80 research outputs found
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
Quantum entanglement of spin-1 bosons with coupled ground states in optical lattices
We examine particle entanglement, characterized by pseudo-spin squeezing, of
spin-1 bosonic atoms with coupled ground states in a one-dimensional optical
lattice. Both the superfluid and Mott-insulator phases are investigated
separately for ferromagnetic and antiferromagnetic interactions. Mode
entanglement is also discussed in the Mott insulating phase. The role of a
small but nonzero angle between the polarization vectors of counter-propagating
lasers forming the optical lattice on quantum correlations is investigated as
well.Comment: 18 pages, 8 figures. To be published in Journal of Physics
Effect of next-nearest-neighbour hopping on Bose–Einstein condensation in optical lattices
Model Hierarchies for Understanding Atmospheric Circulation
This is the final version. Available from Wiley via the DOI in this record.In this review, we highlight the complementary relationship between simple and comprehensive models in addressing key scientific questions to describe Earth’s atmospheric circulation. The systematic
representation of models in steps, or hierarchies, connects our understanding from idealized systems
to comprehensive models, and ultimately the observed atmosphere. We define three interconnected
principles that can be used to characterize the model hierarchies of the atmosphere. We explore
the rich diversity within the governing equations in the dynamical hierarchy, the ability to isolate
and understand atmospheric processes in the process hierarchy, and the importance of the physical
domain and resolution in the hierarchy of scale.
We center our discussion on the large scale circulation of the atmosphere and its interaction with
clouds and convection, focusing on areas where simple models have had a significant impact. Our
confidence in climate model projections of the future is based on our efforts to ground the climate
predictions in fundamental physical understanding. This understanding is, in part, possible due to
the hierarchies of idealized models that afford the simplicity required for understanding complex
systems.Natural Environment Research Council (NERC)US National Science FoundationUS Department of Energy Office of Biological and Environmental ResearchNatural Science and Engineering Research Council of CanadaAustralian Research CouncilSimons FoundationGerman Ministry of Education and Research (BMBF)FONA: Research for Sustainable DevelopmentState Research Agency of Spai
Thermometry with spin-dependent lattices
We propose a method for measuring the temperature of strongly correlated
phases of ultracold atom gases confined in spin-dependent optical lattices. In
this technique, a small number of "impurity" atoms--trapped in a state that
does not experience the lattice potential--are in thermal contact with atoms
bound to the lattice. The impurity serves as a thermometer for the system
because its temperature can be straightforwardly measured using time-of-flight
expansion velocity. This technique may be useful for resolving many open
questions regarding thermalization in these isolated systems. We discuss the
theory behind this method and demonstrate proof-of-principle experiments,
including the first realization of a 3D spin-dependent lattice in the strongly
correlated regime.Comment: 22 pages, 8 figures v2: Several references added; Section on heating
rates updated to include dipole fluctuation terms; Section added on the
limitations of the proposed method. To appear in New Journal of Physic
Mean-field phase diagram of disordered bosons in a lattice at non-zero temperature
Bosons in a periodic lattice with on-site disorder at low but non-zero
temperature are considered within a mean-field theory. The criteria used for
the definition of the superfluid, Mott insulator and Bose glass are analysed.
Since the compressibility does never vanish at non-zero temperature, it can not
be used as a general criterium. We show that the phases are unambiguously
distinguished by the superfluid density and the density of states of the
low-energy exitations. The phase diagram of the system is calculated. It is
shown that even a tiny temperature leads to a significant shift of the boundary
between the Bose glass and superfluid
Phase diagram of two-component bosons on an optical lattice
We present a theoretical analysis of the phase diagram of two--component
bosons on an optical lattice. A new formalism is developed which treats the
effective spin interactions in the Mott and superfluid phases on the same
footing. Using the new approach we chart the phase boundaries of the broken
spin symmetry states up to the Mott to superfluid transition and beyond. Near
the transition point, the magnitude of spin exchange can be very large, which
facilitates the experimental realization of spin-ordered states. We find that
spin and quantum fluctuations have a dramatic effect on the transition making
it first order in extended regions of the phase diagram. For Mott states with
even occupation we find that the competition between effective Heisenberg
exchange and spin-dependent on--site interaction leads to an additional phase
transition from a Mott insulator with no broken symmetries into a spin-ordered
insulator
Vortex Pinning and the Non-Hermitian Mott Transition
The boson Hubbard model has been extensively studied as a model of the zero
temperature superfluid/insulator transition in Helium-4 on periodic substrates.
It can also serve as a model for vortex lines in superconductors with a
magnetic field parallel to a periodic array of columnar pins, due to a formal
analogy between the vortex lines and the statistical mechanics of quantum
bosons. When the magnetic field has a component perpendicular to the pins, this
analogy yields a non-Hermitian boson Hubbard model. At integer filling, we find
that for small transverse fields, the insulating phase is preserved, and the
transverse field is exponentially screened away from the boundaries of the
superconductor. At larger transverse fields, a ``superfluid'' phase of tilted,
entangled vortices appears. The universality class of the transition is found
to be that of vortex lines entering the Meissner phase at H_{c1}, with the
additional feature that the direction of the tilted vortices at the transition
bears a non-trivial relationship to the direction of the applied magnetic
field. The properties of the Mott Insulator and flux liquid phases with tilt
are also discussed.Comment: 20 pages, 12 figures included in text; to appear in Physical Review
Electrocrystallization of zinc on copper single crystal planes under the influence of an asymmetric alternating current
Mixtures of Bosonic and Fermionic Atoms in Optical Lattices
We discuss the theory of mixtures of Bosonic and Fermionic atoms in periodic
potentials at zero temperature. We derive a general Bose--Fermi Hubbard
Hamiltonian in a one--dimensional optical lattice with a superimposed harmonic
trapping potential. We study the conditions for linear stability of the mixture
and derive a mean field criterion for the onset of a Bosonic superfluid
transition. We investigate the ground state properties of the mixture in the
Gutzwiller formulation of mean field theory, and present numerical studies of
finite systems. The Bosonic and Fermionic density distributions and the onset
of quantum phase transitions to demixing and to a Bosonic Mott--insulator are
studied as a function of the lattice potential strength. The existence is
predicted of a disordered phase for mixtures loaded in very deep lattices. Such
a disordered phase possessing many degenerate or quasi--degenerate ground
states is related to a breaking of the mirror symmetry in the lattice.Comment: 11 pages, 8 figures; added discussions; conclusions and references
expande
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