9,568 research outputs found
Collaborative Filtering via Group-Structured Dictionary Learning
Structured sparse coding and the related structured dictionary learning
problems are novel research areas in machine learning. In this paper we present
a new application of structured dictionary learning for collaborative filtering
based recommender systems. Our extensive numerical experiments demonstrate that
the presented technique outperforms its state-of-the-art competitors and has
several advantages over approaches that do not put structured constraints on
the dictionary elements.Comment: A compressed version of the paper has been accepted for publication
at the 10th International Conference on Latent Variable Analysis and Source
Separation (LVA/ICA 2012
Single Superconducting Split-Ring Resonator Electrodynamics
We investigate the microwave electrodynamic properties of a single
superconducting thin film split-ring resonator (SRR). The experiments were
performed in an all-Nb waveguide, with Nb wires and Nb SRRs. Transmission data
showed a high-Q stopband for a single Nb SRR ( at 4.2 K)
below , and no such feature for a Cu SRR, or closed Nb loops, of similar
dimensions. Adding SRRs increased the bandwidth, but decreased the insertion
loss of the features. Placing the Nb SRR into an array of wires produced a
single, elementary negative-index passband ( at 4.2 K).
Changes in the features due to the superconducting kinetic inductance were
observed. Models for the SRR permeability, and the wire dielectric response,
were used to fit the data.Comment: 4 pages, 3 figures, RevTex, submitted to Applied Physics Letters.
Updated version includes mention of bianisotropy, better looking figures, and
different temperature dat
Teleportation scheme implementing contextually the Universal Optimal Quantum Cloning Machine and the Universal Not Gate. Complete experimental realization
By a significant modification of the standard protocol of quantum state
Teleportation two processes ''forbidden'' by quantum mechanics in their exact
form, the Universal NOT gate and the Universal Optimal Quantum Cloning Machine,
have been implemented contextually and optimally by a fully linear method. In
particular, the first experimental demonstration of the Tele-UNOT Gate, a novel
quantum information protocol has been reported (cfr. quant-ph/0304070). A
complete experimental realization of the protocol is presented here.Comment: 11 pages, 3 figure
Low EUV Luminosities Impinging on Protoplanetary Disks
The amount of high-energy stellar radiation reaching the surface of
protoplanetary disks is essential to determine their chemistry and physical
evolution. Here, we use millimetric and centimetric radio data to constrain the
EUV luminosity impinging on 14 disks around young (~2-10Myr) sun-like stars.
For each object we identify the long-wavelength emission in excess to the dust
thermal emission, attribute that to free-free disk emission, and thereby
compute an upper limit to the EUV reaching the disk. We find upper limits lower
than 10 photons/s for all sources without jets and lower than photons/s for the three older sources in our sample. These latter
values are low for EUV-driven photoevaporation alone to clear out
protoplanetary material in the timescale inferred by observations. In addition,
our EUV upper limits are too low to reproduce the [NeII] 12.81 micron
luminosities from three disks with slow [NeII]-detected winds. This indicates
that the [NeII] line in these sources primarily traces a mostly neutral wind
where Ne is ionized by 1 keV X-ray photons, implying higher photoevaporative
mass loss rates than those predicted by EUV-driven models alone. In summary,
our results suggest that high-energy stellar photons other than EUV may
dominate the dispersal of protoplanetary disks around sun-like stars.Comment: Accepted for publication to The Astrophysical Journa
Grain growth in the envelopes and disks of Class I protostars
We present new 3 mm ATCA data of two Class I Young Stellar Objects in the
Ophiucus star forming region: Elias29 and WL12. For our analysis we compare
them with archival 1.1 mm SMA data. In the (u,v) plane the two sources present
a similar behavior: a nearly constant non-zero emission at long baselines,
which suggests the presence of an unresolved component and an increase of the
fluxes at short baselines, related to the presence of an extended envelope. Our
data analysis leads to unusually low values of the spectral index , which may indicate that mm-sized dust grains have already formed
both in the envelopes and in the disk-like structures at such early stages. To
explore the possible scenarios for the interpretation of the sources we perform
a radiative transfer modeling using a Monte Carlo code, in order to take into
account possible deviations from the Rayleigh-Jeans and optically thin regimes.
Comparison between the model outputs and the observations indicates that dust
grains may form aggregates up to millimeter size already in the inner regions
of the envelopes of Class I YSOs. Moreover, we conclude that the embedded
disk-like structures in our two Class Is are probably very compact, in
particular in the case of WL12, with outer radii down to tens of AU.Comment: 12 pages, 8 figures, Accepted for publication in A&
Off-equilibrium confined dynamics in a glassy system with level-crossing states
We study analytically the dynamics of a generalized p-spin model, starting
with a thermalized initial condition. The model presents birth and death of
states, hence the dynamics (even starting at equilibrium) may go out of
equilibrium when the temperature is varied. We give a full description of this
constrained out of equilibrium behavior and we clarify the connection to the
thermodynamics by computing (sub-dominant) TAP states, constrained to the
starting equilibrium configuration.Comment: 10 pages, 3 figures; longer version with appendi
Water-induced correlation between single ions imaged at the solid–liquid interface
When immersed into ​water, most solids develop a surface charge, which is neutralized by an accumulation of dissolved counterions at the interface. Although the density distribution of counterions perpendicular to the interface obeys well-established theories, little is known about counterions’ lateral organization at the surface of the solid. Here we show, by using atomic force microscopy and computer simulations, that single hydrated metal ions can spontaneously form ordered structures at the surface of homogeneous solids in aqueous solutions. The structures are laterally stabilized only by ​water molecules with no need for specific interactions between the surface and the ions. The mechanism, studied here for several systems, is controlled by the hydration landscape of both the surface and the adsorbed ions. The existence of discrete ion domains could play an important role in interfacial phenomena such as charge transfer, crystal growth, nanoscale self-assembly and colloidal stability
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