66 research outputs found
Electronic properties of two isomeric charge transfer [2.2]paracyclophanes
The emission spectra and the zero field splitting parameters of the two diastereomeric 4,7-dicyano-12,15-dimethoxy-[2.2] paracyclophanes 3 and 4 in comparison to the corresponding monomers 1 and 2 were investigated in order to study the orientation dependence of charge transfer (CT) interactions. The general results in glasses (PMMA) are: broad structureless emission bands with large spectral overlap between fluorescence and phosphorescence; strong reduction of the zero field splitting parameters D and D* by a factor of two for the pseudo-ortho isomer 3 and by a factor of four for the pseudo-geminal isomer 4 showing the strong effect of the geometrical orientation. In single crystals of the same phanes the zero field parameters were found to be further reduced to about one fifth of the value of the monomers which indicates, in accordance with the emission spectra, an additional intermolecular interaction between adjacent phanes
Pairing in spin polarized two-species fermionic mixtures with mass asymmetry
We discuss on the pairing mechanism of fermions with mismatch in their fermi
momenta due to a mass asymmetry. Using a variational ansatz for the ground
state we also discuss the BCS -BEC crossover of this system. It is shown that
the breached pairing solution with a single fermi surface is stable in the BEC
regime. We also include the temperatures effect on the fermion pairing within
an approximation that is valid for temperatures much below the critical
temperature.Comment: 8 pages and 6 figures, few typos corrected, version to appear in EPJ
Two-dimensional loosely and tightly bound solitons in optical lattices and inverted traps
We study the dynamics of nonlinear localized excitations (solitons) in
two-dimensional (2D) Bose-Einstein condensates (BECs) with repulsive
interactions, loaded into an optical lattice (OL), which is combined with an
external parabolic potential. First, we demonstrate analytically that a broad
(loosely bound, LB) soliton state, based on a 2D Bloch function near the edge
of the Brillouin zone (BZ), has a negative effective mass (while the mass of a
localized state is positive near the BZ center). The negative-mass soliton
cannot be held by the usual trap, but it is safely confined by an inverted
parabolic potential (anti-trap). Direct simulations demonstrate that the LB
solitons (including the ones with intrinsic vorticity) are stable and can
freely move on top of the OL. The frequency of elliptic motion of the
LB-soliton's center in the anti-trapping potential is very close to the
analytical prediction which treats the solition as a quasi-particle. In
addition, the LB soliton of the vortex type features real rotation around its
center. We also find an abrupt transition, which occurs with the increase of
the number of atoms, from the negative-mass LB states to tightly bound (TB)
solitons. An estimate demonstrates that, for the zero-vorticity states, the
transition occurs when the number of atoms attains a critical number N=10^3,
while for the vortex the transition takes place at N=5x10^3 atoms. The
positive-mass LB states constructed near the BZ center (including vortices) can
move freely too. The effects predicted for BECs also apply to optical spatial
solitons in bulk photonic crystals.Comment: 17 pages, 12 figure
Superfluid phase transition and strong-coupling effects in an ultracold Fermi gas with mass imbalance
We investigate the superfluid phase transition and effects of mass imbalance
in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation)
crossover regime of an cold Fermi gas. We point out that the Gaussian
fluctuation theory developed by Nozi\`eres and Schmitt-Rink and the -matrix
theory, that are now widely used to study strong-coupling physics of cold Fermi
gases, give unphysical results in the presence of mass imbalance. To overcome
this problem, we extend the -matrix theory to include higher-order pairing
fluctuations. Using this, we examine how the mass imbalance affects the
superfluid phase transition. Since the mass imbalance is an important key in
various Fermi superfluids, such as K-Li Fermi gas mixture, exciton
condensate, and color superconductivity in a dense quark matter, our results
would be useful for the study of these recently developing superfluid systems.Comment: 7 pages, 4 figures, Proceedings of QFS-201
Analytical solutions for two heteronuclear atoms in a ring trap
We consider two heteronuclear atoms interacting with a short-range
potential and confined in a ring trap. By taking the Bethe-ansatz-type
wavefunction and considering the periodic boundary condition properly, we
derive analytical solutions for the heteronuclear system. The eigen-energies
represented in terms of quasi-momentums can then be determined by solving a set
of coupled equations. We present a number of results, which display different
features from the case of identical atoms. Our result can be reduced to the
well-known Lieb-Liniger solution when two interacting atoms have the same
masses.Comment: 6 pages, 6 figure
Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms
We present the design, implementation and characterization of a dual-species
magneto-optical trap (MOT) for fermionic 6Li and 40K atoms with large atom
numbers. The MOT simultaneously contains 5.2x10^9 6Li-atoms and 8.0x10^9
40K-atoms, which are continuously loaded by a Zeeman slower for 6Li and a
2D-MOT for 40K. The atom sources induce capture rates of 1.2x10^9 6Li-atoms/s
and 1.4x10^9 40K-atoms/s. Trap losses due to light-induced interspecies
collisions of ~65% were observed and could be minimized to ~10% by using low
magnetic field gradients and low light powers in the repumping light of both
atomic species. The described system represents the starting point for the
production of a large-atom number quantum degenerate Fermi-Fermi mixture
Low temperature properties of the fermionic mixtures with mass imbalance in optical lattice
We study the attractive Hubbard model with mass imbalance to clarify low
temperature properties of the fermionic mixtures in the optical lattice. By
combining dynamical mean-field theory with the continuous-time quantum Monte
Carlo simulation, we discuss the competition between the superfluid and density
wave states at half filling. By calculating the energy and the order parameter
for each state, we clarify that the coexisting (supersolid) state, where the
density wave and superfluid states are degenerate, is realized in the system.
We then determine the phase diagram at finite temperatures.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Liberating Efimov physics from three dimensions
When two particles attract via a resonant short-range interaction, three
particles always form an infinite tower of bound states characterized by a
discrete scaling symmetry. It has been considered that this Efimov effect
exists only in three dimensions. Here we review how the Efimov physics can be
liberated from three dimensions by considering two-body and three-body
interactions in mixed dimensions and four-body interaction in one dimension. In
such new systems, intriguing phenomena appear, such as confinement-induced
Efimov effect, Bose-Fermi crossover in Efimov spectrum, and formation of
interlayer Efimov trimers. Some of them are observable in ultracold atom
experiments and we believe that this study significantly broadens our horizons
of universal Efimov physics.Comment: 17 pages, 5 figures, contribution to a special issue of Few-Body
Systems devoted to Efimov Physic
p-wave phase shift and scattering length of Li
We have calculated the p-wave phase shifts and scattering length of Li.
For this we solve the partial wave Schr\"odinger equation and analyze the
validity of adopting the semiclassical solution to evaluate the constant
factors in the solution. Unlike in the wave case, the semiclassical
solution does not provide unique value of the constants. We suggest an
approximate analytic solution, which provides reliable results in special
cases. Further more, we also use the variable phase method to evaluate the
phase shifts. The p-wave scattering lengths of Cs and Cs are
calculated to validate the schemes followed. Based on our calculations, the
value of the wave scattering length of Li is .Comment: 10 figure
Dynamics of positive- and negative-mass solitons in optical lattices and inverted traps
We study the dynamics of one-dimensional solitons in the attractive and
repulsive Bose-Einstein condensates (BECs) loaded into an optical lattice (OL),
which is combined with an external parabolic potential. First, we demonstrate
analytically that, in the repulsive BEC, where the soliton is of the gap type,
its effective mass is \emph{negative}. This gives rise to a prediction for the
experiment: such a soliton cannot be not held by the usual parabolic trap, but
it can be captured (performing harmonic oscillations) by an anti-trapping
inverted parabolic potential. We also study the motion of the soliton a in long
system, concluding that, in the cases of both the positive and negative mass,
it moves freely, provided that its amplitude is below a certain critical value;
above it, the soliton's velocity decreases due to the interaction with the OL.
At a late stage, the damped motion becomes chaotic. We also investigate the
evolution of a two-soliton pulse in the attractive model. The pulse generates a
persistent breather, if its amplitude is not too large; otherwise, fusion into
a single fundamental soliton takes place. Collisions between two solitons
captured in the parabolic trap or anti-trap are considered too. Depending on
their amplitudes and phase difference, the solitons either perform stable
oscillations, colliding indefinitely many times, or merge into a single
soliton. Effects reported in this work for BECs can also be formulated for
optical solitons in nonlinear photonic crystals. In particular, the capture of
the negative-mass soliton in the anti-trap implies that a bright optical
soliton in a self-defocusing medium with a periodic structure of the refractive
index may be stable in an anti-waveguide.Comment: 22pages, 9 figures, submitted to Journal of Physics
- …