9,716 research outputs found
Berry phase modification to the energy spectrum of excitons
By quantizing the semiclassical motion of excitons, we show that the Berry
curvature can cause an energy splitting between exciton states with opposite
angular momentum. This splitting is determined by the Berry curvature flux
through the -space area spanned by the relative motion of the
electron-hole pair in the exciton wave function. Using the gapped
two-dimensional Dirac equation as a model, we show that this splitting can be
understood as an effective spin-orbit coupling effect. In addition, there is
also an energy shift caused by other "relativistic" terms. Our result reveals
the limitation of the venerable hydrogenic model of excitons, and highlights
the importance of the Berry curvature in the effective mass approximation.Comment: 4.5 pages, 2 figures, reference updated and minor change
The Photometric Investigation of V921 Her using the Lunar-based Ultraviolet Telescope of Chang'e-3 mission
The light curve of V921 Her in ultraviolet band observed by the Lunar-based
Ultraviolet Telescope (LUT) is analyzed by the Wilson-Devinney code. Our
solutions conclude that V921 Her is an early type marginal contact binary
system with an additional close-in component. The binary system is under poor
thermal contact with a temperature difference of nearly between the two
components. The close-in component contributes about of the total
luminosity in the triple system. Combining the radial velocity study together
with our photometric solutions, the mass of the primary star and secondary one
are calculated to be , . The evolutionary scenario of V921 Her is discussed.
All times of light minimum of V921 Her available in the bibliography are taken
into account and the curve is analyzed for the first time. The most
probable fitting results are discussed in the paper, which also confirm the
existence of a third component ( year) around the binary system. The
period of V921 Her is also undergoing a continuously rapid increase at a rate
of , which may due to mass
transfer from the less massive component to the more massive one
Efficient Availability Attacks against Supervised and Contrastive Learning Simultaneously
Availability attacks can prevent the unauthorized use of private data and
commercial datasets by generating imperceptible noise and making unlearnable
examples before release. Ideally, the obtained unlearnability prevents
algorithms from training usable models. When supervised learning (SL)
algorithms have failed, a malicious data collector possibly resorts to
contrastive learning (CL) algorithms to bypass the protection. Through
evaluation, we have found that most of the existing methods are unable to
achieve both supervised and contrastive unlearnability, which poses risks to
data protection. Different from recent methods based on contrastive error
minimization, we employ contrastive-like data augmentations in supervised error
minimization or maximization frameworks to obtain attacks effective for both SL
and CL. Our proposed AUE and AAP attacks achieve state-of-the-art worst-case
unlearnability across SL and CL algorithms with less computation consumption,
showcasing prospects in real-world applications
Game-Theoretic Unlearnable Example Generator
Unlearnable example attacks are data poisoning attacks aiming to degrade the
clean test accuracy of deep learning by adding imperceptible perturbations to
the training samples, which can be formulated as a bi-level optimization
problem. However, directly solving this optimization problem is intractable for
deep neural networks. In this paper, we investigate unlearnable example attacks
from a game-theoretic perspective, by formulating the attack as a nonzero sum
Stackelberg game. First, the existence of game equilibria is proved under the
normal setting and the adversarial training setting. It is shown that the game
equilibrium gives the most powerful poison attack in that the victim has the
lowest test accuracy among all networks within the same hypothesis space, when
certain loss functions are used. Second, we propose a novel attack method,
called the Game Unlearnable Example (GUE), which has three main gradients. (1)
The poisons are obtained by directly solving the equilibrium of the Stackelberg
game with a first-order algorithm. (2) We employ an autoencoder-like generative
network model as the poison attacker. (3) A novel payoff function is introduced
to evaluate the performance of the poison. Comprehensive experiments
demonstrate that GUE can effectively poison the model in various scenarios.
Furthermore, the GUE still works by using a relatively small percentage of the
training data to train the generator, and the poison generator can generalize
to unseen data well. Our implementation code can be found at
https://github.com/hong-xian/gue
Data-Dependent Stability Analysis of Adversarial Training
Stability analysis is an essential aspect of studying the generalization
ability of deep learning, as it involves deriving generalization bounds for
stochastic gradient descent-based training algorithms. Adversarial training is
the most widely used defense against adversarial example attacks. However,
previous generalization bounds for adversarial training have not included
information regarding the data distribution. In this paper, we fill this gap by
providing generalization bounds for stochastic gradient descent-based
adversarial training that incorporate data distribution information. We utilize
the concepts of on-average stability and high-order approximate Lipschitz
conditions to examine how changes in data distribution and adversarial budget
can affect robust generalization gaps. Our derived generalization bounds for
both convex and non-convex losses are at least as good as the uniform
stability-based counterparts which do not include data distribution
information. Furthermore, our findings demonstrate how distribution shifts from
data poisoning attacks can impact robust generalization
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