214 research outputs found
Stochastic Gross-Pitaevsky Equation for BEC via Coarse-Grained Effective Action
We sketch the major steps in a functional integral derivation of a new set of
Stochastic Gross-Pitaevsky equations (GPE) for a Bose-Einstein condensate (BEC)
confined to a trap at zero temperature with the averaged effects of
non-condensate modes incorporated as stochastic sources. The closed-time-path
(CTP) coarse-grained effective action (CGEA) or the equivalent influence
functional method is particularly suitable because it can account for the full
back-reaction of the noncondensate modes on the condensate dynamics
self-consistently. The Langevin equations derived here containing nonlocal
dissipation together with colored and multiplicative noises are useful for a
stochastic (as distinguished from say, a kinetic) description of the
nonequilibrium dynamics of a BEC. This short paper contains original research
results not yet published anywhere.Comment: 6 page
Magnetic and magnetoelectric studies in pure and cation doped BiFeO3
We report magnetic and magnetoelectric studies on BiFeO3 and divalent cation
(A) suvtitute Bi0.7A0.3FeO3 (A = Sr,Ba, and Sr0.5Ba0.5). It is shown that the
rapid increase of magnetization at the Neel temperature (TN = 642 K) is
suppressed in the co-doped compound A = Sr0.5Ba0.5. All the divalent subtituted
compounds show enhanced magnetization and hysteresis loop. Both longitudinal
and transverse magnetoelectric coefficients were measured using the dynamical
lock-in technique. The co-doped compound shows the highest magnetoelectric
coefficient at room temperature although it is not the compound with the
highest saturation magnetization. It is found that as the size of the A-site
cation increses, the transverse magnetoelectric coeffient increases and exceeds
the longitudinal magnetoelectric coefficient. It is suggested that changes in
magnetic domain structure and magnetostriction are possible reasons for the
observed changes in the magnetoelectric coefficients.Comment: 16 pages, 6 figur
Infrared and THz studies of polar phonons and improper magnetodielectric effect in multiferroic BFO3 ceramics
BFO3 ceramics were investigated by means of infrared reflectivity and time
domain THz transmission spectroscopy at temperatures 20 - 950 K, and the
magnetodielectric effect was studied at 10 - 300 K, with the magnetic field up
to 9 T. Below 175 K, the sum of polar phonon contributions into the
permittivity corresponds to the value of measured permittivity below 1 MHz. At
higher temperatures, a giant low-frequency permittivity was observed, obviously
due to the enhanced conductivity and possible Maxwell-Wagner contribution.
Above 200 K the observed magnetodielectric effect is caused essentially through
the combination of magnetoresistance and the Maxwell-Wagner effect, as recently
predicted by Catalan (Appl. Phys. Lett. 88, 102902 (2006)). Since the
magnetodielectric effect does not occur due to a coupling of polarization and
magnetization as expected in magnetoferroelectrics, we call it improper
magnetodielectric effect. Below 175 K the magnetodielectric effect is by
several orders of magnitude lower due to the decreased conductivity. Several
phonons exhibit gradual softening with increasing temperature, which explains
the previously observed high-frequency permittivity increase on heating. The
observed non-complete phonon softening seems to be the consequence of the
first-order nature of the ferroelectric transition.Comment: subm. to PRB. revised version according to referees' report
Collapsing Bose-Einstein condensates beyond the Gross-Pitaevskii approximation
We analyse quantum field models of the bosenova experiment, in which
Rb Bose-Einstein condensates were made to collapse by switching their
atomic interactions from repulsive to attractive. Specifically, we couple the
lowest order quantum field correlation functions to the Gross-Pitaevskii
function, and solve the resulting dynamical system numerically. Comparing the
computed collapse times with the experimental measurements, we find that the
calculated times are much larger than the measured values. The addition of
quantum field corrections does not noticeably improve the agreement compared to
a pure Gross-Pitaevskii theory.Comment: 8 pages, 4 figure
Transient cavities and the excess chemical potentials of hard-spheroid solutes in dipolar hard sphere solvents
Monte Carlo computer simulations are used to study transient cavities and the
solvation of hard-spheroid solutes in dipolar hard sphere solvents. The
probability distribution of spheroidal cavities in the solvent is shown to be
well described by a Gaussian function, and the variations of fit parameters
with cavity elongation and solvent properties are analyzed. The excess chemical
potentials of hard-spheroid solutes with aspect ratios in the range , and with volumes between one and twenty times that of a solvent
molecule, are presented. It is shown that for a given molecular volume and
solvent dipole moment (or temperature) a spherical solute has the lowest excess
chemical potential and hence the highest solubility, while a prolate solute
with aspect ratio should be more soluble than an oblate solute with aspect
ratio . For a given solute molecule, the excess chemical potential
increases with increasing temperature; this same trend is observed in the case
of hydrophobic solvation. To help interpret the simulation results, comparison
is made with a scaled-particle theory that requires prior knowledge of a
solute-solvent interfacial tension and the pure-solvent equation of state,
which parameters are obtained from simulation results for spherical solutes.
The theory shows excellent agreement with simulation results over the whole
range of solute elongations considered.Comment: 10 pages, 10 figure
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