84,214 research outputs found
Relationship between ferroelectricity and Dzyaloshinskii-Moriya interaction in multiferroics and the effect of bond-bending
We studied the microscopic mechanism of multiferroics, in particular with the
"spin current" model (Hosho Katsura, Naoto Nagaosa and Aleander V. Balatsky,
Phys. Rev. Lett. 95, 057205 (2005)). Starting from a system with helical spin
configuration, we solved for the forms of the electron wave functions and
analyzed their characteristics. The relation between ferroelectricity and
Dzyaloshinskii-Moriya interaction (I. Dzyaloshinskii, J. Phys. Chem. Solids 4,
241 (1958) and T. Moriya, Phys. Rev. 120, 91 (1960)) is clearly established.
There is also a simple relation between the electric polarization and the wave
vector of magnetic orders. Finally, we show that the bond-bending exists in
transition metal oxides can enhance ferroelectricity.Comment: 14 pages, 3 figures. acceptby Physical Review
Problems with the Newton-Schr\"odinger Equations
We examine the origin of the Newton-Schr\"odinger equations (NSEs) that play
an important role in alternative quantum theories (AQT), macroscopic quantum
mechanics and gravity-induced decoherence. We show that NSEs for individual
particles do not follow from general relativity (GR) plus quantum field theory
(QFT). Contrary to what is commonly assumed, the NSEs are not the weak-field
(WF), non-relativistic (NR) limit of the semi-classical Einstein equation (SCE)
(this nomenclature is preferred over the `M\/oller-Rosenfeld equation') based
on GR+QFT. The wave-function in the NSEs makes sense only as that for a mean
field describing a system of particles as , not that
of a single or finite many particles. From GR+QFT the gravitational
self-interaction leads to mass renormalization, not to a non-linear term in the
evolution equations of some AQTs. The WF-NR limit of the gravitational
interaction in GR+QFT involves no dynamics. To see the contrast, we give a
derivation of the equation (i) governing the many-body wave function from
GR+QFT and (ii) for the non-relativistic limit of quantum electrodynamics
(QED). They have the same structure, being linear, and very different from
NSEs. Adding to this our earlier consideration that for gravitational
decoherence the master equations based on GR+QFT lead to decoherence in the
energy basis and not in the position basis, despite some AQTs desiring it for
the `collapse of the wave function', we conclude that the origins and
consequences of NSEs are very different, and should be clearly demarcated from
those of the SCE equation, the only legitimate representative of semiclassical
gravity, based on GR+QFT.Comment: 18 pages. Invited paper for the Focus Issue on 'Gravitational quantum
physics' in New Journal of Physic
Geometry, thermodynamics, and finite-size corrections in the critical Potts model
We establish an intriguing connection between geometry and thermodynamics in
the critical q-state Potts model on two-dimensional lattices, using the q-state
bond-correlated percolation model (QBCPM) representation. We find that the
number of clusters of the QBCPM has an energy-like singularity for q different
from 1, which is reached and supported by exact results, numerical simulation,
and scaling arguments. We also establish that the finite-size correction to the
number of bonds, has no constant term and explains the divergence of related
quantities as q --> 4, the multicritical point. Similar analyses are applicable
to a variety of other systems.Comment: 12 pages, 6 figure
Quantum Brownian motion of multipartite systems and their entanglement dynamics
We solve the model of N quantum Brownian oscillators linearly coupled to an
environment of quantum oscillators at finite temperature, with no extra
assumptions about the structure of the system-environment coupling. Using a
compact phase-space formalism, we give a rather quick and direct derivation of
the master equation and its solutions for general spectral functions and
arbitrary temperatures. Since our framework is intrinsically nonperturbative,
we are able to analyze the entanglement dynamics of two oscillators coupled to
a common scalar field in previously unexplored regimes, such as off resonance
and strong coupling.Comment: 10 pages, 6 figure
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