54 research outputs found
Bosons condensed in two modes with flavour-changing interaction
A quantum model is considered for bosons populating two orthogonal
single-particle modes with tunable energy separation in the presence of
flavour-changing contact interaction. The quantum ground state is well
approximated as a coherent superposition (for zero temperature) or a mixture
(at low temperature) of two quasi-classical states. In a mean field
description, the systems realizes one of these states via spontaneous symmetry
breaking. Both mean field states, in a certain parameter range, possess finite
angular momentum and exhibit broken time-reversal symmetry in contrast to the
quantum ground state. The phase diagram is explored at the mean-field level and
by direct diagonalisation. The nature of the quantum ground state at zero and
finite temperature is analyzed by means of the Penrose Onsager criterion. One
of three possible phases shows fragmentation on the single-particle level
together with a finite pair order parameter. Thermal and quantum fluctuations
are characterized with respect to regions of universal scaling behavior. The
non-equilibrium dynamics shows a sharp transition between a self-trapping and a
pair-tunneling regime. A recently realized experimental implementation is
discussed with bosonic atoms condensed in the two inequivalent energy minima
of the second band of a bipartite two-dimensional optical lattice.Comment: 12 pages, 12 figures, extended and revise
Effective time-independent description of optical lattices with periodic driving
For a periodically driven quantum system an effective time-independent
Hamiltonian is derived with an eigen-energy spectrum, which in the regime of
large driving frequencies approximates the quasi-energies of the corresponding
Floquet Hamiltonian. The effective Hamiltonian is evaluated for the case of
optical lattice models in the tight-binding regime subjected to strong periodic
driving. Three scenarios are considered: a periodically shifted one-dimensional
(1D) lattice, a two-dimensional (2D) square lattice with inversely phased
temporal modulation of the well depths of adjacent lattice sites, and a 2D
lattice subjected to an array of microscopic rotors commensurate with its
plaquette structure. In case of the 1D scenario the rescaling of the tunneling
energy, previously considered by Eckardt et al. in Phys. Rev. Lett. 95, 260404
(2005), is reproduced. The 2D lattice with well depth modulation turns out as a
generalization of the 1D case. In the 2D case with staggered rotation, the
expression previously found in the case of weak driving by Lim et al. in Phys.
Rev. Lett. 100, 130402 (2008) is generalized, such that its interpretation in
terms of an artificial staggered magnetic field can be extended into the regime
of strong driving.Comment: 10 pages, 5 figure
Artificial Staggered Magnetic Field for Ultracold Atoms in Optical Lattices
A time-dependent optical lattice with staggered particle current in the
tight-binding regime was considered that can be described by a time-independent
effective lattice model with an artificial staggered magnetic field. The low
energy description of a single-component fermion in this lattice at
half-filling is provided by two copies of ideal two-dimensional massless Dirac
fermions. The Dirac cones are generally anisotropic and can be tuned by the
external staggered flux \p. For bosons, the staggered flux modifies the
single-particle spectrum such that in the weak coupling limit, depending on the
flux \p, distinct superfluid phases are realized. Their properties are
discussed, the nature of the phase transitions between them is establised, and
Bogoliubov theory is used to determine their excitation spectra. Then the
generalized superfluid-Mott-insulator transition is studied in the presence of
the staggered flux and the complete phase diagram is established. Finally, the
momentum distribution of the distinct superfluid phases is obtained, which
provides a clear experimental signature of each phase in ballistic expansion
experiments.Comment: 14 pages, 5 figure
Magnetic Trapping of Metastable Calcium Atoms
Metastable calcium atoms, produced in a magneto-optic trap (MOT) operating
within the singlet system, are continuously loaded into a magnetic trap formed
by the magnetic quadrupole field of the MOT. At MOT temperatures of 3 mK and
240 ms loading time we observe 1.1 x 10^8 magnetically trapped 3P2 atoms at
densities of 2.4 x 10^8 cm^-3 and temperatures of 0.61 mK. In a modified scheme
we first load a MOT for metastable atoms at a temperature of 0.18 mK and
subsequently release these atoms into the magnetic trap. In this case 240 ms of
loading yields 2.4 x 10^8 trapped 3P2 atoms at a peak density of 8.7 x 10^10
cm^-3 and a temperature of 0.13 mK. The temperature decrease observed in the
magnetic trap for both loading schemes can be explained only in part by trap
size effects.Comment: 4 figure
Optimizing the production of metastable calcium atoms in a magneto-optical trap
We investigate the production of long lived metastable (3P2, n=4) calcium
atoms in a magneto-optical trap operating on the 1S0 to 1P1 transition at 423
nm. For excited 1P1-atoms a weak decay channel into the triplet states 3P2 and
3P1 exists via the singlet 1D2 (n=3) state. The undesired 3P1-atoms decay back
to the ground state within 0.4 ms and can be fully recaptured if the
illuminated trap volume is sufficiently large. We obtain a flux of above 10^10
atoms/s into the 3P2-state. We find that our MOT life time of 23 ms is mainly
limited by this loss channel and thus the 3P2-production is not hampered by
inelasic collisions. If we close the loss channel by repumping the 1D2-atoms
with a 671 nm laser back into the MOT cycling transition, a non-exponential 72
ms trap decay is observed indicating the presence of inelastic two-body
collisions between 1S0 and 1P1 atoms.Comment: 4 pages incl. 3 figures, submitted to Applied Physics
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