88 research outputs found
Dynamical mean-field theory for bosons
We discuss the recently developed bosonic dynamical mean-field (B-DMFT)
framework, which maps a bosonic lattice model onto the selfconsistent solution
of a bosonic impurity model with coupling to a reservoir of normal and
condensed bosons. The effective impurity action is derived in several ways: (i)
as an approximation to the kinetic energy functional of the lattice problem,
(ii) using a cavity approach, and (iii) by using an effective medium approach
based on adding a one-loop correction to the selfconsistently defined
condensate. To solve the impurity problem, we use a continuous-time Monte Carlo
algorithm based on a sampling of a perturbation expansion in the hybridization
functions and the condensate wave function. As applications of the formalism we
present finite temperature B-DMFT phase diagrams for the bosonic Hubbard model
on a 3d cubic and 2d square lattice, the condensate order parameter as a
function of chemical potential, critical exponents for the condensate, the
approach to the weakly interacting Bose gas regime for weak repulsions, and the
kinetic energy as a function of temperature.Comment: 26 pages, 19 figure
Non-perturbative renormalization-group approach to zero-temperature Bose systems
We use a non-perturbative renormalization-group technique to study
interacting bosons at zero temperature. Our approach reveals the instability of
the Bogoliubov fixed point when and yields the exact infrared
behavior in all dimensions within a rather simple theoretical framework.
It also enables to compute the low-energy properties in terms of the parameters
of a microscopic model. In one-dimension and for not too strong interactions,
it yields a good picture of the Luttinger-liquid behavior of the superfluid
phase.Comment: v1) 6 pages, 8 figures; v2) added references; v3) corrected typo
Phase separation of Bose-Einstein condensates
The zero-temperature system of two dilute overlapping Bose-Einstein
condensates is unstable against long wavelength excitations if the interaction
strength between the distinguishable bosons exceeds the geometric mean of the
like-boson interaction strengths. If the condensates attract each other, the
instability is similar to the instability of the negative scattering length
condensates. If the condensates repel, they separate spatially into condensates
of equal pressure. We estimate the boundary size, surface tension and energy of
the phase separated condensate system and we discuss the implications for
double condensates in atomic traps.Comment: 11 pages, 1 figur
Infrared behavior of interacting bosons at zero temperature
We review the infrared behavior of interacting bosons at zero temperature.
After a brief discussion of the Bogoliubov approximation and the breakdown of
perturbation theory due to infrared divergences, we present two approaches that
are free of infrared divergences -- Popov's hydrodynamic theory and the
non-perturbative renormalization group -- and allow us to obtain the exact
infrared behavior of the correlation functions. We also point out the
connection between the infrared behavior in the superfluid phase and the
critical behavior at the superfluid--Mott-insulator transition in the
Bose-Hubbard model.Comment: 8 pages, 4 figures. Proceedings of the 19th International Laser
Physics Workshop, LPHYS'10 (Foz do Iguacu, Brazil, July 5-9, 2010
Theory of Bose-Einstein condensation for trapped atoms
We outline the general features of the conventional mean-field theory for the
description of Bose-Einstein condensates at near zero temperatures. This
approach, based on a phenomenological model, appears to give excellent
agreement with experimental data. We argue, however, that such an approach is
not rigorous and cannot contain the full effect of collisional dynamics due to
the presence of the mean-field. We thus discuss an alternative microscopic
approach and explain, within our new formalism, the physical origin of these
effects. Furthermore, we discuss the potential formulation of a consistent
finite-temperature mean-field theory, which we claim necessiates an analysis
beyond the conventional treatment.Comment: 12 pages. To appear in Phil. Trans. R. Soc. Lond. A 355 (1997
Ground state at high density
Weak limits as the density tends to infinity of classical ground states of
integrable pair potentials are shown to minimize the mean-field energy
functional. By studying the latter we derive global properties of high-density
ground state configurations in bounded domains and in infinite space. Our main
result is a theorem stating that for interactions having a strictly positive
Fourier transform the distribution of particles tends to be uniform as the
density increases, while high-density ground states show some pattern if the
Fourier transform is partially negative. The latter confirms the conclusion of
earlier studies by Vlasov (1945), Kirzhnits and Nepomnyashchii (1971), and
Likos et al. (2007). Other results include the proof that there is no Bravais
lattice among high-density ground states of interactions whose Fourier
transform has a negative part and the potential diverges or has a cusp at zero.
We also show that in the ground state configurations of the penetrable sphere
model particles are superposed on the sites of a close-packed lattice.Comment: Note adde
Influence of the Aggregation of a Carbazole Thiophene Cyanoacrylate Sensitizer on Sensitized Photocurrents on ZnO Single Crystals
Dye sensitization of zinc oxide single
crystals by a carbazole
thiophene cyanoacrylate (MK-2) sensitizer deposited from THF and mixtures
of THF and water was investigated. AFM images show the formation of
larger aggregates, with the maximum size of 20–30 nm from mixtures
of THF and water, compared with 8–12 nm from pure THF. Sensitized
photocurrent spectra were correlated with the morphological results
from AFM imaging and indicate that aggregation in water results in
less efficient sensitization of the ZnO substrate. The presence of
the aggregation in solution due to water content was confirmed by
absorbance and fluorescence spectroscopies
Electrogenerated Chemiluminescence of BODIPY, Ru(bpy)<sub>3</sub><sup>2+</sup>, and 9,10-Diphenylanthracene Using Interdigitated Array Electrodes
Interdigitated array electrodes (IDAs)
were used to produce steady-state electrogenerated chemiluminescence
(ECL) by annihilation of oxidized and reduced forms of a substituted
boron dipyrromethene (BODIPY) dye, 9,10-diphenylanthracene (DPA),
and rutheniumÂ(II) trisÂ(bypiridine) (RuÂ(bpy)<sub>3</sub><sup>2+</sup>). Digital simulations were in good agreement with the experimentally
obtained currents and light outputs. Coreactant experiments, using
tri-n-propylamine and benzoyl peroxide as a sacrificial homogeneous
reductant or oxidant, show currents corresponding to electrode reactions
of the dyes and not the oxidation or reduction of the coreactants.
The results show that interdigitated arrays can produce stable ECL
where the light intensity is magnified due to the larger currents
as a consequence of feedback between generator and collector electrodes
in the IDA. The light output for ECL is around 100 times higher than
that obtained with regular planar electrodes with similar area
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