendroctonus frontalis comple
Irregular Wakimoto modules and the Casimir connection
We study some non-highest weight modules over an affine Kac-Moody algebra at
non-critical level. Roughly speaking, these modules are non-commutative
localizations of some non-highest weight "vacuum" modules. Using free field
realization, we embed some rings of differential operators in endomorphism
rings of our modules.
These rings of differential operators act on a localization of the space of
coinvariants of any module over the Kac-Moody algebra with respect to a certain
level subalgebra. In a particular case this action is identified with the
Casimir connection.Comment: Final version, available at Springerlink.co
The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?
The X-ray source 2S0114+650=LSI+65 010 is a binary system containing a B-type
primary and a low mass companion believed to be a neutron star. The system has
three reported periodicities: the orbital period, P{orb}~11.6 d, X-ray flaring
with P{flare}~2.7 hr, and a "superorbital" X-ray periodicity P{super}~30.7 d.
The objective of this paper is to show that the puzzling periodicities in the
system may be explained in the context of scenarios in which tidal interactions
drive oscillations in the B-supergiant star. We calculate the solution of the
equations of motion for one layer of small surface elements distributed along
the equator of the star, as they respond to the forces due to gas pressure,
centrifugal, coriolis, viscous forces, and the gravitational forces of both
stars. This calculation provides variability timescales that can be compared
with the observations. In addition, we use observational data obtained at the
Observatorio Astron\'omico Nacional en San Pedro M\'artir (OAN/SPM) between
1993-2004 to determine which of the periodicities may be present in the optical
region. We suggest that the tidal oscillations lead to a structured stellar
wind which, when fed to the neutron star, produces the X-ray modulations. The
connection between the stellar oscillations and the modulation of the mass
ejection may lie in the shear energy dissipation generated by the tangential
motions that are produced by the tidal interaction, particularly in the tidal
bulge region. The tidal oscillation scenario weakens the case for 2S0114+650
containing a magnetar descendent.Comment: 12 pages, 14 figure
Dynamical differential equations compatible with rational qKZ equations
For the Lie algebra we introduce a system of differential operators
called the dynamical operators. We prove that the dynamical differential
operators commute with the rational quantized Knizhnik-Zamolodchikov
difference operators. We describe the transformations of the dynamical
operators under the natural action of the Weyl group.Comment: 7 pages, AmsLaTe
High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations
Structural evolution of a prospective hydrogen storage material, ammonia
borane NH3BH3, has been studied at high pressures up to 12 GPa and at low
temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a
disordered I4mm structure reversibly transforms into a new ordered phase. Its
Cmc21 structure was solved from the diffraction data, the positions of N and B
atoms and the orientation of NH3 and BH3 groups were finally assigned with the
help of density functional theory calculations. Group-theoretical analysis
identifies a single two-component order parameter, combining ordering and
atomic displacement mechanisms, which link an orientationally disordered parent
phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a
generic phase diagram for NH3BH3, mapping three experimentally found and one
predicted (P21) phases as a function of temperature and pressure, along with
the evolution of the corresponding structural distortions. Ammonia borane
belongs to the class of improper ferroelastics and we show that both
temperature- and pressure-induced phase transitions can be driven to be of the
second order. The role of N-H...H-B dihydrogen bonds and other intermolecular
interactions in the stability of NH3BH3 polymorphs is examined.Comment: 23 pages, 7 figure
Finite strain Landau theory of high pressure phase transformations
The properties of materials near structural phase transitions are often
successfully described in the framework of Landau theory. While the focus is
usually on phase transitions, which are induced by temperature changes
approaching a critical temperature T-c, here we will discuss structural phase
transformations driven by high hydrostatic pressure, as they are of major
importance for understanding processes in the interior of the earth. Since at
very high pressures the deformations of a material are generally very large,
one needs to apply a fully nonlinear description taking physical as well as
geometrical nonlinearities (finite strains) into account. In particular it is
necessary to retune conventional Landau theory to describe such phase
transitions. In Troster et al (2002 Phys. Rev. Lett. 88 55503) we constructed a
Landau-type free energy based on an order parameter part, an order
parameter-(finite) strain coupling and a nonlinear elastic term. This model
provides an excellent and efficient framework for the systematic study of phase
transformations for a wide range of materials up to ultrahigh pressures
Limits of Gaudin algebras, quantization of bending flows, Jucys--Murphy elements and Gelfand--Tsetlin bases
Gaudin algebras form a family of maximal commutative subalgebras in the
tensor product of copies of the universal enveloping algebra U(\g) of a
semisimple Lie algebra \g. This family is parameterized by collections of
pairwise distinct complex numbers . We obtain some new commutative
subalgebras in U(\g)^{\otimes n} as limit cases of Gaudin subalgebras. These
commutative subalgebras turn to be related to the hamiltonians of bending flows
and to the Gelfand--Tsetlin bases. We use this to prove the simplicity of
spectrum in the Gaudin model for some new cases.Comment: 11 pages, references adde
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