Impact of spin-orbit coupling on the magnetism of Sr3MIrO6 (M = Ni, Co)

Using density functional calculations, we demonstrate that the spin-orbit coupling (SOC) of the Ir4+ ion plays an essential role in determining the antiferromagnetism of the hexagonal spin-chain system Sr3MIrO6 (M = Ni, Co) by tuning the crystal-field level sequence and altering the Ir-M inter-orbital interactions. The SOC splits the e'_{g} doublet of the octahedral Ir4+ ion (t_{2g}^5) in a trigonal crystal field, and the single t_{2g} hole resides on the e'_{g} upper branch and gives rise to the antiferromagnetic superexchange. In absence of the SOC, however, the single t_{2g} hole would occupy the a_{1g} singlet instead, which would mediate an unreal ferromagnetic exchange due to a direct a_{1g} hopping along the Ir-M chain. We also find that the Ni2+ and Co2+ ions are both in a high-spin state and moreover the Co2+ ion carries a huge orbital moment. This work well accounts for the recent experiments and magnifies again the significance of the SOC in iridates.Comment: 5 pages, 5 figure

Ferrimagnetism in the double perovskite Ca2FeOsO6: a density functional study

Using density functional calculations, we find that the newly synthesized Ca$_2$FeOsO$_6$ has the high-spin Fe$^{3+}$ ($3d^5$)-Os$^{5+}$ ($5d^3$) state. The octahedral Os$^{5+}$ ion has a large intrinsic exchange splitting, and its $t_{2g\uparrow}^3$ configuration makes the spin-orbit coupling ineffective. Moreover, there is a strong antiferromagnetic (AF) coupling between the neighboring Fe$^{3+}$ ($S$ = 5/2) and Os$^{5+}$ ($S$ = -3/2), but the AF couplings within both the fcc Fe$^{3+}$ and Os$^{5+}$ sublattices are one order of magnitude weaker. Therefore, a magnetic frustration is suppressed and a stable ferrimagnetic (FiM) ground state appears. This FiM order is due to the virtual hopping of the $t_{2g}$ electrons from Os$^{5+}$ ($t_{2g\downarrow}^3$) to Fe$^{3+}$ ($t_{2g\uparrow}^3e_{g\uparrow}^2$). However, if the experimental bended Fe$^{3+}$-O$^{2-}$-Os$^{5+}$ exchange path gets straight, the $e_g$ hopping from Fe$^{3+}$ ($t_{2g\uparrow}^3e_{g\uparrow}^2$) to Os$^{5+}$ ($t_{2g\uparrow}^3$) would be facilitated and then a ferromagnetic (FM) coupling would occur.Comment: 5 pages, 5 figure

Sequentially Aggregated Convolutional Networks

Modern deep networks generally implement a certain form of shortcut connections to alleviate optimization difficulties. However, we observe that such network topology alters the nature of deep networks. In many ways, these networks behave similarly to aggregated wide networks. We thus exploit the aggregation nature of shortcut connections at a finer architectural level and place them within wide convolutional layers. We end up with a sequentially aggregated convolutional (SeqConv) layer that combines the benefits of both wide and deep representations by aggregating features of various depths in sequence. The proposed SeqConv serves as a drop-in replacement of regular wide convolutional layers and thus could be handily integrated into any backbone network. We apply SeqConv to widely adopted backbones including ResNet and ResNeXt, and conduct experiments for image classification on public benchmark datasets. Our ResNet based network with a model size of ResNet-50 easily surpasses the performance of the 2.35$\times$ larger ResNet-152, while our ResNeXt based model sets a new state-of-the-art accuracy on ImageNet classification for networks with similar model complexity. The code and pre-trained models of our work are publicly available at https://github.com/GroupOfAlchemists/SeqConv.Comment: To appear in ICCV 2019 worksho

Color Assessment and Transfer for Web Pages

Colors play a particularly important role in both designing and accessing Web pages. A well-designed color scheme improves Web pages' visual aesthetic and facilitates user interactions. As far as we know, existing color assessment studies focus on images; studies on color assessment and editing for Web pages are rare. This paper investigates color assessment for Web pages based on existing online color theme-rating data sets and applies this assessment to Web color edit. This study consists of three parts. First, we study the extraction of a Web page's color theme. Second, we construct color assessment models that score the color compatibility of a Web page by leveraging machine learning techniques. Third, we incorporate the learned color assessment model into a new application, namely, color transfer for Web pages. Our study combines techniques from computer graphics, Web mining, computer vision, and machine learning. Experimental results suggest that our constructed color assessment models are effective, and useful in the color transfer for Web pages, which has received little attention in both Web mining and computer graphics communities.Comment: 10page

Giant magnetic anisotropy of Co, Ru, and Os adatoms on MgO (001) surface

Large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. Here, using ligand-field level diagrams and density functional calculations, we well explain the very recent discovery [I. G. Rau et al., Science 344, 988 (2014)] that an individual Co adatom on a MgO (001) surface has a large MAE of more than 60 meV. More importantly, we predict that a giant MAE up to 110 meV could be realized for Ru adatoms on MgO (001), and even more for the Os adatoms (208 meV). This is a joint effect of the special ligand field, orbital multiplet, and significant spin-orbit interaction, in the intermediate-spin state of the Ru or Os adatoms on top of the surface oxygens. The giant MAE could provide a route to atomic scale memory.Comment: 5 pages, 4 figures, and 1 tabl

Long-range magnetic interaction and frustration in double perovskite Sr2_{2}NiIrO6_{6}

Sr$_{2}$NiIrO$_{6}$ would be a ferromagnetic (FM) insulator in terms of the common superexchange mechanism between the first nearest neighboring (1NN) magnetic ions Ni$^{2+}$ ($t_{2g}^{6}e_{g}^{2}$) and Ir$^{6+}$ ($t_{2g}^{3}$). However, the observed antiferromagnetic (AF) order questions this viewpoint. In this work, we present first-principles calculations and find that while the 1NN Ni$^{2+}$-Ir$^{6+}$ exchange is indeed FM, the 2NN and 3NN couplings in the fcc Ir (and Ni) sublattice are AF. Moreover, the 2NN AF Ir-Ir coupling turns out to be even stronger than the 1NN FM Ni-Ir coupling, thus giving rise to a magnetic frustration. Sr$_{2}$NiIrO$_{6}$ hence becomes a distorted low-temperature antiferromagnet. Naturally, a very similar magnetic property in Sr$_{2}$ZnIrO$_{6}$ can be explained by the frustrated AF coupling in the fcc Ir$^{6+}$ sublattice. This work highlights the long-range magnetic interaction of the delocalized $5d$ electrons, and also addresses why the spin-orbit coupling is ineffective here.Comment: 5 pages, 5 figures, 1 tabl

Detecting Adversarial Examples via Key-based Network

Though deep neural networks have achieved state-of-the-art performance in visual classification, recent studies have shown that they are all vulnerable to the attack of adversarial examples. Small and often imperceptible perturbations to the input images are sufficient to fool the most powerful deep neural networks. Various defense methods have been proposed to address this issue. However, they either require knowledge on the process of generating adversarial examples, or are not robust against new attacks specifically designed to penetrate the existing defense. In this work, we introduce key-based network, a new detection-based defense mechanism to distinguish adversarial examples from normal ones based on error correcting output codes, using the binary code vectors produced by multiple binary classifiers applied to randomly chosen label-sets as signatures to match normal images and reject adversarial examples. In contrast to existing defense methods, the proposed method does not require knowledge of the process for generating adversarial examples and can be applied to defend against different types of attacks. For the practical black-box and gray-box scenarios, where the attacker does not know the encoding scheme, we show empirically that key-based network can effectively detect adversarial examples generated by several state-of-the-art attacks.Comment: 6 page

Periodic Oscillations of Blood Cell Populations in Chronic Myelogenous Leukemia

We develop some techniques to prove analytically the existence and stability of long period oscillations of stem cell populations in the case of periodic chronic myelogenous leukemia. Such a periodic oscillation $p_\infty$ can be analytically constructed when the hill coefficient involved in the nonlinear feedback is infinite, and we show it is possible to obtain a contractive returning map (for the semiflow defined by the modeling functional differential equation) in a closed and convex cone containing $p_\infty$ when the hill coefficient is large, and the fixed point of such a contractive map gives the long period oscillation previously observed both numerically and experimentally.Comment: 23 page

Improvement on Fermionic properties and new isotope production in molecular dynamics simulations

By considering momentum transfer in the Fermi constraint procedure, the stability of the initial nuclei and fragments produced in heavy-ion collisions can be further improved in the quantum molecular dynamics simulations. The case of the phase space occupation probability larger than one is effectively reduced with the proposed procedure. Simultaneously, the energy conservation can be better described for both individual nuclei and heavy-ion reactions. With the revised version of the improved quantum molecular dynamics (ImQMD) model, the fusion excitation functions of $^{16}$O+$^{186}$W and the central collisions of Au+Au at 35 AMeV are re-examined. The fusion cross sections at sub-barrier energies and the charge distribution of fragments are relatively better reproduced due to the reduction of spurious nucleon emission. The charge and isotope distribution of fragments in Xe+Sn, U+U and Zr+Sn at intermediate energies are also predicted. More unmeasured extremely neutron-rich fragments with $Z=16-28$ are observed in the central collisions of $^{238}$U+$^{238}$U than that of $^{96}$Zr+$^{124}$Sn, which indicates that multi-fragmentation of U+U may offer a fruitful pathway to new neutron-rich isotopes.Comment: 8 figures, to appear in J. Phys.

Efficient Human Pose Estimation with Depthwise Separable Convolution and Person Centroid Guided Joint Grouping

In this paper, we propose efficient and effective methods for 2D human pose estimation. A new ResBlock is proposed based on depthwise separable convolution and is utilized instead of the original one in Hourglass network. It can be further enhanced by replacing the vanilla depthwise convolution with a mixed depthwise convolution. Based on it, we propose a bottom-up multi-person pose estimation method. A rooted tree is used to represent human pose by introducing person centroid as the root which connects to all body joints directly or hierarchically. Two branches of sub-networks are used to predict the centroids, body joints and their offsets to their parent nodes. Joints are grouped by tracing along their offsets to the closest centroids. Experimental results on the MPII human dataset and the LSP dataset show that both our single-person and multi-person pose estimation methods can achieve competitive accuracies with low computational costs