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Xu Zhang

Publications and source records attributed to Xu Zhang.

At least 469 records · Page 26Linked to original sources
arXiv2017

Hyperbolic metamaterial as a tunable near-field spatial filter for the implementation of the active plasmon injection loss compensation scheme

We present how to physically realize the auxiliary source described in the recently introduced active plasmon injection loss compensation scheme for enhanced near-field superlensing. Particularly, we show that the characteristics of the auxiliary source described in the active plasmon injection scheme including tunable narrow-band and selective amplification via convolution can be realized by using a hyperbolic metamaterial functioning as a near-field spatial filter. Besides loss compensation, the proposed near-field spatial filter can be useful for real-time high resolution edge detection.

physics.optics↗
arXiv2017

Charge Transport in Hybrid Halide Perovskites

Charge transport is crucial to the performance of hybrid halide perovskite solar cells. A theoretical model based on large polarons is proposed to elucidate charge transport properties in the perovskites. Critical new physical insights are incorporated into the model, including the recognitions that acoustic phonons as opposed to optical phonons are responsible for the scattering of the polarons; these acoustic phonons are fully excited due to the "softness" of the perovskites, and the temperature-dependent dielectric function underlies the temperature dependence of charge carrier mobility. This work resolves key controversies in literature and forms a starting point for more rigorous first-principles predictions of charge transport.

cond-mat.mtrl-sci↗
arXiv2017

Tropical Principal Component Analysis and its Application to Phylogenetics

Principal component analysis is a widely-used method for the dimensionality reduction of a given data set in a high-dimensional Euclidean space. Here we define and analyze two analogues of principal component analysis in the setting of tropical geometry. In one approach, we study the Stiefel tropical linear space of fixed dimension closest to the data points in the tropical projective torus; in the other approach, we consider the tropical polytope with a fixed number of vertices closest to the data points. We then give approximative algorithms for both approaches and apply them to phylogenetics, testing the methods on simulated phylogenetic data and on an empirical dataset of Apicomplexa genomes.

math.CO↗
arXiv2017

Field-free perpendicular magnetization switching through domain wall motion in Pt/Co/Cr racetracks by spin orbit torques with the assistance of accompanying Joule heating effect

Heavy metal/ferromagnetic layers with perpendicular magnetic anisotropy (PMA) have potential applications for high-density information storage in racetrack memories and nonvolatile magnetic random access memories. Writing and erasing of information in these devices are carried out by domain wall (DW) motion and deterministic magnetization switching via electric current generated spin orbital torques (SOTs) with an assistance of in-plane bias field to break the symmetry. Improvements in energy efficiency could be obtained when the switching of perpendicular magnetization is controlled by an electric current generated SOTs without the in-plane bias fields. Here, we report on reversible electric-current-driven magnetization switching through DW motion in Pt/Co/Cr trilayers with PMA at room temperature due to the formation of homochiral Neel-type domain, in which an in-plane effective Dzyaloshinskii-Moriya interaction field exists. Fully deterministic magnetic magnetization switching in this trilayers is based on the enhancement of SOTs from a dedicated design of Pt/Co/Cr structures with two heavy metals Pt and Cr which show the opposite sign of spin Hall angles. We also demonstrated that the simultaneously accompanying Joule heating effect also plays a key role for field-free magnetization switching through the decrease of the propagation field.

physics.app-ph↗
arXiv2017

The Boltzmann equation with incoming boundary condition: global solutions and Navier-Stokes limit

We consider the Boltzmann equations with cutoff collision kernels in bounded domains. For the initial data with finite physical bounds, we prove the existence of global-in-time renormalized solutions in the sense of DiPerna-Lions endowed with incoming boundary condition. Moreover, we justify the limit as the Knudsen number $ε\rightarrow 0$ to Leray solutions of the incompressible Navier-Stokes-Fourier equations with homogeneous Dirichlet conditions from renormalized solutions of the scaled Boltzmann equations when the incoming data are close to the global Maxwellian in the sense of the boundary relative entropy of order $O(ε^3)$

math.AP↗
arXiv2017

Current induced magnetization switching in PtCoCr structures with enhanced perpendicular magnetic anisotropy and spin-orbit torques

Magnetic trilayers having large perpendicular magnetic anisotropy (PMA) and high spin-orbit torques (SOTs) efficiency are the key to fabricate nonvolatile magnetic memory and logic devices. In this work, PMA and SOTs are systematically studied in Pt/Co/Cr stacks as a function of Cr thickness. An enhanced perpendicular anisotropy field around 10189 Oe is obtained and is related to the interface between Co and Cr layers. In addition, an effective spin Hall angle up to 0.19 is observed due to the improved antidamping-like torque by employing dissimilar metals Pt and Cr with opposite signs of spin Hall angles on opposite sides of Co layer. Finally, we observed a nearly linear dependence between spin Hall angle and longitudinal resistivity from their temperature dependent properties, suggesting that the spin Hall effect may arise from extrinsic skew scattering mechanism. Our results indicate that 3d transition metal Cr with a large negative spin Hall angle could be used to engineer the interfaces of trilayers to enhance PMA and SOTs.

physics.app-ph↗
arXiv2017

Learning Spread-out Local Feature Descriptors

We propose a simple, yet powerful regularization technique that can be used to significantly improve both the pairwise and triplet losses in learning local feature descriptors. The idea is that in order to fully utilize the expressive power of the descriptor space, good local feature descriptors should be sufficiently "spread-out" over the space. In this work, we propose a regularization term to maximize the spread in feature descriptor inspired by the property of uniform distribution. We show that the proposed regularization with triplet loss outperforms existing Euclidean distance based descriptor learning techniques by a large margin. As an extension, the proposed regularization technique can also be used to improve image-level deep feature embedding.

cs.CV↗
arXiv2017

Superconvergence of immersed finite volume methods for one-dimensional interface problems

In this paper, we introduce a class of high order immersed finite volume methods (IFVM) for one-dimensional interface problems. We show the optimal convergence of IFVM in H1 and L2 norms. We also prove some superconvergence results of IFVM. To be more precise, the IFVM solution is superconvergent of order p+2 at the roots of generalized Lobatto polynomials, and the flux is superconvergent of order p+1 at generalized Gauss points on each element including the interface element. Furthermore, for diffusion interface problems, the convergence rates for IFVM solution at the mesh points and the flux at generalized Gaussian points can both be raised to 2p. These superconvergence results are consistent with those for the standard finite volume methods. Numerical examples are provided to confirm our theoretical analysis.

math.NA↗
arXiv2017

Active plasmon injection scheme for subdiffraction imaging with imperfect negative index flat lens

We present an active physical implementation of the recently introduced plasmon injection loss compensation scheme for Pendry's non-ideal negative index flat lens in the presence of realistic material losses and signal-dependent noise. In this active implementation, we propose to use a physically convolved external auxiliary source for signal amplification and suppression of the noise in the imaging system. In comparison with the previous passive implementations of the plasmon injection scheme for sub-diffraction limited imaging, where an inverse filter post-processing is used, the active implementation proposed here allows for deeper subwavelength imaging far beyond the passive post-processing scheme by extending the loss compensation to even higher spatial frequencies.

physics.optics↗
arXiv2017

An improved immersed finte element particle-in-cell method for plasma simulation

The particle-in-cell (PIC) method has been widely used for plasma simulation, because of its noise-reduction capability and moderate computational cost. The immersed finite element (IFE) method is efficient for solving interface problems on Cartesian meshes, which is desirable for PIC method. The combination of these two methods provides an effective tool for plasma simulation with complex interface/boundary. This paper introduces an improved IFE-PIC method that enhances the performance in both IFE and PIC aspects. For the electric field solver, we adopt the newly developed partially penalized IFE method with enhanced accuracy. For PIC implementation, we introduce a new interpolation technique to ensure the conservation of the charge. Numerical examples are provided to demonstrate the features of the improved IFE-PIC method.

math.NA↗
arXiv2017

Compressed Sensing with Prior Information via Maximizing Correlation

Compressed sensing (CS) with prior information concerns the problem of reconstructing a sparse signal with the aid of a similar signal which is known beforehand. We consider a new approach to integrate the prior information into CS via maximizing the correlation between the prior knowledge and the desired signal. We then present a geometric analysis for the proposed method under sub-Gaussian measurements. Our results reveal that if the prior information is good enough, then the proposed approach can improve the performance of the standard CS. Simulations are provided to verify our results.

cs.IT↗
arXiv2017

Photon-phonon parametric oscillation induced by the quadratic coupling in an optomechanical resonator

A direct photon-phonon parametric effect of the quadratic coupling on the mean-field dynamics of an optomechanical resonator in the large-scale-movement regime is found and investigated. Under a weak pumping power, the mechanical resonator damps to steady state with a nonlinear static response sensitively modified by the quadratic coupling. When the driving powerincreases beyond the static energy balance, the steady states lose their stabilities via Hopf bifurcations and the resonator produces stable self-sustained oscillation(limit-circle behavior) of discrete energies with step-like amplitudes due to the parametric effect of the quadratic coupling, which can be understood roughly by the power balance between gain and loss on the resonator. A further increase of the pumping power can induce chaotic dynamic of the resonator via a typical routine of period-doubling bifurcation but which can be stabilized by the parametric effect through an inversion bifurcation process back to limit-circle states. The bifurcation-to-inverse-bifurcation transitions are numerically verified by the maximal Lyapunov exponents of the dynamics and which indicate an efficient way to suppress the chaotic behavior of the optomechanical resonator by the quadratic coupling. Furthermore, the parametric effect of the quadratic coupling on the dynamic transitions of an optomechanical resonator can be conveniently detected or traced by the output power spectrum of the cavity field.

nlin.CD↗
arXiv2017

Normal State Gap in Parent Cuprate Pr2CuO4+/-δ

We present a tunneling study on single crystalline parent cuprate thin films, i.e. a series of Pr2CuO4+/-δ(PCO) with tunable superconducting transition temperature. The zero-bias anomaly of differential conductance, well reported in the normal state of R2-xCexCuO4 (R = Pr, Nd, La) and named as normal state gap (NSG), is observed in the Ce-free samples. This NSG behaves quite robust against the magnetic field up to 16 T, but fades away with increasing the temperature. Most importantly, we find that the magnitude of the NSG becomes larger with increasing point-contact junction resistance on the superconducting films, which is further enhanced in the non-superconducting samples of more oxygen disorders. The origination of NSG can be understood in the framework of Altshuler-Aronov-Lee (AAL) theory, where the disorder-induced electron-electron interactions suppress the density of states and thereby result in a soft Coulomb gap.

cond-mat.supr-con↗
arXiv2017

Faddeev fixed center approximation to $π\bar{K} K^*$ system and the $π_1(1600)$

We investigate the three-body system of $π\bar{K} K^*$ by using the fixed-center approximation to the Faddeev equation, taking the interaction between $π$ and $\bar{K}$, $π$ and $K^*$, and $\bar{K}$ and $K^*$ from the chiral unitary approach. The study is made assuming scattering of a $π$ on a $\bar{K} K^*$ cluster, which is known to generate the $f_1(1285)$ state. The resonant structure around $1650$ MeV shows up in the modulus squared of the $π$-$(\bar{K} K^*)_{f_1(1285)}$ scattering amplitude and suggests that a $π$-$(\bar{K} K^*)_{f_1(1285)}$ state, with "exotic" quantum numbers $J^{PC} = 1^{-+}$, can be formed. This state can be identified as the observed $π_1(1600)$ resonance. We suggest that this is the origin of the present $π_1(1600)$ resonance and propose to look at the $πf_1(1285)$ mode in future experiments to clarify the issue.

hep-ph↗
arXiv2017

Superconvergence of Immersed Finite Element Methods for Interface Problems

In this article, we study superconvergence properties of immersed finite element methods for the one dimensional elliptic interface problem. Due to low global regularity of the solution, classical superconvergence phenomenon for finite element methods disappears unless the discontinuity of the coefficient is resolved by partition. We show that immersed finite element solutions inherit all desired superconvergence properties from standard finite element methods without requiring the mesh to be aligned with the interface. In particular, on interface elements, superconvergence occurs at roots of generalized orthogonal polynomials that satisfy both orthogonality and interface jump conditions.

math.NA↗
arXiv2017

Fault Tolerant Control of Automotive Air Conditioning Systems using a GIMC Structure

Although model-based fault tolerant control (FTC) has become prevalent in various engineering fields, its application to air-conditioning systems is limited due to the lack of control-oriented models to characterize the phase change of refrigerant in the vapor compression cycle. The emergence of moving boundary method (MBM) illuminates a promising way for FTC design. In this paper, we exploit a control-oriented nonlinear model comparable to MBM to design an FTC framework with a generalized internal model control (GIMC) approach. A fault detector and isolator (FDI) is developed to identify potential actuator and sensor faults. A fault compensator is employed to compensate these faults if detected. Comprehensive simulations are carried out to evaluate the developed FTC framework with promising results. Plant variations are explicitly considered to enhance the gain-scheduled FTC developments.

eess.SY↗
arXiv2016

Supercloseness analysis and polynomial preserving recovery for a class of weak Galerkin method

In this paper, we analyze convergence and supercloseness properties of a class of weak Galerkin (WG) finite element methods for solving second-order elliptic problems. It is shown that the WG solution is superclose to the Lagrange type interpolation using Lobatto points. This supercloseness behavior is obtained through some newly designed stabilization terms. A post-processing technique using the polynomial preserving recovery (PPR) is introduced for WG approximation. Superconvergence analysis is carried out for the PPR approximation. Numerical examples are provided to verify our theoretical results.

math.NA↗