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Jianbo Wang

Publications and source records attributed to Jianbo Wang.

At least 37 records · Page 2Linked to original sources

Elliptic genera of level $N$ for complete intersections

We study the elliptic genera of level $N$ at the cusps of $Γ_1(N)$ for any complete intersection. These genera are described as the summations of generalized binomial coefficients, where each generalized binomial coefficient is related to the dimension and multi-degree of complete intersection. For complete intersection $X_n(\underline{d})$, write $c_1(X_n(\underline{d}))=c_1x$, where $x\in H^2(X_n(\underline{d});\mathbb{Z})\cong\mathbb{Z}$ is a generator. We mainly discuss the values of the elliptic genera of level $N$ for $X_n(\underline{d})$ in the case of $c_1>0, =0$ or $<0$. In particular, the values about the Todd genus, $\hat{A}$-genus and $A_k$-genus of $X_n(\underline{d})$ can be derived from the elliptic genera of level $N$.

math.AT↗

Direct Imaging of Resonant Phonon-Magnon Coupling

Direct detection of phonons is important for the investigation of information interconversion between the resonantly coupled magnons and phonons. Here we report resonant coupling of magnons and phonons, which can be directly visualized by using micro focused Brillouin light scattering in Ni/LiNbO3 hybrid heterostructures. The patterns of surface acoustic wave phonons, originating from the interference between the original wave ψ0(A_0,k) and reflected wave ψ1(A_1,-k), can be modulated by magnetic field due to the magnon-phonon coupling. By analyzing the information of phonons obtained from Brillouin spectroscopy, the properties of the magnon system (Ni film), e.g., ferromagnetic resonance field and resonance linewidth can be determined. The results provide spatially resolved information about phonon manipulation and detection in a coupled magnon-phonon system.

physics.app-ph↗

Interaction between defect and skyrmion in nanodisk

Magnetic skyrmions are topologically protected stable magnetization configurations, which are expected to be a promising candidate as information carrier, while defect is inevitable and plays an important role on the stabilization and movement of skyrmion. In this paper, we investigated the influence of a point defect and a ring defect on the stabilization and dynamics of skyrmion in the nanodisk, where the defect are acquired by local modification magnetic material parameters. Considering the combined action of geometry confinement and pinning effect, we demonstrate the preferred skyrmion position as a function of defect type and distance between skyrmion and a point defect. We also show the confinement effect on skyrmion size in the presence of a ring defect due to circular symmetry. Finally, in the application of spin transfer nano-oscillator, we show that the skyrmion can be pinned or rotate in the nanodisk, the oscillator frequency can be modified in a large variation by the ring defect. These findings provide a complete understanding of interaction between skyrmion and defect in the confined geometry and may provide a good strategy for the design of skyrmion oscillators.

cond-mat.mes-hall↗

The Hirzebruch genera of complete intersections

Following Brooks's calculation of the $\hat{A}$-genus of complete intersections, a new and more computable formula about the $\hat{A}$-genus and $α$-invariant will be described as polynomials of multi-degree and dimension. We also give an iterated formula of $\hat{A}$-genus and the necessary and sufficient conditions for the vanishing of $\hat{A}$-genus of complex even dimensional spin complete intersections. Finally, we obtain a general formula about the Hirzebruch genus of complete intersections, and calculate some classical Hirzebruch genera as examples.

math.AT↗

Examples of diffeomorphic complete intersections with different Hodge numbers

In this paper, we give three pairs of complex 3-dim complete intersections and a pair of complex 5-dim complete intersections, and every pair of them is diffeomorphic but with different Hodge numbers. Moreover, the diffeomorphic complex 3-dim complete intersections have different Chern mumbers $c_1^3, c_1c_2$.

math.DG↗

Understanding Electricity-Theft Behavior via Multi-Source Data

Electricity theft, the behavior that involves users conducting illegal operations on electrical meters to avoid individual electricity bills, is a common phenomenon in the developing countries. Considering its harmfulness to both power grids and the public, several mechanized methods have been developed to automatically recognize electricity-theft behaviors. However, these methods, which mainly assess users' electricity usage records, can be insufficient due to the diversity of theft tactics and the irregularity of user behaviors. In this paper, we propose to recognize electricity-theft behavior via multi-source data. In addition to users' electricity usage records, we analyze user behaviors by means of regional factors (non-technical loss) and climatic factors (temperature) in the corresponding transformer area. By conducting analytical experiments, we unearth several interesting patterns: for instance, electricity thieves are likely to consume much more electrical power than normal users, especially under extremely high or low temperatures. Motivated by these empirical observations, we further design a novel hierarchical framework for identifying electricity thieves. Experimental results based on a real-world dataset demonstrate that our proposed model can achieve the best performance in electricity-theft detection (e.g., at least +3.0% in terms of F0.5) compared with several baselines. Last but not least, our work has been applied by the State Grid of China and used to successfully catch electricity thieves in Hangzhou with a precision of 15% (an improvement form 0% attained by several other models the company employed) during monthly on-site investigation.

cs.CY↗

Phonon Transport Controlled by Ferromagnetic Resonance

The resonant coupling of phonons and magnons is important for the interconversion of phononic and spin degrees of freedom. We studied the phonon transmission in LiNbO3 manipulated by the dynamic magnetization in a Ni thin film. It was observed that the phonons could be absorbed strongly through resonant magnon-phonon coupling, which was realized by optimizing the interfacial coupling between Ni and LiNbO3. The line shapes of phonon transmission were further investigated considering the magnon-phonon interconversion in the elastically driven ferromagnetic resonance process. The results promote unique routes for phonon manipulation and detection in the presence of magnetization dynamics.

physics.app-ph↗

Spin current pumped by resonant skyrmion

Spin pumping is a widely recognized method to generate the spin current in the spintronics, which is acknowledged as a fundamentally dynamic process equivalent to the spin-transfer torque. In this work, we theoretically verify that the oscillating spin current can be pumped from the microwave-motivated breathing skyrmion. The skyrmion spin pumping can be excited by a relatively low frequency compared with the ferromagnetic resonance (FMR) and the current density is larger than the ordinary FMR spin pumping. Based on the skyrmion spin pumping, we build a high reading-speed racetrack memory model whose reading speed is an order of magnitude higher than the SOT (spin-orbit torque) /STT (spin-transfer torque) skyrmion racetrack. Our work explored the spin pumping phenomenon in the skyrmion, and it may contribute to the applications of the skyrmion-based device.

cond-mat.mes-hall↗

Gilbert damping of [Co/Pd]n/Py multilayer thin films

Understanding the Gilbert damping in exchange-coupled multilayer materials is particularly important to develop future fast switching spintronics devices. Here, we report an experimental investigation of temperature-dependent Gilbert damping in [Co/Pd]n/Py multilayer films of varying the number of Co/Pd repetitions by ferromagnetic resonance. The results demonstrate that three independent contributions to the Gilbert damping are identified, namely the intrinsic Gilbert damping, the inhomogeneous linewidth broadening and the two-magnon scattering contribution. Of particular interest, the two-magnon scattering intensity increases as the enlargement of number repetitions of Co/Pd due to the larger pinning effect at the interface between Py and the Co/Pd layers. The Gilbert damping increases monotonically as the temperature decreases from 300K to 50K. Our findings open the door to comprehend the physical origin of the Gilbert damping in ultrathin exchange-coupled multilayer films.

physics.app-ph↗

Dynamics of a magnetic skyrmionium in an anisotropy gradient

Magnetic skyrmionium is a novel magnetization configuration with zero skyrmion number, which is composed by two skyrmions with opposite skyrmion number. Here, we study the dynamics of skyrmionium under an anisotropy gradient. We find that the skyrmionium can be efficiently driven by an anisotropy gradient with moving straightly along the direction of gradient. The skyrmion Hall angle for skyrmionium is close to zero which is much smaller than that of skyrmion. while the speed is much larger. We also demonstrate that the skyrmionium motion depends on the damping cofficient, and the skyrmionium stabilization in the motion can be modulated by narrowing the width of the nanowire. Our work shows a efficient driven method for skyrmionium, which may be promising in the application of skyrmionium based racetrack memory.

cond-mat.mes-hall↗

Atomistic manipulation of reversible oxidation and reduction in Ag by electron beam

Employing electrons for direct control of nanoscale reaction is highly desirable since it provides fabrication of nanostructures with different properties at atomic resolution and with flexibility of dimension and location. Here, applying in situ transmission electron microscopy, we show the reversible oxidation and reduction kinetics in Ag, well controlled by changing the dose rate of electron beam. Aberration-corrected high-resolution transmission electron microscopy observation reveals that O atoms are preferably inserted and extracted along the {111} close-packed planes of Ag, leading to the nucleation and decomposition of nanoscale Ag2O islands on the Ag substrate. By controlling electron beam size and dose rate, we demonstrated fabrication of an array of 3 nm Ag2O nanodots in an Ag matrix. Our results open up a new pathway to manipulate atomistic reaction with electron beam towards the precise fabrication of nanostructures for device applications.

cond-mat.mtrl-sci↗

Understanding current-driven dynamics of magnetic Néel walls in heavy metal/ferromagnetic metal/oxide trilayers

We consider analytically current-driven dynamics of magnetic Néel walls in heavy metal/ferromagnetic metal/oxide trilayers where strong spin-orbit coupling and interfacial Dzyaloshinskii-Moriya interaction (i-DMI) coexist. We show that field-like spin-orbit torque (FL-SOT) with effective field along $\mathbf{n}\times\hat{\mathbf{J}}$ ($\mathbf{n}$ being the interface normal and $\hat{\mathbf{J}}$ being the charge current direction) and i-DMI induced torque can both lead to Walker breakdown suppression meanwhile leaving the wall mobility (velocity versus current density) unchanged. However, i-DMI itself can not induce the "universal absence of Walker breakdown" (UAWB) while FL-SOT exceeding a certain threshold can. Finitely-enlarged Walker limits before UAWB are theoretically calculated and well explain existing data. In addition, change in wall mobility and even its sign-inversion can be understood only if the anti-damping-like (ADL) SOT is appended. For Néel walls in ferromagnetic-metal layer with both perpendicular and in-plane anisotropies, we have calculated the respective modifications of wall mobility under the coexistence of spin-transfer torque, SOTs and i-DMI. Analytics shows that in trilayers with perpendicular anisotropy strong enough spin Hall angle and appropriate sign of i-DMI parameter can lead to sign-inversion in wall mobility even under small enough current density, while in those with in-plane anisotropy this only occurs for current density in a specific range.

cond-mat.mes-hall↗

Field-tuned spin excitation spectrum of $kπ$-skyrmion

We study spin-wave excitation modes of $kπ$ skyrmion in a magnetic nanodot under an external magnetic field along $z$-direction using micromagnetic simulations based on Landau-Lifshitz-Gilbert equation. We find that a transition of $kπ$ skyrmion to other skyrmion-like structures appears under some critical external field, the corresponding spin-wave spectra are simulated for each state in the process of applying magnetic field. For skyrmion, the frequencies of excitation modes increases and then decreases with the low frequency modes splitting at a critical magnetic field. In addition to the well known two in-plane rotational modes and a out-of-plane breathing mode of skyrmion, a higher number of excitation modes are found with increasing $k$ ($k=2, 3$). The excitation modes vary as a function of magnetic field, and the excitation frequencies for different modes exhibit a rapid or slight change depending on the field induced change of magnetization profile. Our study indicates the rich spin-wave excitations for $kπ$ skyrmion and opens a possibility in theoretical or experimental investigation of magnonics application.

cond-mat.mes-hall↗

Current-induced motion of twisted skyrmions

Twisted skyrmions, whose helicity angles are different from that of Bloch skyrmions and Néel skyrmions, have already been demonstrated in experiments recently. In this work, we first contrast the magnetic structure and origin of the twisted skyrmion with other three types of skyrmion including Bloch skyrmion, Néel skyrmion and antiskyrmion. Following, we investigate the dynamics of twisted skyrmions driven by the spin transfer toque (STT) and the spin Hall effect (SHE) by using micromagnetic simulations. It is found that the spin Hall angle of the twisted skyrmion is related to the dissipative force tensor and the Gilbert damping both for the motions induced by the STT and the SHE, especially for the SHE induced motion, the skyrmion Hall angle depends substantially on the skyrmion helicity. At last, we demonstrate that the trajectory of the twisted skyrmion can be controlled in a two dimensional plane with a Gilbert damping gradient. Our results provide the understanding of current-induced motion of twisted skyrmions, which may contribute to the applications of skyrmion-based racetrack memories.

physics.app-ph↗

Directional Spin-wave Propagation in the Skyrmion Chain

The Dzyaloshinskii-Moriya interaction (DMI), favoring a chiral spin structure like the skyrmion, gives rise to the nonreciprocal propagation of spin waves. We investigate the propagation of spin waves in a nanostripe with the presence of a skyrmion chain by using micromagnetic simulations. Through applying a microwave locally, it is found that when the interval between skyrmions is large enough, the spin waves can be separated to the counter direction according to different frequencies. While for the tightly arranged skyrmions, the skyrmion chain with strong interactions between skyrmions becomes a channel for spin waves, which is around the frequency of skyrmion breathing and exhibit a characteristic of directional propagation. This work opens a vista for skyrmion-based spin wave devices.

cond-mat.mes-hall↗

Metastable magnetic bubble in [Co/Pd]4/Py multilayers

Magnetic bubbles are topologically spin textures that offering the interesting physics and great promise for next-generation information storage technologies. The main obstacles so far are that magnetic bubbles are generated with no field stimuli in new material systems at room temperature. Here, we report the observation of individual magnetic bubbles and its high frequency measurement at room temperature in an exchange-coupled [Co/Pd]4/Py multilayers. We demonstrate that the emergence of magnetic bubbles at remanence can be tuned by the in-plane tilted magnetic field (roughly 3°) along the film plane at room temperature. High frequency results indicate that the presence of magnetic bubbles leads to broadening of the magnetic permeability spectrum lines (due to the non-uniformity of the magnetic moments). Our findings open the door to the bubble-based spintronics at room temperature in exchange-coupled magnetic multilayers.

physics.app-ph↗

Magnetic field direction dependent antiskyrmion motion with microwave electric fields

Magnetic skyrmions are regarded as promising information candidates in future spintronic devices, which have been investigated theoretically and experimentally in isotropic system. Recently, the sta- bilization of antiskyrmions in the presence of anisotropic Dzyaloshinskii-Moriya interaction and its dynamics driven by current have been investigated. Here, we report the antiskyrmion motion with the combined action of the in-plane magnetic field and microwave electric fields. The in-plane mag- netic field breaks the rotation symmetry of the antiskyrmion, and perpendicular microwave electric field induces the pumping of magnetic anisotropy, leading to antiskyrmion breathing mode. With above two effects, the antiskyrmion propagates with a desired trajectory. Antiskyrmion propagation velocity depends on the frequency, amplitude of anisotropy pumping, and damping constant as well as strength of in-plane field, which reaches the maximum value when the frequency of microwave electric field is in consist with the resonance frequency of antiskyrmion. Moreover, we show that the antiskyrmion propagation depends on the direction of magnetic field, where the antiskyrmion Hall angle can be suppressed or enhanced. At a critical direction of magnetic field, the Hall angle is zero. Our results introduce a possible application of antiskyrmion in antiskyrmion-based spintronic devices with lower energy consumption.

cond-mat.mes-hall↗

Image Semantic Transformation: Faster, Lighter and Stronger

We propose Image-Semantic-Transformation-Reconstruction-Circle(ISTRC) model, a novel and powerful method using facenet's Euclidean latent space to understand the images. As the name suggests, ISTRC construct the circle, able to perfectly reconstruct images. One powerful Euclidean latent space embedded in ISTRC is FaceNet's last layer with the power of distinguishing and understanding images. Our model will reconstruct the images and manipulate Euclidean latent vectors to achieve semantic transformations and semantic images arthimetic calculations. In this paper, we show that ISTRC performs 10 high-level semantic transformations like "Male and female","add smile","open mouth", "deduct beard or add mustache", "bigger/smaller nose", "make older and younger", "bigger lips", "bigger eyes", "bigger/smaller mouths" and "more attractive". It just takes 3 hours(GTX 1080) to train the models of 10 semantic transformations.

cs.CV↗