arXiv subjects

Xin Ma

Publications and source records attributed to Xin Ma.

At least 145 records · Page 8Linked to original sources

arXiv2019

A brief introduction to the Grey Machine Learning

This paper presents a brief introduction to the key points of the Grey Machine Learning (GML) based on the kernels. The general formulation of the grey system models have been firstly summarized, and then the nonlinear extension of the grey models have been developed also with general formulations. The kernel implicit mapping is used to estimate the nonlinear function of the GML model, by extending the nonparametric formulation of the LSSVM, the estimation of the nonlinear function of the GML model can also be expressed by the kernels. A short discussion on the priority of this new framework to the existing grey models and LSSVM have also been discussed in this paper. And the perspectives and future orientations of this framework have also been presented.

stat.ML↗

arXiv2019

Accurate Target Localization by using Artificial Pinnae of brown long-eared bat

Echolocating bats locate the targets by echolocation. Many theoretical frameworks have been suggested the abilities of bats are related to the shapes of bats ears, but few artificial bat-like ears have been made to mimic the abilities, the difficulty of which lies in the determination of the elevation angle of the target. In this study, we present a device with artificial bat pinnae modeling by the ears of brown long-eared bat (Plecotus auritus) which can accurately estimate the elevation angle of the aerial target by virtue of active sonar. An artificial neural-network with the labeled data obtained from echoes as the trained and tested data is used and optimized by a tenfold cross-validation technique. A decision method we named sliding window averaging algorithm is designed for getting the estimation results of elevation. At last, a right-angle pinnae construction is designed for determining direction of the target. The results show a higher accuracy for the direction determination of the single target. The results also demonstrate that for the Plecotus auritus bat, not only the binaural shapes, but the binaural relative orientations also play important roles in the target localization.

eess.SP↗

arXiv2019

The Extension Dimension of Abelian Categories

Let $\A$ be an abelian category having enough projective objects and enough injective objects. We prove that if $\A$ admits an additive generating object, then the extension dimension and the weak resolution dimension of $\A$ are identical, and they are at most the representation dimension of $\A$ minus two. By using it, for a right Morita ring $\La$, we establish the relation between the extension dimension of the category $\mod \La$ of finitely generated right $Λ$-modules and the representation dimension as well as the right global dimension of $Λ$. In particular, we give an upper bound for the extension dimension of $\mod Λ$ in terms of the projective dimension of certain class of simple right $Λ$-modules and the radical layer length of $Λ$. In addition, we investigate the behavior of the extension dimension under some ring extensions and recollements.

math.RT↗

arXiv2019

Strong damping-like spin-orbit torque and tunable Dzyaloshinskii-Moriya interaction generated by low-resistivity Pd$_{1-x}$Pt$_x$ alloys

Despite their great promise for providing a pathway for very efficient and fast manipulation of magnetization at the nanoscale, spin-orbit torque (SOT) operations are currently energy inefficient due to a low damping-like SOT efficiency per unit current bias, and/or the very high resistivity of the spin Hall materials. Here, we report an advantageous spin Hall material, Pd1-xPtx, which combines a low resistivity with a giant spin Hall effect as evidenced through the use of three independent SOT ferromagnetic detectors. The optimal Pd0.25Pt0.75 alloy has a giant internal spin Hall ratio of >0.47 (damping-like SOT efficiency of ~ 0.26 for all three ferromagnets) and a low resistivity of ~57.5 μΩ cm at 4 nm thickness. Moreover, we find the Dzyaloshinskii-Moriya interaction (DMI), the key ingredient for the manipulation of chiral spin arrangements (e.g. magnetic skyrmions and chiral domain walls), is considerably strong at the Pd1-xPtx/Fe0.6Co0.2B0.2 interface when compared to that at Ta/Fe0.6Co0.2B0.2 or W/Fe0.6Co0.2B0.2 interfaces and can be tuned by a factor of 5 through control of the interfacial spin-orbital coupling via the heavy metal composition. This work establishes a very effective spin current generator that combines a notably high energy efficiency with a very strong and tunable DMI for advanced chiral spintronics and spin torque applications.

cond-mat.mtrl-sci↗

arXiv2018

Crossbar-aware neural network pruning

Crossbar architecture based devices have been widely adopted in neural network accelerators by taking advantage of the high efficiency on vector-matrix multiplication (VMM) operations. However, in the case of convolutional neural networks (CNNs), the efficiency is compromised dramatically due to the large amounts of data reuse. Although some mapping methods have been designed to achieve a balance between the execution throughput and resource overhead, the resource consumption cost is still huge while maintaining the throughput. Network pruning is a promising and widely studied leverage to shrink the model size. Whereas, previous work didn`t consider the crossbar architecture and the corresponding mapping method, which cannot be directly utilized by crossbar-based neural network accelerators. Tightly combining the crossbar structure and its mapping, this paper proposes a crossbar-aware pruning framework based on a formulated L0-norm constrained optimization problem. Specifically, we design an L0-norm constrained gradient descent (LGD) with relaxant probabilistic projection (RPP) to solve this problem. Two grains of sparsity are successfully achieved: i) intuitive crossbar-grain sparsity and ii) column-grain sparsity with output recombination, based on which we further propose an input feature maps (FMs) reorder method to improve the model accuracy. We evaluate our crossbar-aware pruning framework on median-scale CIFAR10 dataset and large-scale ImageNet dataset with VGG and ResNet models. Our method is able to reduce the crossbar overhead by 44%-72% with little accuracy degradation. This work greatly saves the resource and the related energy cost, which provides a new co-design solution for mapping CNNs onto various crossbar devices with significantly higher efficiency.

cs.CV↗

arXiv2018

Single spin sensing of domain wall structure and dynamics in a thin film skyrmion host

Skyrmions are nanoscale magnetic structures with features promising for future low-power memory or logic devices. In this work, we demonstrate novel scanning techniques based on nitrogen vacancy center magnetometry that simultaneously probe both the magnetic dynamics and structure of room temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the handedness of the Dzyaloshinskii-Moriya interaction in this material and extract the helicity angle of the skyrmion bubbles. Our measurements also show that the skyrmion bubbles in this material change size in discrete steps, dependent on the local pinning environment, with their average size determined dynamically as their domain walls hop between pinning sites. In addition, an increase in magnetic field noise is observed near all skyrmion bubble domain walls. These measurements highlight the importance of interactions between internal degrees of freedom of skyrmion bubble domain walls and pinning sites in thin film systems. Our observations have relevance for future devices based on skyrmion bubbles where pinning interactions will determine important aspects of current-driven motion.

cond-mat.mes-hall↗

arXiv2018

Batch Normalization Sampling

Deep Neural Networks (DNNs) thrive in recent years in which Batch Normalization (BN) plays an indispensable role. However, it has been observed that BN is costly due to the reduction operations. In this paper, we propose alleviating this problem through sampling only a small fraction of data for normalization at each iteration. Specifically, we model it as a statistical sampling problem and identify that by sampling less correlated data, we can largely reduce the requirement of the number of data for statistics estimation in BN, which directly simplifies the reduction operations. Based on this conclusion, we propose two sampling strategies, "Batch Sampling" (randomly select several samples from each batch) and "Feature Sampling" (randomly select a small patch from each feature map of all samples), that take both computational efficiency and sample correlation into consideration. Furthermore, we introduce an extremely simple variant of BN, termed as Virtual Dataset Normalization (VDN), that can normalize the activations well with few synthetical random samples. All the proposed methods are evaluated on various datasets and networks, where an overall training speedup by up to 20% on GPU is practically achieved without the support of any specialized libraries, and the loss on accuracy and convergence rate are negligible. Finally, we extend our work to the "micro-batch normalization" problem and yield comparable performance with existing approaches at the case of tiny batch size.

cs.LG↗

arXiv2018

Application of the novel fractional grey model FAGMO(1,1,k) to predict China's nuclear energy consumption

At present, the energy structure of China is shifting towards cleaner and lower amounts of carbon fuel, driven by environmental needs and technological advances. Nuclear energy, which is one of the major low-carbon resources, plays a key role in China's clean energy development. To formulate appropriate energy policies, it is necessary to conduct reliable forecasts. This paper discusses the nuclear energy consumption of China by means of a novel fractional grey model FAGMO(1,1,k). The fractional accumulated generating matrix is introduced to analyse the fractional grey model properties. Thereafter, the modelling procedures of the FAGMO(1,1,k) are presented in detail, along with the transforms of its optimal parameters. A stochastic testing scheme is provided to validate the accuracy and properties of the optimal parameters of the FAGMO(1,1,k). Finally, this model is used to forecast China's nuclear energy consumption and the results demonstrate that the FAGMO(1,1,k) model provides accurate prediction, outperforming other grey models.

stat.AP↗

arXiv2018

In-memory multiplication engine with SOT-MRAM based stochastic computing

Processing-in-memory (PIM) turns out to be a promising solution to breakthrough the memory wall and the power wall. While prior PIM designs yield successful implementation of bitwise Boolean logic operations locally in memory, it is difficult to accomplish the multiplication (MUL) instruction in a fast and efficient manner. In this paper, we propose a new stochastic computing (SC) design to perform MUL with in-memory operations. Instead of using the stochastic number generators (SNGs), we harness the inherent stochasticity in the memory write behavior of the magnetic random access memory (MRAM). Each memory bit serves as an SC engine, performs MUL on operands in the form of write voltage pulses, and stores the MUL outcome in-situ. The proposed design provides up to 4x improvement in performance compared with conversational SC approaches, and achieves 18x speedup over implementing MUL with only in-memory bitwise Boolean logic operations.

cs.AR↗

arXiv2018

Correlation between Dzyaloshinskii Moriya interaction and spin mixing conductance at antiferromagnet / ferromagnet interface

The rich interaction phenomena at antiferromagnet (AFM)/ ferromagnet (FM) interfaces are key ingredients in AFM spintronics, where many underlying mechanisms remain unclear. Here we report a correlation observed between interfacial Dzyaloshinskii-Moriya interaction (DMI) Ds and effective spin mixing conductance g at IrMn/CoFeB interface. Both Ds and g are quantitatively determined with Brillouin light scattering measurements, and increase with IrMn thickness in the range of 2.5~7.5 nm. Such correlation likely originates from the AFM-states-mediated spin-flip transitions in FM, which promote both interfacial DMI and spin pumping effect. Our findings provide deeper insight into the AFM-FM interfacial coupling for future spintronic design.

cond-mat.mtrl-sci↗

arXiv2018

Invariant ergodic measures and the classification of crossed product $C^\ast$-algebras

Let $α: G\curvearrowright X$ be a minimal free continuous action of an infinite countable amenable group on an infinite compact metrizable space. In this paper, under the hypothesis that the invariant ergodic probability Borel measure space $E_G(X)$ is compact and zero-dimensional, we show that the action $α$ has the small boundary property. This partially answers an open problem in dynamical systems that asks whether a minimal free action of an amenable group has the small boundary property if its space $M_G(X)$ of invariant Borel probability measures forms a Bauer simplex. In addition, under the same hypothesis, we show that dynamical comparison implies almost finiteness, which was shown by Kerr to imply that the crossed product is $\mathcal{Z}$-stable. Finally, we discuss some rank properties and provide two classifiability results for crossed products, one of which is based on the work of Elliott and Niu.

math.DS↗

arXiv2017

Torsion Pairs in Recollements of Abelian Categories

For a recollement $(\mathcal{A},\mathcal{B},\mathcal{C})$ of abelian categories, we show that torsion pairs in $\mathcal{A}$ and $\mathcal{C}$ can induce torsion pairs in $\mathcal{B}$; and the converse holds true under certain conditions.

math.RT↗

arXiv2017

Scaling of Dzyaloshinskii Moriya interaction at heavy metal and ferromagnetic metal interfaces

The Dzyaloshinskii Moriya Interaction (DMI) at the heavy metal (HM) and ferromagnetic metal (FM) interface has been recognized as a key ingredient in spintronic applications. Here we investigate the chemical trend of DMI on the 5d band filling (5d^3~5d^10) of the HM element in HM/CoFeB/MgO multilayer thin films. DMI is quantitatively evaluated by measuring asymmetric spin wave dispersion using Brillouin light scattering. Sign reversal and 20 times modification of the DMI coefficient D have been measured as the 5d HM element is varied. The chemical trend can be qualitatively understood by considering the 5d and 3d bands alignment at the HM/FM interface and the subsequent orbital hybridization around the Fermi level. Furthermore, a positive correlation is observed between DMI and spin mixing conductance at the HM/FM interfaces. Our results provide new insights into the interfacial DMI for designing future spintronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Dzyaloshinskii Moriya interaction across antiferromagnet / ferromagnet interface

The antiferromagnet (AFM) / ferromagnet (FM) interfaces are of central importance in recently developed pure electric or ultrafast control of FM spins, where the underlying mechanisms remain unresolved. Here we report the direct observation of Dzyaloshinskii Moriya interaction (DMI) across the AFM/FM interface of IrMn/CoFeB thin films. The interfacial DMI is quantitatively measured from the asymmetric spin wave dispersion in the FM layer using Brillouin light scattering. The DMI strength is enhanced by a factor of 7 with increasing IrMn layer thickness in the range of 1- 7.5 nm. Our findings provide deeper insight into the coupling at AFM/FM interface and may stimulate new device concepts utilizing chiral spin textures such as magnetic skyrmions in AFM/FM heterostructures.

cond-mat.mtrl-sci↗

arXiv2016

Trion Valley Coherence in Monolayer Semiconductors

The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control of this quantum degree of freedom. The charged exciton (trion) in TMDs is an intriguing alternative to the neutral exciton for control of valley pseudospin because of its long spontaneous recombination lifetime, its robust valley polarization, and its coupling to residual electronic spin. Trion valley coherence has however been unexplored due to experimental challenges in accessing it spectroscopically. In this work, we employ ultrafast two-dimensional coherent spectroscopy to resonantly generate and detect trion valley coherence in monolayer MoSe$_2$ demonstrating that it persists for a few-hundred femtoseconds. We conclude that the underlying mechanisms limiting trion valley coherence are fundamentally different from those applicable to exciton valley coherence. Based on these observations, we suggest possible strategies for extending valley coherence times in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2016

Interfacial Control of Dzyaloshinskii Moriya Interaction in Heavy Metal_Ferromagnetic Metal Thin Film Heterostructures

The interfacial Dzyaloshinskii Moriya Interaction (DMI) in ultrathin magnetic thin film heterostructures provides a new approach for controlling spin textures on mesoscopic length scales. Here we investigate the dependence of the interfacial DMI constant D on a Pt wedge insertion layer in Ta_CoFeB_Pt(wedge)_MgO thin films by observing the asymmetric spin wave dispersion using Brillouin light scattering. Continuous tuning of D by more than a factor of three is realized by inserting less than one monolayer of Pt. The observations provide new insights for designing magnetic thin film heterostructures with tailored D for controlling skyrmions and magnetic domain wall chirality and dynamics.

cond-mat.mtrl-sci↗

arXiv2016

Impact of Grain Boundaries on Efficiency and Stability of Organic-Inorganic Trihalide Perovskites

Organic-inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies (PCEs). To further improve the device performance, however, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the first quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide (MAPbI3) thin films with different PCEs by light-stimulated microwave impedance microscopy. The intrinsic photo-response is largely uniform across grains and grain boundaries, which is direct evidence on the inherently benign nature of microstructures in the perovskite thin films. In contrast, the carrier mobility and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation due to water diffusion through the capping layer begins with the disintegration of large grains rather than the nucleation and propagation from grain boundaries. Our findings provide new insights to improve the electro-optical properties of MAPbI3 thin films towards large-scale commercialization.

cond-mat.mtrl-sci↗

arXiv2016

Magnons and Phonons Optically Driven Out of Local Equilibrium in a Magnetic Insulator

Magnons are the energy quanta of fundamental spin excitations, namely spin waves, and they can make a considerable contribution to energy transport in some magnetic materials in a similar manner as lattice vibration waves or phonons. The coupling and possible non-equilibrium between magnons and other energy carriers have been used to explain several recently discovered thermally driven spin transport and energy conversion phenomena. Here, we report experiments in which local non-equilibrium between magnons and phonons in a single crystalline bulk magnetic insulator, Y3Fe5O12 (yttrium iron garnet, or YIG), has been created optically within a focused laser spot and probed directly with the use of micro-Brillouin light scattering (BLS). By analyzing the experimental results with a thermally induced magnon diffusion model, we obtain the magnon diffusion length of thermal magnons. By explicitly establishing non-equilibrium between magnons and phonons, our studies represent an important step toward a quantitative understanding of various spin-heat coupling phenomena.

cond-mat.mtrl-sci↗