926 research outputs found
The K\"ahler-Ricci flow with positive bisectional curvature
We show that the K\"ahler-Ricci flow on a manifold with positive first Chern
class converges to a K\"ahler-Einstein metric assuming positive bisectional
curvature and certain stability conditions.Comment: 15 page
Enhanced osteoblast adhesion on nanostructured selenium compacts for anti-cancer orthopedic applications
Metallic bone implants possess numerous problems limiting their long-term efficacy, such as poor prolonged osseointegration, stress shielding, and corrosion under in vivo environments. Such problems are compounded for bone cancer patients since numerous patients receive orthopedic implants after cancerous bone resection. Unfortunately, current orthopedic materials were not originally developed to simultaneously increase healthy bone growth (as in traditional orthopedic implant applications) while inhibiting cancerous bone growth. The long-term objective of the present research is to investigate the use of nano-rough selenium to prevent bone cancer from re-occurring while promoting healthy bone growth for this select group of cancer patients. Selenium is a well known anti-cancer chemical. However, what is not known is how healthy bone cells interact with selenium. To determine this, selenium, spherical or semispherical shots, were pressed into cylindrical compacts and these compacts were then etched using 1N NaOH to obtain various surface structures ranging from the micron, submicron to nano scales. Changes in surface chemistry were also analyzed. Through these etching techniques, results of this study showed that biologically inspired surface roughness values were created on selenium compacts to match that of natural bone roughness. Moreover, results showed that healthy bone cell adhesion increased with greater nanometer selenium roughness (more closely matching that of titanium). In this manner, this study suggests that nano-rough selenium should be further tested for orthopedic applications involving bone cancer treatment
Selenium nanoparticles inhibit Staphylococcus aureus growth
Staphylococcus aureus is a key bacterium commonly found in numerous infections. S. aureus infections are difficult to treat due to their biofilm formation and documented antibiotic resistance. While selenium has been used for a wide range of applications including anticancer applications, the effects of selenium nanoparticles on microorganisms remain largely unknown to date. The objective of this in vitro study was thus to examine the growth of S. aureus in the presence of selenium nanoparticles. Results of this study provided the first evidence of strongly inhibited growth of S. aureus in the presence of selenium nanoparticles after 3, 4, and 5 hours at 7.8, 15.5, and 31 μg/mL. The percentage of live bacteria also decreased in the presence of selenium nanoparticles. Therefore, this study suggests that selenium nanoparticles may be used to effectively prevent and treat S. aureus infections and thus should be further studied for such applications
Quantum Geometric Oscillations in Two-Dimensional Flat-Band Solids
Two-dimensional van der Waals heterostructures can be engineered into
artificial superlattices that host flat bands with significant Berry curvature
and provide a favorable environment for the emergence of novel electron
dynamics. In particular, the Berry curvature can induce an oscillating
trajectory of an electron wave packet transverse to an applied static electric
field. Though analogous to Bloch oscillations, this novel oscillatory behavior
is driven entirely by quantum geometry in momentum space instead of band
dispersion. While the orbits of Bloch oscillations can be localized by
increasing field strength, the size of the geometric orbits saturates to a
nonzero plateau in the strong-field limit. In non-magnetic materials, the
geometric oscillations are even under inversion of the applied field, whereas
the Bloch oscillations are odd, a property that can be used to distinguish
these two co-existing effects.Comment: 6 + 7 pages, 2 figures. Comments are greatly appreciated
Protected Fermionic Zero Modes in Periodic Gauge Fields
It is well-known that macroscopically-normalizable zero-energy wavefunctions
of spin- particles in a two-dimensional inhomogeneous magnetic
field are spin-polarized and exactly calculable with degeneracy equaling the
number of flux quanta linking the whole system. Extending this argument to
massless Dirac fermions subjected to magnetic fields that have \textit{zero}
net flux but are doubly periodic in real space, we show that there exist
\textit{only two} Bloch-normalizable zero-energy eigenstates, one for each spin
flavor. This result is immediately relevant to graphene multilayer systems
subjected to doubly-periodic strain fields, which at low energies, enter the
Hamiltonian as periodic pseudo-gauge vector potentials. Furthermore, we explore
various related settings including nonlinearly-dispersing band structure models
and systems with singly-periodic magnetic fields.Comment: 9 pages, 1 figure. Comments are very appreciate
Boundary Modes from Periodic Magnetic and Pseudomagnetic Fields in Graphene
Single-layer graphenes subject to periodic lateral strains are artificial
crystals that can support boundary spectra with an intrinsic polarity. These
are analyzed by comparing the effects of periodic magnetic fields and
strain-induced pseudomagnetic fields that respectively break and preserve
time-reversal symmetry. In the former case, a Chern classification of the
superlattice minibands with zero total magnetic flux enforces {\it single}
counter-propagating modes traversing each bulk gap on opposite boundaries of a
nanoribbon. For the pseudomagnetic field, pairs of counter-propagating modes
migrate to the {\it same} boundary where they provide well-developed
valley-helical transport channels on a single zigzag edge. We discuss possible
schemes for implementing this situation and their experimental signatures.Comment: 5+12 pages; 3+6 figures; version accepted to Physical Review Letter
Factorizing LambdaMART for cold start recommendations
Recommendation systems often rely on point-wise loss metrics such as the mean
squared error. However, in real recommendation settings only few items are
presented to a user. This observation has recently encouraged the use of
rank-based metrics. LambdaMART is the state-of-the-art algorithm in learning to
rank which relies on such a metric. Despite its success it does not have a
principled regularization mechanism relying in empirical approaches to control
model complexity leaving it thus prone to overfitting.
Motivated by the fact that very often the users' and items' descriptions as
well as the preference behavior can be well summarized by a small number of
hidden factors, we propose a novel algorithm, LambdaMART Matrix Factorization
(LambdaMART-MF), that learns a low rank latent representation of users and
items using gradient boosted trees. The algorithm factorizes lambdaMART by
defining relevance scores as the inner product of the learned representations
of the users and items. The low rank is essentially a model complexity
controller; on top of it we propose additional regularizers to constraint the
learned latent representations that reflect the user and item manifolds as
these are defined by their original feature based descriptors and the
preference behavior. Finally we also propose to use a weighted variant of NDCG
to reduce the penalty for similar items with large rating discrepancy.
We experiment on two very different recommendation datasets, meta-mining and
movies-users, and evaluate the performance of LambdaMART-MF, with and without
regularization, in the cold start setting as well as in the simpler matrix
completion setting. In both cases it outperforms in a significant manner
current state of the art algorithms
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