1,077 research outputs found
Ultracold atoms in optical lattices
Bosonic atoms trapped in an optical lattice at very low temperatures, can be
modeled by the Bose-Hubbard model. In this paper, we propose a slave-boson
approach for dealing with the Bose-Hubbard model, which enables us to
analytically describe the physics of this model at nonzero temperatures. With
our approach the phase diagram for this model at nonzero temperatures can be
quantified.Comment: 29 pages, 10 figure
Aggregation of ecological indicators for mapping aquatic nature quality : overview of existing methods and case studies
Indicators for aquatic nature quality are calculated using ecological monitoring data from individual sampling stations. For reporting purposes, these results need to be aggregated and scaled up to higher levels (catchment area, country). This report provides an overview of different existing spatial aggregation methods for this purpose, including an evaluation of their suitability for aquatic ecological indicators. So-called „model-based„ methods, consisting of some sort of „kriging¿ step followed by calculation of the arithmetic mean, appeared to be the most appropriate. Application of these methods to multimetric indicators of aquatic macroinvertebrates in two Dutch subcatchment areas confirmed their suitability. However, the methods that were used were based on aggregation (using kriging) over Euclidian (straight), distances. It is recommended to conduct further research on the suitability of interpolation through stream networks, i.e., through the waterways themselves
Quantum phases in a resonantly-interacting Bose-Fermi mixture
We consider a resonantly-interacting Bose-Fermi mixture of K and
Rb atoms in an optical lattice. We show that by using a red-detuned
optical lattice the mixture can be accurately described by a generalized
Hubbard model for K and Rb atoms, and K-Rb
molecules. The microscopic parameters of this model are fully determined by the
details of the optical lattice and the interspecies Feshbach resonance in the
absence of the lattice. We predict a quantum phase transition to occur in this
system already at low atomic filling fraction, and present the phase diagram as
a function of the temperature and the applied magnetic field.Comment: 4 pages, 3 figure
Modelling and experiments of self-reflectivity under femtosecond ablation conditions
We present a numerical model which describes the propagation of a single
femtosecond laser pulse in a medium of which the optical properties dynamically
change within the duration of the pulse. We use a Finite Difference Time Domain
(FDTD) method to solve the Maxwell's equations coupled to equations describing
the changes in the material properties. We use the model to simulate the
self-reflectivity of strongly focused femtosecond laser pulses on silicon and
gold under laser ablation condition. We compare the simulations to experimental
results and find excellent agreement.Comment: 11 pages, 8 figure
Mean-field theory for Bose-Hubbard Model under a magnetic field
We consider the superfluid-insulator transition for cold bosons under an
effective magnetic field. We investigate how the applied magnetic field affects
the Mott transition within mean field theory and find that the critical hopping
strength , increases with the applied field. The increase in the
critical hopping follows the bandwidth of the Hofstadter butterfly at the given
value of the magnetic field. We also calculate the magnetization and superfluid
density within mean field theory.Comment: 11 pages, 7 figures, published versio
Mott insulators in an optical lattice with high filling factors
We discuss the superfluid to Mott insulator transition of an atomic Bose gas
in an optical lattice with high filling factors. We show that also in this
multi-band situation, the long-wavelength physics is described by a single-band
Bose-Hubbard model. We determine the many-body renormalization of the tunneling
and interaction parameters in the effective Bose-Hubbard Hamiltonian, and
consider the resulting model at nonzero temperatures. We show that in
particular for a one or two-dimensional optical lattice, the Mott insulator
phase is more difficult to realize than anticipated previously.Comment: 5 pages, 3 figures, title changed, major restructuring, resubmitted
to PR
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