944 research outputs found
Chaoticity without thermalisation in disordered lattices
We study chaoticity and thermalization in Bose-Einstein condensates in
disordered lattices, described by the discrete nonlinear Schr\"odinger equation
(DNLS). A symplectic integration method allows us to accurately obtain both the
full phase space trajectories and their maximum Lyapunov exponents (mLEs),
which characterize their chaoticity. We find that disorder destroys ergodicity
by breaking up phase space into subsystems that are effectively disjoint on
experimentally relevant timescales, even though energetically, classical
localisation cannot occur. This leads us to conclude that the mLE is a very
poor ergodicity indicator, since it is not sensitive to the trajectory being
confined to a subregion of phase space. The eventual thermalization of a BEC in
a disordered lattice cannot be predicted based only on the chaoticity of its
phase space trajectory
Supersolid phases of dipolar bosons in optical lattices with a staggered flux
We present the theoretical mean-field zero-temperature phase diagram of a
Bose-Einstein condensate (BEC) with dipolar interactions loaded into an optical
lattice with a staggered flux. Apart from uniform superfluid, checkerboard
supersolid and striped supersolid phases, we identify several supersolid phases
with staggered vortices, which can be seen as combinations of supersolid phases
found in earlier work on dipolar BECs and a staggered-vortex phase found for
bosons in optical lattices with staggered flux. By allowing for different
phases and densities on each of the four sites of the elementary plaquette,
more complex phase patterns are found.Comment: 11 pages; added references, minor changes in tex
Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices
We consider ultracold bosons in a 2D square optical lattice described by the
Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is
applied to the system, which shakes the lattice along one of the diagonals. The
effect of the shaking is to renormalize the nearest-neighbor hopping
coefficients, which can be arbitrarily reduced, can vanish, or can even change
sign, depending on the shaking parameter. It is therefore necessary to account
for higher-order hopping terms, which are renormalized differently by the
shaking, and introduce anisotropy into the problem. We show that the
competition between these different hopping terms leads to finite-momentum
condensates, with a momentum that may be tuned via the strength of the shaking.
We calculate the boundaries between the Mott-insulator and the different
superfluid phases, and present the time-of-flight images expected to be
observed experimentally. Our results open up new possibilities for the
realization of bosonic analogs of the FFLO phase describing inhomogeneous
superconductivity.Comment: 7 pages, 7 figure
Joint Deep Modeling of Users and Items Using Reviews for Recommendation
A large amount of information exists in reviews written by users. This source
of information has been ignored by most of the current recommender systems
while it can potentially alleviate the sparsity problem and improve the quality
of recommendations. In this paper, we present a deep model to learn item
properties and user behaviors jointly from review text. The proposed model,
named Deep Cooperative Neural Networks (DeepCoNN), consists of two parallel
neural networks coupled in the last layers. One of the networks focuses on
learning user behaviors exploiting reviews written by the user, and the other
one learns item properties from the reviews written for the item. A shared
layer is introduced on the top to couple these two networks together. The
shared layer enables latent factors learned for users and items to interact
with each other in a manner similar to factorization machine techniques.
Experimental results demonstrate that DeepCoNN significantly outperforms all
baseline recommender systems on a variety of datasets.Comment: WSDM 201
Are Dutch Skylarks partial migrants? Ring recovery data and radio-telemetry suggest local coexistence of contrasting migration strategies
In recent years, Skylarks Alauda arvensis have undergone dramatic population declines in many European countries. Evidence exists for deteriorating conditions during the breeding season, but little is known about the situation during the rest of the annual cycle. Here we use two approaches to test if the Dutch breeding population of Skylarks consists of resident and/or migratory individuals. First, we present an analysis of ring recoveries from the Dutch Ringing Centre "Vogeltrekstation". Out of 25 recoveries, 12 Skylarks were resident in winter, 10 migrated and three were classified as probable migrants. Resident birds were accompanied during winter by birds from northern and eastern Europe. Very limited natal and breeding dispersal recorded in the same dataset suggests that our results were not influenced by long dispersal distances. Next, we compared these results to a local radio-telemetry study in the northern Netherlands. During two different years we equipped a total of 27 Skylarks from a breeding population with radio-transmitters and followed them during the subsequent winter. Four birds were found to winter locally. Out of 23 individuals that we did not find in winter, 14 returned in the following breeding season to the study area, all with a working transmitter, suggesting that they wintered outside our study area. Two ring recoveries of birds from the same study population indeed showed migration to south-west Europe. Based on these two lines of evidence, we conclude local coexistence of a resident and a migrant strategy in Dutch Skylarks. The findings of our study are important for the planning of conservation efforts, as we can only protect this rapidly declining species when we know their behaviour and whereabouts throughout the entire annual cycle
Three-dimensional single roughness element in a turbulent boundary layer, A
CER65-HT-VAS-73.October 1965.Includes bibliographical references (page 8).Prepared for U.S. Army Research Grant.Under grant DA-AMC-28-043-64-G-9
Kinetic models of ion transport through a nanopore
Kinetic equations for the stationary state distribution function of ions
moving through narrow pores are solved for a number of one-dimensional models
of single ion transport. Ions move through pores of length , under the
action of a constant external field and of a concentration gradient. The
interaction of single ions with the confining pore surface and with water
molecules inside the pore are modelled by a Fokker-Planck term in the kinetic
equation, or by uncorrelated collisions with thermalizing centres distributed
along the pore. The temporary binding of ions to polar residues lining the pore
is modelled by stopping traps or energy barriers. Analytic expressions for the
stationary ion current through the pore are derived for several versions of the
model, as functions of key physical parameters. In all cases, saturation of the
current at high fields is predicted. Such simple models, for which results are
analytic, may prove useful in the study of the current/voltage relations of ion
channels through membranes
Pokrovsky-Talapov Model at finite temperature: a renormalization-group analysis
We calculate the finite-temperature shift of the critical wavevector
of the Pokrovsky-Talapov model using a renormalization-group analysis.
Separating the Hamiltonian into a part that is renormalized and one that is
not, we obtain the flow equations for the stiffness and an arbitrary potential.
We then specialize to the case of a cosine potential, and compare our results
to well-known results for the sine-Gordon model, to which our model reduces in
the limit of vanishing driving wavevector Q=0. Our results may be applied to
describe the commensurate-incommensurate phase transition in several physical
systems and allow for a more realistic comparison with experiments, which are
always carried out at a finite temperature
Electrostatics of ions inside the nanopores and trans-membrane channels
A model of a finite cylindrical ion channel through a phospholipid membrane
of width separating two electrolyte reservoirs is studied. Analytical
solution of the Poisson equation is obtained for an arbitrary distribution of
ions inside the trans-membrane pore. The solution is asymptotically exact in
the limit of large ionic strength of electrolyte on the two sides of membrane.
However, even for physiological concentrations of electrolyte, the
electrostatic barrier sizes found using the theory are in excellent agreement
with the numerical solution of the Poisson equation. The analytical solution is
used to calculate the electrostatic potential energy profiles for pores
containing charged protein residues. Availability of a semi-exact interionic
potential should greatly facilitate the study of ionic transport through
nanopores and ion channels
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