497 research outputs found
Non-Gaussian statistics of electrostatic fluctuations of hydration shells
We report the statistics of electric field fluctuations produced by SPC/E
water inside a Kihara solute given as a hard-sphere core with a Lennard-Jones
layer at its surface. The statistics of electric field fluctuations, obtained
from numerical simulations, are studied as a function of the magnitude of a
point dipole placed close to the solute-water interface. The free energy
surface as a function of the electric field projected on the dipole direction
shows a cross-over with the increasing dipole magnitude. While it is a
single-well harmonic function at low dipole values, it becomes a double-well
surface at intermediate dipole moment magnitudes, transforming to a single-well
surface, with a non-zero minimum position, at still higher dipoles. A broad
intermediate region where the interfacial waters fluctuate between the two
minima is characterized by intense field fluctuations, with non-Gaussian
statistics and the variance far exceeding the linear-response expectations. The
excited state of the surface water is found to be lifted above the ground state
by the energy required to break approximately two hydrogen bonds. This state is
pulled down in energy by the external electric field of the solute dipole,
making it readily accessible to thermal excitations. The excited state is a
localized surface defect in the hydrogen-bond network creating a stress in the
nearby network, but otherwise relatively localized in the region closest to the
solute dipole
The polymer phase of the TDAE-C organic ferromagnet
The high-pressure Electron Spin Resonance (ESR) measurements were preformed
on TDAE-C single crystals and stability of the polymeric phase was
established in the parameter space. At 7 kbar the system undergoes a
ferromagnetic to paramagnetic phase transition due to the pressure-induced
polymerization. The polymeric phase remains stable after the pressure release.
The depolymerization of the pressure-induced phase was observed at the
temperature of 520 K. Below room temperature, the polymeric phase behaves as a
simple Curie-type insulator with one unpaired electron spin per chemical
formula. The TDAE donor-related unpaired electron spins, formerly
ESR-silent, become active above the temperature of 320 K and the Curie-Weiss
behavior is re-established.Comment: Submitted to Phys. Rev.
X-Ray Diffuse Scattering Study on Ionic-Pair Displacement Correlations in Relaxor Lead Magnesium Niobate
Ionic-pair equal-time displacement correlations in relaxor lead magnesium
niobate, , have been investigated at room
temperature in terms of an x-ray diffuse scattering technique. Functions of the
distinct correlations have been determined quantitatively. The results show the
significantly strong rhombohedral-polar correlations regarding Pb-O, Mg/Nb-O,
and O-O' pairs. Their spatial distribution forms an ellipse or a sphere with
the radii of 30-80. This observation of local structure in the system
proves precursory presence of the polar microregions in the paraelectric state
which leads to the dielectric dispersion.Comment: 11 pages, 3 figure
Inversion symmetry breaking in noncollinear magnetic phase of a triangular lattice antiferromagnet CuFeO2
Magnetoelectric and magnetoelastic phenomena correlated with a phase
transition into noncollinear magnetic phase have been investigated for single
crystals of CuFeO2 with a frustrated triangular lattice. CuFeO2 exhibits
several long-wavelength magnetic structures related to the spin frustration,
and it is found that finite electric polarization, namely inversion symmetry
breaking, occurs with noncollinear but not at collinear magnetic phases. This
result demonstrates that the noncollinear spin structure is a key role to
induce electric polarization, and suggests that frustrated magnets which often
favor noncollinear configurations can be plausible candidates for
magnetoelectrics with strong magnetoelectric interaction.Comment: 15 page
Spin-lattice coupling in multiferroic Pb(Fe1/2Nb1/2)O3 thin films
We have made magnetization and x-ray diffraction measurements on an epitaxial
Pb(Fe1/2Nb1/2)O3 200 nm film. From the temperature dependence of the
out-of-plane lattice parameter we can assign a Burns' temperature at Td ~ 640
K, a temperature at T* ~ 510 K, related to the appearance of static polar
nanoregions, and an anomaly occurring at 200 K. The latter is precisely the
N\'eel temperature TN determined from magnetization and points to spin-lattice
coupling at TN ~ 200 K. We also observe "weak ferromagnetism" up to 300K and
propose superantiferromagnetic clusters as a plausible scenario to explain this
hysteresis above TN.Comment: 12 pages, 4 figure
Application of elastostatic Green function tensor technique to electrostriction in cubic, hexagonal and orthorhombic crystals
The elastostatic Green function tensor approach, which was recently used to
treat electrostriction in numerical simulation of domain structure formation in
cubic ferroelectrics, is reviewed and extended to the crystals of hexagonal and
orthorhombic symmetry. The tensorial kernels appearing in the expressions for
effective nonlocal interaction of electrostrictive origin are derived
explicitly and their physical meaning is illustrated on simple examples. It is
argued that the bilinear coupling between the polarization gradients and
elastic strain should be systematically included in the Ginzburg-Landau free
energy expansion of electrostrictive materials.Comment: 4 page
Monte Carlo Study of Relaxor Systems: A Minimum Model for Pb(InNb)O}
We examine a simple model for Pb(InNb)O (PIN), which
includes both long-range dipole-dipole interaction and random local anisotropy.
A improved algorithm optimized for long-range interaction has been applied for
efficient large-scale Monte Carlo simulation. We demonstrate that the phase
diagram of PIN is qualitatively reproduced by this minimum model. Some
properties characteristic of relaxors such as nano-scale domain formation, slow
dynamics and dispersive dielectric responses are also examined.Comment: 5 pages, 4 figure
NMR imaging of the soliton lattice profile in the spin-Peierls compound CuGeO_3
In the spin-Peierls compound CuGeO, the commensurate-incommensurate
transition concerning the modulation of atomic position and the local
spin-polarization is fully monitored at T=0 by the application of an external
magnetic field () above a threshold value 13 Tesla. The
solitonic profile of the spin-polarization, as well as its absolute magnitude,
has been precisely imaged from NMR lineshapes obtained for
varying from 0.0015 to 2. This offers a unique possibility
to test quantitatively the various numerical and analytical methods developed
to solve a generic Hamiltonian in 1-D physics, namely strongly interacting
fermions in presence of electron-phonon coupling at arbitrary band filling.Comment: 3 pages, 4 eps figures, RevTeX, submitted to Physical Review Lette
Effects of interatomic interaction on cooperative relaxation of two-level atoms
We study effects of direct interatomic interaction on cooperative processes
in atom-photon dynamics. Using a model of two-level atoms with Ising-type
interaction as an example, it is demonstrated that interparticle interaction
combined with atom-field coupling can introduce additional interatomic
correlations acting as a phase synchronizing factor. For the case of weakly
interacting atoms with , where is the interparticle
coupling constant and is the atomic frequency, dynamical regimes of
cooperative relaxation of atoms are analyzed in Born-Markov approximation both
numerically and using the mean field approximation. We show that interparticle
correlations induced by the direct interaction result in inhibition of
incoherent spontaneous decay leading to the regime of collective pulse
relaxation which differs from superradiance in nature. For superradiant
transition, the synchronizing effect of interatomic interaction is found to
manifest itself in enhancement of superradiance. When the interaction is strong
and , one-partice one-photon transitions are excluded and
transition to the regime of multiphoton relaxation occurs. Using a simple model
of two atoms in a high-Q single mode cavity we show that such transition is
accompanied by Rabi oscillations involving many-atom multiphoton states.
Dephasing effect of dipole-dipole interaction and solitonic mechanism of
relaxation are discussed.Comment: 34 pages, 8 figure
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