74,230 research outputs found
A System Exhibiting Toroidal Order
A two dimensional system of discs upon which a triangle of spins are mounted
is shown to undergo a sequence of interesting phase transitions as the
temperature is lowered. We are mainly concerned with the `solid' phase in which
bond orientational order but not positional order is long ranged. As the
temperature is lowered in the `solid' phase, the first phase transition
involving the orientation or toroidal charge of the discs is into a `gauge
toroid' phase in which the product of a magnetic toroidal parameter and an
orientation variable (for the discs) orders but due to a local gauge symmetry
these variables themselves do not individually order. Finally, in the lowest
temperature phase the gauge symmetry is broken and toroidal order and
orientational order both develop. In the `gauge toroidal' phase time reversal
invariance is broken and in the lowest temperature phase inversion symmetry is
also broken. In none of these phases is there long range order in any Fourier
component of the average spin. A definition of the toroidal magnetic moment
of the th plaquette is proposed such that the magnetostatic
interaction between plaquettes and is proportional to .
Symmetry considerations are used to construct the magnetoelectric free energy
and thereby to deduce which coefficients of the linear magnetoelectric tensor
are allowed to be nonzero. In none of the phases does symmetry permit a
spontaneous polarization.Comment: 9 pages, 6 figure
Landau Theory of Tilting of Oxygen Octahedra in Perovskites
The list of possible commensurate phases obtained from the parent tetragonal
phase of Ruddlesden-Popper systems, ABC for general
due to a single phase transition involving the reorienting of octahedra of C
(oxygen) ions is reexamined using a Landau expansion. This expansion allows for
the nonlinearity of the octahedral rotations and the rotation-strain coupling.
It is found that most structures allowed by symmetry are inconsistent with the
constraint of rigid octahedra which dictates the form of the quartic terms in
the Landau free energy. For ABC our analysis allows only 10 (see Table
III) of the 41 structures listed by Hatch {\it et al.} which are allowed by
general symmetry arguments. The symmetry of rotations for RP systems with
is clarified. Our list of possible structures in Table VII excludes many
structures allowed in previous studies.Comment: 21 pages, 21 figures. An elaboration of arXiv:1012.512
Charge and Spin Ordering in the Mixed Valence Compound LuFe2O4
Landau theory and symmetry considerations lead us to propose an explanation
for several seemingly paradoxical behaviors of charge ordering (CO) and spin
ordering (SO) in the mixed valence compound LuFe2O4. Both SO and CO are highly
frustrated. We analyze a lattice gas model of CO within mean field theory and
determine the magnitude of several of the phenomenological interactions. We
show that the assumption of a continuous phase transition at which CO or SO
develops implies that both CO and SO are incommensurate. To explain how
ferroelectric fluctuations in the charge disordered phase can be consistent
with an antiferroelectric ordered phase, we invoke an electron-phonon
interaction in which a low energy (20meV) zone-center transverse phonon plays a
key role. The energies of all the zone-center phonons are calculated from first
principles. We give a Landau analysis which explains SO and we discuss a model
of interactions which stabilizes the SO state, if it is assumed commensurate.
However, we suggest a high resolution experimental determination to see whether
this phase is really commensurate, as believed up to now. The applicability of
representation analysis is discussed. A tentative explanation for the
sensitivity of the CO state to an applied magnetic field in field-cooled
experiments is given.Comment: 36 pages, 17 figure
Spin Dynamics of Trimers on a Distorted Kagom\'e Lattice
We treat the ground state, elementary excitations, and neutron scattering
cross section for a system of trimers consisting of three tightly bound spins
1/2 on a distorted Kagom\'e lattice, subject to isotropic nearest neighbor
(usually antiferromagnetic) Heisenberg interactions. The interactions between
trimers are assumed to be weak compared to the intra trimer interactions. We
compare the spin-wave excitation spectrum of trimers with that obtained from
standard spin-wave theory and attribute the differences at low energy to the
fact that the trimer formulation includes exactly the effects of intra-trimer
zero point motion. Application to existing systems is briefly discussed.Comment: 20 pages, 10 figure
Nonlocal Modulation of Entangled Photons
We consider ramifications of the use of high speed light modulators to
questions of correlation and measurement of time-energy entangled photons.
Using phase modulators, we find that temporal modulation of one photon of an
entangled pair, as measured by correlation in the frequency domain, may be
negated or enhanced by modulation of the second photon. Using amplitude
modulators we describe a Fourier technique for measurement of biphoton wave
functions with slow detectors
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