8,399 research outputs found
Stable nontrivial Z2 topology in ultrathin Bi (111) films: a first-principles study
Recently, there have been intense efforts in searching for new topological
insulator (TI) materials. Based on first-principles calculations, we find that
all the ultrathin Bi (111) films are characterized by a nontrivial Z2 number
independent of the film thickness, without the odd-even oscillation of
topological triviality as commonly perceived. The stable nontrivial Z2 topology
is retained by the concurrent band gap inversions at multiple
time-reversal-invariant k-points and associated with the intermediate
inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further
indicate that the presence of metallic surface states in thick Bi(111) films
can be effectively removed by surface adsorption.Comment: 5 pages, 3 figure
Joint Channel-and-Data Estimation for Large-MIMO Systems with Low-Precision ADCs
The use of low precision (e.g., 1-3 bits) analog-to-digital convenors (ADCs)
in very large multiple-input multiple-output (MIMO) systems is a technique to
reduce cost and power consumption. In this context, nevertheless, it has been
shown that the training duration is required to be {\em very large} just to
obtain an acceptable channel state information (CSI) at the receiver. A
possible solution to the quantized MIMO systems is joint channel-and-data (JCD)
estimation. This paper first develops an analytical framework for studying the
quantized MIMO system using JCD estimation. In particular, we use the
Bayes-optimal inference for the JCD estimation and realize this estimator
utilizing a recent technique based on approximate message passing. Large-system
analysis based on the replica method is then adopted to derive the asymptotic
performances of the JCD estimator. Results from simulations confirm our
theoretical findings and reveal that the JCD estimator can provide a
significant gain over conventional pilot-only schemes in the quantized MIMO
system.Comment: 7 pages, 4 figure
A Novel Counterfactual Data Augmentation Method for Aspect-Based Sentiment Analysis
Aspect-based-sentiment-analysis (ABSA) is a fine-grained sentiment evaluation
task, which analyzes the emotional polarity of the evaluation aspects.
Generally, the emotional polarity of an aspect exists in the corresponding
opinion expression, whose diversity has great impact on model's performance. To
mitigate this problem, we propose a novel and simple counterfactual data
augmentation method to generate opinion expressions with reversed sentiment
polarity. In particular, the integrated gradients are calculated to locate and
mask the opinion expression. Then, a prompt combined with the reverse
expression polarity is added to the original text, and a Pre-trained language
model (PLM), T5, is finally was employed to predict the masks. The experimental
results shows the proposed counterfactual data augmentation method performs
better than current augmentation methods on three ABSA datasets, i.e. Laptop,
Restaurant, and MAMS.Comment: Camera-ready for ACML 202
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