421 research outputs found
Magnetic Phase Transition of the Perovskite-type Ti Oxides
Properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state is hardly changed so that the spin-exchange coupling along the
c-axis changes nearly continuously from positive to negative and takes
approximately zero at the phase boundary. The resultant strong
two-dimensionality in the spin coupling causes a rapid suppression of the
critical temperature as is observed experimentally.Comment: 9 pages, 5 figure
Response of an atomic Bose-Einstein condensate to a rotating elliptical trap
We investigate numerically the response of an atomic Bose-Einstein condensate
to a weakly-elliptical rotating trap over a large range of rotation
frequencies. We analyse the quadrupolar shape oscillation excited by rotation,
and discriminate between its stable and unstable regimes. In the latter case,
where a vortex lattice forms, we compare with experimental observations and
find good agreement. By examining the role of thermal atoms in the process, we
infer that the process is temperature-independent, and show how terminating the
rotation gives control over the number of vortices in the lattice. We also
study the case of critical rotation at the trap frequency, and observe large
centre-of-mass oscillations of the condensate.Comment: 14 pages, 8 figure
Theory of Feshbach molecule formation in a dilute gas during a magnetic field ramp
Starting with coupled atom-molecule Boltzmann equations, we develop a
simplified model to understand molecule formation observed in recent
experiments. Our theory predicts several key features: (1) the effective
adiabatic rate constant is proportional to density; (2) in an adiabatic ramp,
the dependence of molecular fraction on magnetic field resembles an error
function whose width and centroid are related to the temperature; (3) the
molecular production efficiency is a universal function of the initial phase
space density, the specific form of which we derive for a classical gas. Our
predictions show qualitative agreement with the data from [Hodby et al, Phys.
Rev. Lett. {\bf{94}}, 120402 (2005)] without the use of adjustable parameters
Rate limit for photoassociation of a Bose-Einstein condensate
We simulate numerically the photodissociation of molecules into noncondensate
atom pairs that accompanies photoassociation of an atomic Bose-Einstein
condensate into a molecular condensate. Such rogue photodissociation sets a
limit on the achievable rate of photoassociation. Given the atom density \rho
and mass m, the limit is approximately 6\hbar\rho^{2/3}/m. At low temperatures
this is a more stringent restriction than the unitary limit of scattering
theory.Comment: 5 pgs, 18 refs., 3 figs., submitted to Phys. Rev. Let
Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach
The properties and mechanism of the magnetic phase transition of the
perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle
distortion, are studied theoretically by using the effective spin and
pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the
A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs
as the Ti-O-Ti bond angle is decreased. Through this phase transition, the
orbital state changes only little whereas the spin-exchange coupling along the
c-axis is expected to change from positive to negative nearly continuously and
approaches zero at the phase boundary. The resultant strong two-dimensionality
in the spin coupling causes rapid suppression of the critical temperature, as
observed experimentally. It may induce large quantum fluctuations in this
region.Comment: 13 pages, 15 figure
Luttinger model approach to interacting one-dimensional fermions in a harmonic trap
A model of interacting one--dimensional fermions confined to a harmonic trap
is proposed. The model is treated analytically to all orders of the coupling
constant by a method analogous to that used for the Luttinger model. As a first
application, the particle density is evaluated and the behavior of Friedel
oscillations under the influence of interactions is studied. It is found that
attractive interactions tend to suppress the Friedel oscillations while strong
repulsive interactions enhance the Friedel oscillations significantly. The
momentum distribution function and the relation of the model interaction to
realistic pair interactions are also discussed.Comment: 12 pages latex, 1 eps-figure in 1 tar file, extended Appendix, added
and corrected references, new eq. (53), corrected typos, accepted for PR
Free Expansion of a Weakly-interacting Dipolar Fermi Gas
We theoretically investigate a polarized dipolar Fermi gas in free expansion.
The inter-particle dipolar interaction deforms phase-space distribution in trap
and also in the expansion. We exactly predict the minimal quadrupole
deformation in the expansion for the high-temperature Maxwell-Boltzmann and
zero-temperature Thomas-Fermi gases in the Hartree-Fock and Landau-Vlasov
approaches. In conclusion, we provide a proper approach to develop the
time-of-flight method for the weakly-interacting dipolar Fermi gas and also
reveal a scaling law associated with the Liouville's theorem in the long-time
behaviors of the both gases
Feshbach resonances in ultracold K(39)
We discover several magnetic Feshbach resonances in collisions of ultracold
K(39) atoms, by studying atom losses and molecule formation. Accurate
determination of the magnetic-field resonance locations allows us to optimize a
quantum collision model for potassium isotopes. We employ the model to predict
the magnetic-field dependence of scattering lengths and of near-threshold
molecular levels. Our findings will be useful to plan future experiments on
ultracold potassium atoms and molecules.Comment: 7 pages, 6 figure
Ferromagnetism in a lattice of Bose condensates
We show that an ensemble of spinor Bose-Einstein condensates confined in a
one dimensional optical lattice can undergo a ferromagnetic phase transition
and spontaneous magnetization arises due to the magnetic dipole-dipole
interaction. This phenomenon is analogous to ferromagnetism in solid state
physics, but occurs with bosons instead of fermions.Comment: 4 pages, 2 figure
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