49 research outputs found
Revealing Superfluid--Mott-Insulator Transition in an Optical Lattice
We study (by an exact numerical scheme) the single-particle density matrix of
ultracold atoms in an optical lattice with a parabolic confining
potential. Our simulation is directly relevant to the interpretation and
further development of the recent pioneering experiment by Greiner et al. In
particular, we show that restructuring of the spatial distribution of the
superfluid component when a domain of Mott-insulator phase appears in the
system, results in a fine structure of the particle momentum distribution. This
feature may be used to locate the point of the superfluid--Mott-insulator
transition.Comment: 4 pages (12 figures), Latex. (A Latex macro is corrected
Linear and nonlinear susceptibilities of a decoherent two-level system
The linear and nonlinear dynamical susceptibilities of a two level system are
calculated as it undergoes a transition to a decoherent state. Analogously to
the Glover-Tinkham-Ferrell sum rule of superconductivity, spectral weight in
the linear susceptibility is continuously transferred from a finite frequency
resonance to nearly zero frequency, corresponding to a broken symmetry in the
thermodynamic limit. For this reason, the behavior of the present model (the
Mermin model) differs significantly from the spin-boson model. The third order
nonlinear susceptibility, corresponding to two-photon absorption, has an
unexpected non-monotonic behavior as a function of the environmental coupling,
reaching a maximum within the decoherent phase of the model. Both linear and
nonlinear susceptibilities may be expressed in a universal form.Comment: 10 pages, 9 figure
Suppression of Quantum Phase Interference in Molecular Magnets Fe₈ with Dipolar-Dipolar Interaction
Renormalized tunnel splitting with a finite distribution in the biaxial spin
model for molecular magnets is obtained by taking into account the dipolar
interaction of enviromental spins. Oscillation of the resonant tunnel splitting
with a transverse magnetic field along the hard axis is smeared by the finite
distribution which subsequently affects the quantum steps of hysteresis curve
evaluated in terms of the modified Landau-Zener model of spin flipping induced
by the sweeping field. We conclude that the dipolar-dipolar interaction drives
decoherence of quantum tunnelling in molcular magnets Fe₈, which explains
why the quenching points of tunnel spliting between odd and even resonant
tunnelling predcited theoretically were not observed experimentally.Comment: 5 pages including 3 figure and 1 table. To appear in Physical Review
'Hole-digging' in ensembles of tunneling Molecular Magnets
The nuclear spin-mediated quantum relaxation of ensembles of tunneling
magnetic molecules causes a 'hole' to appear in the distribution of internal
fields in the system. The form of this hole, and its time evolution, are
studied using Monte Carlo simulations. It is shown that the line-shape of the
tunneling hole in a weakly polarised sample must have a Lorentzian lineshape-
the short-time half-width in all experiments done so far should be
, the half-width of the nuclear spin multiplet. After a time
, the single molecule tunneling relaxation time, the hole width begins
to increase rapidly. In initially polarised samples the disintegration of
resonant tunneling surfaces is found to be very fast.Comment: 4 pages, 5 figure
Self-Trapping of Polarons in the Rashba-Pekar Model
We performed quantum Monte Carlo study of the exciton-polaron model which
features the self-trapping phenomenon when the coupling strength and/or
particle momentum is varied. For the first time accurate data for energy,
effective mass, the structure of the polaronic cloud, dispersion law, and
spectral function are available throughout the crossover region. We observed
that self-trapping can not be reduced to hybridization of two states with
different lattice deformation, and that at least three states are involved in
the crossover from light- to heavy-mass regimes.Comment: 5 pages, 5 figures, Accepted to Phys. Rev. B Rapid Communication