631 research outputs found
Improving Negative Sampling for Word Representation using Self-embedded Features
Although the word-popularity based negative sampler has shown superb
performance in the skip-gram model, the theoretical motivation behind
oversampling popular (non-observed) words as negative samples is still not well
understood. In this paper, we start from an investigation of the gradient
vanishing issue in the skipgram model without a proper negative sampler. By
performing an insightful analysis from the stochastic gradient descent (SGD)
learning perspective, we demonstrate that, both theoretically and intuitively,
negative samples with larger inner product scores are more informative than
those with lower scores for the SGD learner in terms of both convergence rate
and accuracy. Understanding this, we propose an alternative sampling algorithm
that dynamically selects informative negative samples during each SGD update.
More importantly, the proposed sampler accounts for multi-dimensional
self-embedded features during the sampling process, which essentially makes it
more effective than the original popularity-based (one-dimensional) sampler.
Empirical experiments further verify our observations, and show that our
fine-grained samplers gain significant improvement over the existing ones
without increasing computational complexity.Comment: Accepted in WSDM 201
Gated Class-Attention with Cascaded Feature Drift Compensation for Exemplar-free Continual Learning of Vision Transformers
In this paper we propose a new method for exemplar-free class incremental
training of ViTs. The main challenge of exemplar-free continual learning is
maintaining plasticity of the learner without causing catastrophic forgetting
of previously learned tasks. This is often achieved via exemplar replay which
can help recalibrate previous task classifiers to the feature drift which
occurs when learning new tasks. Exemplar replay, however, comes at the cost of
retaining samples from previous tasks which for some applications may not be
possible. To address the problem of continual ViT training, we first propose
gated class-attention to minimize the drift in the final ViT transformer block.
This mask-based gating is applied to class-attention mechanism of the last
transformer block and strongly regulates the weights crucial for previous
tasks. Secondly, we propose a new method of feature drift compensation that
accommodates feature drift in the backbone when learning new tasks. The
combination of gated class-attention and cascaded feature drift compensation
allows for plasticity towards new tasks while limiting forgetting of previous
ones. Extensive experiments performed on CIFAR-100, Tiny-ImageNet and
ImageNet100 demonstrate that our method outperforms existing exemplar-free
state-of-the-art methods without the need to store any representative exemplars
of past tasks
Characterising Alzheimer's Disease with EEG-based Energy Landscape Analysis
Alzheimer's disease (AD) is one of the most common neurodegenerative
diseases, with around 50 million patients worldwide. Accessible and
non-invasive methods of diagnosing and characterising AD are therefore urgently
required. Electroencephalography (EEG) fulfils these criteria and is often used
when studying AD. Several features derived from EEG were shown to predict AD
with high accuracy, e.g. signal complexity and synchronisation. However, the
dynamics of how the brain transitions between stable states have not been
properly studied in the case of AD and EEG data. Energy landscape analysis is a
method that can be used to quantify these dynamics. This work presents the
first application of this method to both AD and EEG. Energy landscape assigns
energy value to each possible state, i.e. pattern of activations across brain
regions. The energy is inversely proportional to the probability of occurrence.
By studying the features of energy landscapes of 20 AD patients and 20 healthy
age-matched counterparts, significant differences were found. The dynamics of
AD patients' brain networks were shown to be more constrained - with more local
minima, less variation in basin size, and smaller basins. We show that energy
landscapes can predict AD with high accuracy, performing significantly better
than baseline models.Comment: 11 pages, 7 figure
Modeling realistic multiphase flows using a non-orthogonal multiple-relaxation-time lattice Boltzmann method
In this paper, we develop a three-dimensional multiple-relaxation-time
lattice Boltzmann method (MRT-LBM) based on a set of non-orthogonal basis
vectors. Compared with the classical MRT-LBM based on a set of orthogonal basis
vectors, the present non-orthogonal MRT-LBM simplifies the transformation
between the discrete velocity space and the moment space, and exhibits better
portability across different lattices. The proposed method is then extended to
multiphase flows at large density ratio with tunable surface tension, and its
numerical stability and accuracy are well demonstrated by some benchmark cases.
Using the proposed method, a practical case of a fuel droplet impacting on a
dry surface at high Reynolds and Weber numbers is simulated and the evolution
of the spreading film diameter agrees well with the experimental data.
Furthermore, another realistic case of a droplet impacting on a
super-hydrophobic wall with a cylindrical obstacle is reproduced, which
confirms the experimental finding of Liu \textit{et al.} [``Symmetry breaking
in drop bouncing on curved surfaces," Nature communications 6, 10034 (2015)]
that the contact time is minimized when the cylinder radius is comparable with
the droplet cylinder.Comment: 19 pages, 11 figure
A Study of background for IXPE
Focal plane X-ray polarimetry is intended for relatively bright sources with
a negligible impact of background. However this might not be always possible
for IXPE (Imaging X-ray Polarimetry Explorer) when observing faint extended
sources like supernova remnants. We present for the first time the expected
background of IXPE by Monte Carlo simulation and its impact on real
observations of point and extended X-ray sources. The simulation of background
has been performed by Monte Carlo based on GEANT4 framework. The spacecraft and
the detector units have been modeled, and the expected background components in
IXPE orbital environment have been evaluated. We studied different background
rejection techniques based on the analysis of the tracks collected by the Gas
Pixel Detectors on board IXPE. The estimated background is about 2.9 times
larger than the requirement, yet it is still negligible when observing point
like sources. Albeit small, the impact on supernova remnants indicates the need
for a background subtraction for the observation of the extended sources.Comment: 16 pages, 16 figure
Dielectric Breakdown in Chemical Vapor Deposited Hexagonal Boron Nitride
Insulating films are essential in multiple electronic devices because they can provide essential functionalities, such as capacitance effects and electrical fields. Two-dimensional (2D) layered materials have superb electronic, physical, chemical, thermal, and optical properties, and they can be effectively used to provide additional performances, such as flexibility and transparency. 2D layered insulators are called to be essential in future electronic devices, but their reliability, degradation kinetics, and dielectric breakdown (BD) process are still not understood. In this work, the dielectric breakdown process of multilayer hexagonal boron nitride (h-BN) is analyzed on the nanoscale and on the device level, and the experimental results are studied via theoretical models. It is found that under electrical stress, local charge accumulation and charge trapping/detrapping are the onset mechanisms for dielectric BD formation. By means of conductive atomic force microscopy, the BD event was triggered at several locations on the surface of different dielectrics (SiO2, HfO2, Al2O3, multilayer h-BN, and monolayer h-BN); BD-induced hillocks rapidly appeared on the surface of all of them when the BD was reached, except in monolayer h-BN. The high thermal conductivity of h-BN combined with the one-atom-thick nature are genuine factors contributing to heat dissipation at the BD spot, which avoids self-accelerated and thermally driven catastrophic BD. These results point to monolayer h-BN as a sublime dielectric in terms of reliability, which may have important implications in future digital electronic devices.Fil: Jiang, Lanlan. Soochow University; ChinaFil: Shi, Yuanyuan. Soochow University; China. University of Stanford; Estados UnidosFil: Hui, Fei. Soochow University; China. Massachusetts Institute of Technology; Estados UnidosFil: Tang, Kechao. University of Stanford; Estados UnidosFil: Wu, Qian. Soochow University; ChinaFil: Pan, Chengbin. Soochow University; ChinaFil: Jing, Xu. Soochow University; China. University of Texas at Austin; Estados UnidosFil: Uppal, Hasan. University of Manchester; Reino UnidoFil: Palumbo, FĂ©lix Roberto Mario. ComisiĂłn Nacional de EnergĂa AtĂłmica; Argentina. Universidad TecnolĂłgica Nacional; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Lu, Guangyuan. Chinese Academy of Sciences; RepĂşblica de ChinaFil: Wu, Tianru. Chinese Academy of Sciences; RepĂşblica de ChinaFil: Wang, Haomin. Chinese Academy of Sciences; RepĂşblica de ChinaFil: Villena, Marco A.. Soochow University; ChinaFil: Xie, Xiaoming. Chinese Academy of Sciences; RepĂşblica de China. ShanghaiTech University; ChinaFil: McIntyre, Paul C.. University of Stanford; Estados UnidosFil: Lanza, Mario. Soochow University; Chin
Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers
Atomically precise Au24M(SR)18 clusters were used as singlet-oxygen photosensitizers. Comprehensive kinetic analysis provided insights into the mechanism and driving-force dependence of the quenching of 1O2 by gold nanoclusters
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