78 research outputs found
ElfStore: A Resilient Data Storage Service for Federated Edge and Fog Resources
Edge and fog computing have grown popular as IoT deployments become
wide-spread. While application composition and scheduling on such resources are
being explored, there exists a gap in a distributed data storage service on the
edge and fog layer, instead depending solely on the cloud for data persistence.
Such a service should reliably store and manage data on fog and edge devices,
even in the presence of failures, and offer transparent discovery and access to
data for use by edge computing applications. Here, we present Elfstore, a
first-of-its-kind edge-local federated store for streams of data blocks. It
uses reliable fog devices as a super-peer overlay to monitor the edge
resources, offers federated metadata indexing using Bloom filters, locates data
within 2-hops, and maintains approximate global statistics about the
reliability and storage capacity of edges. Edges host the actual data blocks,
and we use a unique differential replication scheme to select edges on which to
replicate blocks, to guarantee a minimum reliability and to balance storage
utilization. Our experiments on two IoT virtual deployments with 20 and 272
devices show that ElfStore has low overheads, is bound only by the network
bandwidth, has scalable performance, and offers tunable resilience.Comment: 24 pages, 14 figures, To appear in IEEE International Conference on
Web Services (ICWS), Milan, Italy, 201
Stochastic Mean-Field Theory for the Disordered Bose-Hubbard Model
We investigate the effect of diagonal disorder on bosons in an optical
lattice described by an Anderson-Hubbard model at zero temperature. It is known
that within Gutzwiller mean-field theory spatially resolved calculations suffer
particularly from finite system sizes in the disordered case, while arithmetic
averaging of the order parameter cannot describe the Bose glass phase for
finite hopping . Here we present and apply a new \emph{stochastic}
mean-field theory which captures localization due to disorder, includes
non-trivial dimensional effects beyond the mean-field scaling level and is
applicable in the thermodynamic limit. In contrast to fermionic systems, we
find the existence of a critical hopping strength, above which the system
remains superfluid for arbitrarily strong disorder.Comment: 6 pages, 6 figure
Mott-Hubbard Transition of Bosons in Optical Lattices with Three-body Interactions
In this paper, the quantum phase transition between superfluid state and
Mott-insulator state is studied based on an extended Bose-Hubbard model with
two- and three-body on-site interactions. By employing the mean-field
approximation we find the extension of the insulating 'lobes' and the existence
of a fixed point in three dimensional phase space. We investigate the link
between experimental parameters and theoretical variables. The possibility to
obverse our results through some experimental effects in optically trapped
Bose-Einstein Condensates(BEC) is also discussed.Comment: 7 pages, 4 figures; to be appear in Phys. Rev.
Some remarks on the coherent-state variational approach to nonlinear boson models
The mean-field pictures based on the standard time-dependent variational
approach have widely been used in the study of nonlinear many-boson systems
such as the Bose-Hubbard model. The mean-field schemes relevant to
Gutzwiller-like trial states , number-preserving states and
Glauber-like trial states are compared to evidence the specific
properties of such schemes. After deriving the Hamiltonian picture relevant to
from that based on , the latter is shown to exhibit a Poisson
algebra equipped with a Weyl-Heisenberg subalgebra which preludes to the
-based picture. Then states are shown to be a superposition of -boson states and the similarities/differences of the -based and
-based pictures are discussed. Finally, after proving that the simple,
symmetric state indeed corresponds to a SU(M) coherent state, a dual
version of states and in terms of momentum-mode operators is
discussed together with some applications.Comment: 16 page
Glassy features of a Bose Glass
We study a two-dimensional Bose-Hubbard model at a zero temperature with
random local potentials in the presence of either uniform or binary disorder.
Many low-energy metastable configurations are found with virtually the same
energy as the ground state. These are characterized by the same blotchy pattern
of the, in principle, complex nonzero local order parameter as the ground
state. Yet, unlike the ground state, each island exhibits an overall random
independent phase. The different phases in different coherent islands could
provide a further explanation for the lack of coherence observed in experiments
on Bose glasses.Comment: 14 pages, 4 figures
Phase diagram of the Bose-Hubbard Model on Complex Networks
Critical phenomena can show unusual phase diagrams when defined in complex
network topologies. The case of classical phase transitions such as the
classical Ising model and the percolation transition has been studied
extensively in the last decade. Here we show that the phase diagram of the
Bose-Hubbard model, an exclusively quantum mechanical phase transition, also
changes significantly when defined on random scale-free networks. We present a
mean-field calculation of the model in annealed networks and we show that when
the second moment of the average degree diverges the Mott-insulator phase
disappears in the thermodynamic limit. Moreover we study the model on quenched
networks and we show that the Mott-insulator phase disappears in the
thermodynamic limit as long as the maximal eigenvalue of the adjacency matrix
diverges. Finally we study the phase diagram of the model on Apollonian
scale-free networks that can be embedded in 2 dimensions showing the extension
of the results also to this case.Comment: (6 pages, 4 figures
Atomic Bose-Fermi mixtures in an optical lattice
A mixture of ultracold bosons and fermions placed in an optical lattice
constitutes a novel kind of quantum gas, and leads to phenomena, which so far
have been discussed neither in atomic physics, nor in condensed matter physics.
We discuss the phase diagram at low temperatures, and in the limit of strong
atom-atom interactions, and predict the existence of quantum phases that
involve pairing of fermions with one or more bosons, or, respectively, bosonic
holes. The resulting composite fermions may form, depending on the system
parameters, a normal Fermi liquid, a density wave, a superfluid liquid, or an
insulator with fermionic domains. We discuss the feasibility for observing such
phases in current experiments.Comment: 4 pages, 1 eps figure, misprints correcte
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
Simulation of gauge transformations on systems of ultracold atoms
We show that gauge transformations can be simulated on systems of ultracold
atoms. We discuss observables that are invariant under these gauge
transformations and compute them using a tensor network ansatz that escapes the
phase problem. We determine that the Mott-insulator-to-superfluid critical
point is monotonically shifted as the induced magnetic flux increases. This
result is stable against the inclusion of a small amount of entanglement in the
variational ansatz.Comment: 14 pages, 6 figure
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
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