44,474 research outputs found
Quantum Simulation of the Hubbard Model: The Attractive Route
We study the conditions under which, using a canonical transformation, the
phases sought after for the repulsive Hubbard model, namely a Mott insulator in
the paramagnetic and anti-ferromagnetic phases, and a putative d-wave
superfluid can be deduced from observations in an optical lattice loaded with a
spin-imbalanced ultra-cold Fermi gas with attractive interactions, thus
realizing the attractive Hubbard model. We show that the Mott insulator and
antiferromagnetic phase of the repulsive Hubbard model are in fact more easy to
observe as a paired, and superfluid phase respectively, in the attractive
Hubbard model. The putative d-wave superfluid phase of the repulsive Hubbard
model doped away from half-filling is related to a d-wave antiferromagnetic
phase for the attractive Hubbard model. We discuss the advantages of this
approach to 'quantum simulate' the Hubbard model in an optical lattice over the
approach that attempts to directly simulate the doped Hubbard model in the
repulsive regime. We also point out a number of technical difficulties of the
proposed approach and, in some cases, suggest possible solutions.Comment: 11 pages, 5 figs. New version as accepted in PRA. We have clarified
the models we are discussing in various places, and expanded on the critical
number estimate to include both K40 and Li6 in section V. Also added
reference
Deconfinement and cold atoms in optical lattices
Despite the fact that by now one dimensional and three dimensional systems of
interacting particles are reasonably well understood, very little is known on
how to go from the one dimensional physics to the three dimensional one. This
is in particular true in a quasi-one dimensional geometry where the hopping of
particles between one dimensional chains or tubes can lead to a dimensional
crossover between a Luttinger liquid and more conventional high dimensional
states. Such a situation is relevant to many physical systems. Recently cold
atoms in optical traps have provided a unique and controllable system in which
to investigate this physics. We thus analyze a system made of coupled one
dimensional tubes of interacting fermions. We explore the observable
consequences, such as the phase diagram for isolated tubes, and the possibility
to realize unusual superfluid phases in coupled tubes systems.Comment: Proceedings of the conference on "Quantum Many Body Theories 13", to
be published by World Scientifi
An Automated Social Graph De-anonymization Technique
We present a generic and automated approach to re-identifying nodes in
anonymized social networks which enables novel anonymization techniques to be
quickly evaluated. It uses machine learning (decision forests) to matching
pairs of nodes in disparate anonymized sub-graphs. The technique uncovers
artefacts and invariants of any black-box anonymization scheme from a small set
of examples. Despite a high degree of automation, classification succeeds with
significant true positive rates even when small false positive rates are
sought. Our evaluation uses publicly available real world datasets to study the
performance of our approach against real-world anonymization strategies, namely
the schemes used to protect datasets of The Data for Development (D4D)
Challenge. We show that the technique is effective even when only small numbers
of samples are used for training. Further, since it detects weaknesses in the
black-box anonymization scheme it can re-identify nodes in one social network
when trained on another.Comment: 12 page
Feshbach resonant scattering of three fermions in one-dimensional wells
We study the weak-tunnelling limit for a system of cold 40K atoms trapped in
a one-dimensional optical lattice close to an s-wave Feshbach resonance. We
calculate the local spectrum for three atoms at one site of the lattice within
a two-channel model. Our results indicate that, for this one-dimensional
system, one- and two-channel models will differ close to the Feshbach
resonance, although the two theories would converge in the limit of strong
Feshbach coupling. We also find level crossings in the low-energy spectrum of a
single well with three atoms that may lead to quantum phase transition for an
optical lattice of many wells. We discuss the stability of the system to a
phase with non-uniform density.Comment: 10 pages, 5 figure
Pseudo-gap features of intrinsic tunneling in (HgBr_2)-Bi2212 single crystals
The c-axis tunneling properties of both pristine Bi2212 and its HgBr
intercalate have been measured in the temperature range 4.2 - 250 K.
Lithographically patterned 7-10 unit-cell heigh mesa structures on the surfaces
of these single crystals were investigated. Clear SIS-like tunneling curves for
current applied in the -axis direction have been observed. The dynamic
conductance dd shows both sharp peaks corresponding to a
superconducting gap edge and a dip feature beyond the gap, followed by a wide
maximum, which persists up to a room temperature. Shape of the temperature
dependence of the {\it c}-axis resistance does not change after the
intercalation suggesting that a coupling between -bilayers has
little effect on the pseudogap.Comment: 6 pages, 5 figures; presented at the Second Int Conf. New3Sc-1999
(Las Vegas, NV
Silicon solar cell development for low temperature and low illumination intensity operation, volume 1 Analysis report, 30 Jun. 1969 - 30 Apr. 1970
Operational performance of solar cell at low temperatures and under low illumination intensit
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