66 research outputs found
Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO_3 versus BiFeO_3
In this article we review and discuss a mechanism for coupling between
electric polarization and magnetization that can ultimately lead to
electric-field switchable magnetization. The basic idea is that a ferroelectric
distortion in an antiferromagnetic material can "switch on" the
Dzyaloshinskii-Moriya interaction which leads to a canting of the
antiferromagnetic sublattice magnetizations, and thus to a net magnetization.
This magnetization M is coupled to the polarization P via a trilinear free
energy contribution of the form P(M x L), where L is the antiferromagnetic
order parameter. In particular, we discuss why such an invariant is present in
R3c FeTiO_3 but not in the isostructural multiferroic BiFeO_3. Finally, we
construct symmetry groups that in general allow for this kind of
ferroelectrically-induced weak ferromagnetism.Comment: 15 pages, 3 images, to appear in J. Phys: Condens. Matter Focus Issue
on Multiferroic
Molecular formations in ultracold mixtures of interacting and noninteracting atomic gases
Atom-molecule equilibrium for molecular formation processes is discussed for
boson-fermion, fermion-fermion, and boson-boson mixtures of ultracold atomic
gases in the framework of quasichemical equilibrium theory. After presentation
of the general formulation, zero-temperature phase diagrams of the
atom-molecule equilibrium states are calculated analytically; molecular, mixed,
and dissociated phases are shown to appear for the change of the binding energy
of the molecules. The temperature dependences of the atom or molecule densities
are calculated numerically, and finite-temperature phase structures are
obtained of the atom-molecule equilibrium in the mixtures. The transition
temperatures of the atom or molecule Bose-Einstein condensations are also
evaluated from these results. Quantum-statistical deviations of the law of mass
action in atom-molecule equilibrium, which should be satisfied in mixtures of
classical Maxwell-Boltzmann gases, are calculated, and the difference in the
different types of quantum-statistical effects is clarified. Mean-field
calculations with interparticle interactions (atom-atom, atom-molecule, and
molecule-molecule) are formulated, where interaction effects are found to give
the linear density-dependent term in the effective molecular binding energies.
This method is applied to calculations of zero-temperature phase diagrams,
where new phases with coexisting local-equilibrium states are shown to appear
in the case of strongly repulsive interactions.Comment: 35 pages, 14 figure
Symmetry of Magnetically Ordered Quasicrystals
The notion of magnetic symmetry is reexamined in light of the recent
observation of long range magnetic order in icosahedral quasicrystals [Charrier
et al., Phys. Rev. Lett. 78, 4637 (1997)]. The relation between the symmetry of
a magnetically-ordered (periodic or quasiperiodic) crystal, given in terms of a
``spin space group,'' and its neutron diffraction diagram is established. In
doing so, an outline of a symmetry classification scheme for magnetically
ordered quasiperiodic crystals is provided. Predictions are given for the
expected diffraction patterns of magnetically ordered icosahedral crystals,
provided their symmetry is well described by icosahedral spin space groups.Comment: 5 pages. Accepted for publication in Phys. Rev. Letter
What is Minimal Model of 3He Adsorbed on Graphite? -Importance of Density Fluctuations in 4/7 Registered Solid -
We show theoretically that the second layer of 3He adsorbed on graphite and
solidified at 4/7 of the first-layer density is close to the fluid-solid
boundary with substantial density fluctuations on the third layer. The solid
shows a translational symmetry breaking as in charge-ordered insulators of
electronic systems. We construct a minimal model beyond the multiple-exchange
Heisenberg model. An unexpectedly large magnetic field required for the
measured saturation of magnetization is well explained by the density
fluctuations. The emergence of quantum spin liquid is understood from the same
mechanism as in the Hubbard model and in \kappa-(ET)_2Cu_2(CN)_3 near the Mott
transitions.Comment: 9 pages, 5 figure
New Symmetries in Crystals and Handed Structures
For over a century, the structure of materials has been described by a
combination of rotations, rotation-inversions and translational symmetries. By
recognizing the reversal of static structural rotations between clockwise and
counterclockwise directions as a distinct symmetry operation, here we show that
there are many more structural symmetries than are currently recognized in
right- or left-handed handed helices, spirals, and in antidistorted structures
composed equally of rotations of both handedness. For example, though a helix
or spiral cannot possess conventional mirror or inversion symmetries, they can
possess them in combination with the rotation reversal symmetry. Similarly, we
show that many antidistorted perovskites possess twice the number of symmetry
elements as conventionally identified. These new symmetries predict new forms
for "roto" properties that relate to static rotations, such as rotoelectricity,
piezorotation, and rotomagnetism. They also enable symmetry-based search for
new phenomena, such as multiferroicity involving a coupling of spins, electric
polarization and static rotations. This work is relevant to structure-property
relationships in all material structures with static rotations such as
minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error
Periodic Vortex Structures in Superfluid 3He-A
We discuss the general properties of periodic vortex arrangements in rotating
superfluids. The different possible structures are classified according to the
symmetry space-groups and the circulation number. We calculate numerically
several types of vortex structures in superfluid 3He-A. The calculations are
done in the Ginzburg-Landau region, but the method is applicable at all
temperatures. A phase diagram of vortices is constructed in the plane formed by
the magnetic field and the rotation velocity. The characteristics of the six
equilibrium vortex solutions are discussed. One of these, the locked vortex 3,
has not been considered in the literature before. The vortex sheet forms the
equilibrium state of rotating 3He-A at rotation velocities exceeding 2.6 rad/s.
The results are in qualitative agreement with experiments.Comment: 13 pages, 7 figures,
http://boojum.hut.fi/research/theory/diagram.htm
Spontaneous Magnetization of the O(3) Ferromagnet at Low Temperatures
We investigate the low-temperature behavior of ferromagnets with a
spontaneously broken symmetry O(3) O(2). The analysis is performed within
the perspective of nonrelativistic effective Lagrangians, where the dynamics of
the system is formulated in terms of Goldstone bosons. Unlike in a
Lorentz-invariant framework (chiral perturbation theory), where loop graphs are
suppressed by two powers of momentum, loops involving ferromagnetic spin waves
are suppressed by three momentum powers. The leading coefficients of the
low-temperature expansion for the partition function are calculated up to order
. In agreement with Dyson's pioneering microscopic analysis of the
cubic ferromagnet, we find that, in the spontaneous magnetization, the
magnon-magnon interaction starts manifesting itself only at order . The
striking difference with respect to the low-temperature properties of the O(3)
antiferromagnet is discussed from a unified point of view, relying on the
effective Lagrangian technique.Comment: 23 pages, 4 figure
The Free Energy of the Quantum Heisenberg Ferromagnet at Large Spin
We consider the spin-S ferromagnetic Heisenberg model in three dimensions, in
the absence of an external field. Spin wave theory suggests that in a suitable
temperature regime the system behaves effectively as a system of
non-interacting bosons (magnons). We prove this fact at the level of the
specific free energy: if and the inverse temperature in such a way that stays constant, we rigorously show that
the free energy per unit volume converges to the one suggested by spin wave
theory. The proof is based on the localization of the system in small boxes and
on upper and lower bounds on the local free energy, and it also provides
explicit error bounds on the remainder.Comment: 11 pages, pdfLate
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