8,256 research outputs found
Decay of super-currents in condensates in optical lattices
In this paper we discuss decay of superfluid currents in boson lattice
systems due to quantum tunneling and thermal activation mechanisms. We derive
asymptotic expressions for the decay rate near the critical current in two
regimes, deep in the superfluid phase and close to the superfluid-Mott
insulator transition. The broadening of the transition at the critical current
due to these decay mechanisms is more pronounced at lower dimensions. We also
find that the crossover temperature below which quantum decay dominates is
experimentally accessible in most cases. Finally, we discuss the dynamics of
the current decay and point out the difference between low and high currents.Comment: Contribution to the special issue of Journal of Superconductivity in
honor of Michael Tinkham's 75th birthda
Superfluid-insulator transition in a moving system of interacting bosons
We analyze stability of superfluid currents in a system of strongly
interacting ultra-cold atoms in an optical lattice. We show that such a system
undergoes a dynamic, irreversible phase transition at a critical phase gradient
that depends on the interaction strength between atoms. At commensurate
filling, the phase boundary continuously interpolates between the classical
modulation instability of a weakly interacting condensate and the equilibrium
quantum phase transition into a Mott insulator state at which the critical
current vanishes. We argue that quantum fluctuations smear the transition
boundary in low dimensional systems. Finally we discuss the implications to
realistic experiments.Comment: updated refernces and introduction, minor correction
Abel-Jacobi maps for hypersurfaces and non commutative Calabi-Yau's
It is well known that the Fano scheme of lines on a cubic 4-fold is a
symplectic variety. We generalize this fact by constructing a closed p-form
with p=2n-4 on the Fano scheme of lines on a (2n-2)-dimensional hypersurface Y
of degree n. We provide several definitions of this form - via the Abel-Jacobi
map, via Hochschild homology, and via the linkage class, and compute it
explicitly for n = 4. In the special case of a Pfaffian hypersurface Y we show
that the Fano scheme is birational to a certain moduli space of sheaves on a
p-dimensional Calabi--Yau variety X arising naturally in the context of
homological projective duality, and that the constructed form is induced by the
holomorphic volume form on X. This remains true for a general non Pfaffian
hypersurface but the dual Calabi-Yau becomes non commutative.Comment: 34 pages; exposition of Hochschild homology expanded; references
added; introduction re-written; some imrecisions, typos and the orbit diagram
in the last section correcte
Exchange bias and interface electronic structure in Ni/Co3O4(011)
A detailed study of the exchange bias effect and the interfacial electronic
structure in Ni/Co3O4(011) is reported. Large exchange anisotropies are
observed at low temperatures, and the exchange bias effect persists to
temperatures well above the Neel temperature of bulk Co3O4, of about 40 K: to
~80 K for Ni films deposited on well ordered oxide surfaces, and ~150 K for Ni
films deposited on rougher Co3O4 surfaces. Photoelectron spectroscopy
measurements as a function of Ni thickness show that Co reduction and Ni
oxidation occur over an extended interfacial region. We conclude that the
exchange bias observed in Ni/Co3O4, and in similar ferromagnetic metallic/Co3O4
systems, is not intrinsic to Co3O4 but rather due to the formation of CoO at
the interface.Comment: 8 pages, 6 figures. Accepted for publication in Physical Review B
Lifetime of double occupancies in the Fermi-Hubbard model
We investigate the decay of artificially created double occupancies in a
repulsive Fermi-Hubbard system in the strongly interacting limit using
diagrammatic many-body theory and experiments with ultracold fermions on
optical lattices. The lifetime of the doublons is found to scale exponentially
with the ratio of the on-site repulsion to the bandwidth. We show that the
dominant decay process in presence of background holes is the excitation of a
large number of particle hole pairs to absorb the energy of the doublon. We
also show that the strongly interacting nature of the background state is
crucial in obtaining the correct estimate of the doublon lifetime in these
systems. The theoretical estimates and the experimental data are in fair
quantitative agreement
Complete moduli of cubic threefolds and their intermediate Jacobians
The intermediate Jacobian map, which associates to a smooth cubic threefold
its intermediate Jacobian, does not extend to the GIT compactification of the
space of cubic threefolds, not even as a map to the Satake compactification of
the moduli space of principally polarized abelian fivefolds. A much better
"wonderful" compactification of the space of cubic threefolds was constructed
by the first and fourth authors --- it has a modular interpretation, and
divisorial normal crossing boundary. We prove that the intermediate Jacobian
map extends to a morphism from the wonderful compactification to the second
Voronoi toroidal compactification of the moduli of principally polarized
abelian fivefolds --- the first and fourth author previously showed that it
extends to the Satake compactification. Since the second Voronoi
compactification has a modular interpretation, our extended intermediate
Jacobian map encodes all of the geometric information about the degenerations
of intermediate Jacobians, and allows for the study of the geometry of cubic
threefolds via degeneration techniques. As one application we give a complete
classification of all degenerations of intermediate Jacobians of cubic
threefolds of torus rank 1 and 2.Comment: 56 pages; v2: multiple updates and clarification in response to
detailed referee's comment
Noise Correlations of Hard-core Bosons: Quantum Coherence and Symmetry Breaking
Noise correlations, such as those observable in the time of flight images of
a released cloud, are calculated for hard-core bosonic (HCB) atoms. We find
that the standard mapping of HCB systems onto spin-1/2 XY models fails in
application to computation of noise correlations due to the contribution of
multiply occupied virtual states in HCB systems. Such states do not exist in
spin models. An interesting manifestation of such states is the breaking of
particle-hole symmetry in HCB. We use noise correlations to explore quantum
coherence of strongly correlated bosons in the fermionized regime with and
without external parabolic confinement. Our analysis points to distinctive new
experimental signatures of the Mott phase.Comment: 17 pages, 6 figures. This is a detailed revised version of
quant-ph/0507153. It has been submitted to Journal of Physics B: the special
edition for the Cortona BEC worksho
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
Tools for loading MEDLINE into a local relational database
BACKGROUND: Researchers who use MEDLINE for text mining, information extraction, or natural language processing may benefit from having a copy of MEDLINE that they can manage locally. The National Library of Medicine (NLM) distributes MEDLINE in eXtensible Markup Language (XML)-formatted text files, but it is difficult to query MEDLINE in that format. We have developed software tools to parse the MEDLINE data files and load their contents into a relational database. Although the task is conceptually straightforward, the size and scope of MEDLINE make the task nontrivial. Given the increasing importance of text analysis in biology and medicine, we believe a local installation of MEDLINE will provide helpful computing infrastructure for researchers. RESULTS: We developed three software packages that parse and load MEDLINE, and ran each package to install separate instances of the MEDLINE database. For each installation, we collected data on loading time and disk-space utilization to provide examples of the process in different settings. Settings differed in terms of commercial database-management system (IBM DB2 or Oracle 9i), processor (Intel or Sun), programming language of installation software (Java or Perl), and methods employed in different versions of the software. The loading times for the three installations were 76 hours, 196 hours, and 132 hours, and disk-space utilization was 46.3 GB, 37.7 GB, and 31.6 GB, respectively. Loading times varied due to a variety of differences among the systems. Loading time also depended on whether data were written to intermediate files or not, and on whether input files were processed in sequence or in parallel. Disk-space utilization depended on the number of MEDLINE files processed, amount of indexing, and whether abstracts were stored as character large objects or truncated. CONCLUSIONS: Relational database (RDBMS) technology supports indexing and querying of very large datasets, and can accommodate a locally stored version of MEDLINE. RDBMS systems support a wide range of queries and facilitate certain tasks that are not directly supported by the application programming interface to PubMed. Because there is variation in hardware, software, and network infrastructures across sites, we cannot predict the exact time required for a user to load MEDLINE, but our results suggest that performance of the software is reasonable. Our database schemas and conversion software are publicly available at
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