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Jian Wang

Publications and source records attributed to Jian Wang.

At least 1,027 records · Page 57Linked to original sources
arXiv2017

Impact of intergrain spin transfer torques due to huge thermal gradients on the performance of heat assisted magnetic recording

Heat assisted magnetic recording (HAMR) is a new technology which uses temporary near field laser heating of the media during write to increase hard disk drive storage density. By using plasmonic antenna embedded in the write head, extremely high thermal gradient are created in the recording media (up to 10K/nm). State of the art HAMR media consists of grains of FePtX ordered alloys exhibiting high perpendicular anisotropy separated by insulating grain boundaries. Nearby the plasmonic antenna, the difference of temperature between two 8nm wide neighboring grains in the media can reach 80K, representing a gigantic thermal gradient of ~40K/nm across the grain boundary. Such situations with much weaker thermal gradient (~1K/nm, already considered as very large) have already been studied in the field of spincaloritronics. There, it was shown that very large spin transfer torques due to thermal gradients can arise in magnetic tunnel junctions which can even yield magnetization switching. Considering that two neighboring grains separated by an insulating grain boundary in a HAMR media can be viewed as a magnetic tunnel junction and that the thermal gradients in HAMR are one to two orders of magnitude larger than those existing in conventional spincaloritronics, one may expect a major impact from these thermal torques on magnetization switching dynamics and therefore on HAMR recording performances. This paper combines theory, experiments aiming at determining the polarization of tunneling electrons across the media grain boundaries, and micromagnetic simulations of recording process. It is shown that the thermal in-plane torque can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on media design are discussed in order to limit the impact of these thermal torques.

physics.app-ph↗
arXiv2017

On bifurcation of eigenvalues along convex symplectic paths

We consider a continuously differentiable curve $t\mapsto γ(t)$ in the space of $2n\times 2n$ real symplectic matrices, which is the solution of the following ODE: $\frac{\mathrm{d}γ}{\mathrm{d}t}(t)=J_{2n}A(t)γ(t), γ(0)\in\operatorname{Sp}(2n,\mathbb{R})$, where $J=J_{2n}\overset{\text{def}}{=}\begin{bmatrix}0 & \operatorname{Id}_n\\-\operatorname{Id}_n & 0\end{bmatrix}$ and $A:t\mapsto A(t)$ is a continuous in the space of $2n\times2n$ real matrices which are symmetric. Under certain convexity assumption (which includes the particular case that $A(t)$ is strictly positive definite for all $t\in\mathbb{R}$), we investigate the dynamics of the eigenvalues of $γ(t)$ when $t$ varies, which are closely related to the stability of such Hamiltonian dynamical systems. We rigorously prove the qualitative behavior of the branching of eigenvalues and explicitly give the first order asymptotics of the eigenvalues. This generalizes classical Krein-Lyubarskii theorem on the analytic bifurcation of the Floquet multipliers under a linear perturbation of the Hamiltonian. As a corollary, we give a rigorous proof of the following statement of Ekeland: $\{t\in\mathbb{R}:γ(t)\text{ has a Krein indefinite eigenvalue of modulus }1\}$ is a discrete set.

math.DS↗
arXiv2017

Rectifying full-counting statistics in a spin Seebeck engine

We investigate the rectification and negative differential effects of full-counting statistics of conjugate thermal spin transport in a spin Seebeck engine. The engine is made by an electron/magnon interface diode driven by a temperature bias, of which the scaled cumulant generating function is formulated in the framework of nonequilibrium Green's function. The strongly fluctuating interfacial electron density of states induced by quantum dot is responsible for these intriguing properties. This work is relevant for designing efficient spin caloritronic engine and can broaden our view in nonequilibrium thermodynamics and the nonlinear phenomenon in quantum transport system.

cond-mat.mes-hall↗
arXiv2017

Premise Selection for Theorem Proving by Deep Graph Embedding

We propose a deep learning-based approach to the problem of premise selection: selecting mathematical statements relevant for proving a given conjecture. We represent a higher-order logic formula as a graph that is invariant to variable renaming but still fully preserves syntactic and semantic information. We then embed the graph into a vector via a novel embedding method that preserves the information of edge ordering. Our approach achieves state-of-the-art results on the HolStep dataset, improving the classification accuracy from 83% to 90.3%.

cs.AI↗
arXiv2017

Stability of heat kernel estimates for symmetric non-local Dirichlet forms

In this paper, we consider symmetric jump processes of mixed-type on metric measure spaces under general volume doubling condition, and establish stability of two-sided heat kernel estimates and heat kernel upper bounds. We obtain their stable equivalent characterizations in terms of the jumping kernels, variants of cut-off Sobolev inequalities, and the Faber-Krahn inequalities. In particular, we establish stability of heat kernel estimates for $α$-stable-like processes even with $α\ge 2$ when the underlying spaces have walk dimensions larger than $2$, which has been one of the major open problems in this area.

math.PR↗
arXiv2017

Heat kernel estimates for time fractional equations

In this paper, we establish existence and uniqueness of weak solutions to general time fractional equations and give their probabilistic representations. We then derive sharp two-sided estimates for fundamental solutions of a family of time fractional equations in metric measure spaces.

math.PR↗
arXiv2017

Elliptic Harnack inequalities for symmetric non-local Dirichlet forms

We study relations and characterizations of various elliptic Harnack inequalities for symmetric non-local Dirichlet forms on metric measure spaces. We allow the scaling function be state-dependent and the state space possibly disconnected. Stability of elliptic Harnack inequalities is established under certain regularity conditions and implication for a priori Hölder regularity of harmonic functions is explored. New equivalent statements for parabolic Harnack inequalities of non-local Dirichlet forms are obtained in terms of elliptic Harnack inequalities.

math.PR↗
arXiv2017

The rigidity of pseudo-rotations on the two-torus and a question of Norton-Sullivan

We show that under certain boundedness condition, a $C^{r}$ conservative irrational pseudo-rotations on $\mathbb{T}^2$ with a generic rotation vector is $C^{r-1}$-rigid. We also obtain $C^0$-rigidity for Hölder pseudo-rotations with similar properties. These provide a partial generalisation of the main results in [B. Bramham, Invent. Math. (2015), no. 2, 561-580; A. Avila, B. Fayad, P. Le Calvez, D. Xu and Z. Zhang, arXiv: 1509.06906v1]. We then use these results to study conservative irrational pseudo-rotations on $\mathbb{T}^2$ with a generic rotation vector that is semi-conjugate to a translation via a semi-conjugacy homotopic to the identity. We show that the conservative centralizers of any such diffeomorphism is isomorphic to a uncountable subgroup of $\mathbb{R}^2/\mathbb{Z}^2$. In connection with a question of Alec Norton and Dennis Sullivan, we describe the topologically linearizable maps within this class using the topology of the conservative centralizer group. In the minimal case, we obtain a precise characterization of topological linearizability for all totally irrational vectors. We also construct a $C^\infty$ conservative and minimal totally irrational pseudo-rotation diffeomorphism that is semi-conjugate to a translation, but is topologically nonlinearizable. This gives a negative answer to the question of Norton and Sullivan in the $C^{\infty}$ category.

math.DS↗
arXiv2017

Deep Metric Learning with Angular Loss

The modern image search system requires semantic understanding of image, and a key yet under-addressed problem is to learn a good metric for measuring the similarity between images. While deep metric learning has yielded impressive performance gains by extracting high level abstractions from image data, a proper objective loss function becomes the central issue to boost the performance. In this paper, we propose a novel angular loss, which takes angle relationship into account, for learning better similarity metric. Whereas previous metric learning methods focus on optimizing the similarity (contrastive loss) or relative similarity (triplet loss) of image pairs, our proposed method aims at constraining the angle at the negative point of triplet triangles. Several favorable properties are observed when compared with conventional methods. First, scale invariance is introduced, improving the robustness of objective against feature variance. Second, a third-order geometric constraint is inherently imposed, capturing additional local structure of triplet triangles than contrastive loss or triplet loss. Third, better convergence has been demonstrated by experiments on three publicly available datasets.

cs.CV↗
arXiv2017

Cloud Computing - Architecture and Applications

In the era of Internet of Things and with the explosive worldwide growth of electronic data volume, and associated need of processing, analysis, and storage of such humongous volume of data, it has now become mandatory to exploit the power of massively parallel architecture for fast computation. Cloud computing provides a cheap source of such computing framework for large volume of data for real-time applications. It is, therefore, not surprising to see that cloud computing has become a buzzword in the computing fraternity over the last decade. This book presents some critical applications in cloud frameworks along with some innovation design of algorithms and architecture for deployment in cloud environment. It is a valuable source of knowledge for researchers, engineers, practitioners, and graduate and doctoral students working in the field of cloud computing. It will also be useful for faculty members of graduate schools and universities.

cs.DC↗
arXiv2017

Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature above 6 K, 2 times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field Bc// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point.

cond-mat.supr-con↗
arXiv2017

Isoperimetric Inequalities for Non-Local Dirichlet Forms

Let $(E,\F,μ)$ be a $\si$-finite measure space. For a non-negative symmetric measure $J(\d x, \d y):=J(x,y) \,μ(\d x)\,μ(\d y)$ on $E\times E,$ consider the quadratic form $$\E(f,f):= \frac{1}{2}\int_{E\times E} (f(x)-f(y))^2 \, J(\d x,\d y)$$ in $L^2(μ)$. We characterize the relationship between the isoperimetric inequality and the super Poincaré inequality associated with $\E$. In particular, sharp Orlicz-Sobolev type and Poincaré type isoperimetric inequalities are derived for stable-like Dirichlet forms on $\R^n$, which include the existing fractional isoperimetric inequality as a special example.

math.PR↗
arXiv2017

Upper Rate Functions of Brownian Motion Type for Symmetric Jump Processes

Let $X$ be a symmetric jump process on $\R^d$ such that the corresponding jumping kernel $J(x,y)$ satisfies $$J(x,y)\le \frac{c}{|x-y|^{d+2}\log^{1+\varepsilon}(e+|x-y|)}$$ for all $x,y\in\R^d$ with $|x-y|\ge1$ and some constants $c,\varepsilon>0$. Under additional mild assumptions on $J(x,y)$ for $|x-y|<1$, we show that $C\sqrt{r\log \log r}$ with some constant $C>0$ is an upper rate function of the process $X$, which enjoys the same form as that for Brownian motions. The approach is based on heat kernel estimates of large time for the process $X$. As a by-product, we also obtain two-sided heat kernel estimates of large time for symmetric jump processes whose jumping kernels are comparable to $$\frac{1}{|x-y|^{d+2+\varepsilon}}$$ for all $x,y\in\R^d$ with $|x-y|\ge1$ and some constant $\varepsilon>0$.

math.PR↗
arXiv2017

A photonic quantum interface for hybrid quantum network

The hybrid quantum network, a universal form of quantum network which is aimed for quantum communication and distributed quantum computation, is that the quantum nodes in it are realized with different physical systems. This universal form of quantum network can combine the advantages and avoid the inherent defects of the different physical system. However, one obstacle standing in the way is the compatible photonic quantum interface. One possible solution is using non-degenerate, narrow-band, entangled photon pairs as the photonic interface. Here, for the first time, we generate nondegenrate narrow-band polarization-entangled photon pairs in cavity-enhanced spontaneous parametric down-conversion process. The bandwidths and central wavelengths of the signal and idler photons are 9 MHz at 935 nm and 9.5 MHz at 880 nm, which are compatible with trapped ion system and solid-state quantum memory system. The entanglement of the photon source is certified by quantum state tomography, showing a fidelity of 89.6% between the generated quantum state with a Bell state. Besides, a strong violation against Bell inequality with 2.36+/-0.03 further confirms the entanglement property of the photon pairs. Our method is suitable for the hybrid quantum network and will take a big step in this field.

quant-ph↗
arXiv2017

Nearly Optimal Bounds for Orthogonal Least Squares

In this paper, we study the orthogonal least squares (OLS) algorithm for sparse recovery. On the one hand, we show that if the sampling matrix $\mathbf{A}$ satisfies the restricted isometry property (RIP) of order $K + 1$ with isometry constant $$ δ_{K + 1} < \frac{1}{\sqrt{K+1}}, $$ then OLS exactly recovers the support of any $K$-sparse vector $\mathbf{x}$ from its samples $\mathbf{y} = \mathbf{A} \mathbf{x}$ in $K$ iterations. On the other hand, we show that OLS may not be able to recover the support of a $K$-sparse vector $\mathbf{x}$ in $K$ iterations for some $K$ if $$ δ_{K + 1} \geq \frac{1}{\sqrt{K+\frac{1}{4}}}. $$

cs.IT↗
arXiv2017

Novel Logical Method for Security Analysis of Electronic Payment Protocols

Electronic payment protocols play a vital role in electronic commerce security, which is essential for secure operation of electronic commerce activities. Formal method is an effective way to verify the security of protocols. But current formal method lacks the description and analysis of timeliness in electronic payment protocols. In order to improve analysis ability, a novel approach to analyze security properties such as accountability, fairness and timeliness in electronic payment protocols is proposed in this paper. This approach extends an existing logical method by adding a concise time expression and analysis method. It enables to describe the event time, and extends the time characteristics of logical inference rules. We analyzed the Netbill protocol with the new approach and found that the fairness of the protocol is not satisfied, due to timeliness problem. The result illustrates the new approach is able to analyze the key properties of electronic payment protocols. Furthermore, the new approach can be introduced to analyze other time properties of cryptographic protocols.

cs.LO↗
arXiv2017

Intrinsic Ultracontractivity of Non-local Dirichlet forms on Unbounded Open Sets

In this paper we consider a large class of symmetric Markov processes $X=(X_t)_{t\ge0}$ on $\R^d$ generated by non-local Dirichlet forms, which include jump processes with small jumps of $α$-stable-like type and with large jumps of super-exponential decay. Let $D\subset \R^d$ be an open (not necessarily bounded and connected) set, and $X^D=(X_t^D)_{t\ge0}$ be the killed process of $X$ on exiting $D$. We obtain explicit criterion for the compactness and the intrinsic ultracontractivity of the Dirichlet Markov semigroup $(P^{D}_t)_{t\ge0}$ of $X^D$. When $D$ is a horn-shaped region, we further obtain two-sided estimates of ground state in terms of jumping kernel of $X$ and the reference function of the horn-shaped region $D$.

math.PR↗