arXiv subjects

Jian-Jun Wang

Publications and source records attributed to Jian-Jun Wang.

5 recordsLinked to original sources

arXiv2026

SHAARP: An Open-Source Package for Analytical and Numerical Modeling of Optical Second Harmonic Generation in Anisotropic Crystals

Optical second harmonic generation is a second-order nonlinear process that combines two photons of a given frequency into a third photon at twice the frequency. Due to the symmetry constraints, it is widely used as a sensitive probe to detect broken inversion symmetry and local polar order. Analytical modeling of the electric-dipole SHG response is essential to extract fundamental properties of materials from experiments. However, complexity builds up dramatically in the analytical model when the probed crystal is of a low bulk crystal symmetry, with a low-symmetry surface orientation, exhibits absorption and dispersion, and consists of multiple interfaces. As a result, there is a largely uneven landscape in the literature on the SHG modeling of new materials, involving numerous approximations and a wide range of (in)accuracies, leading to a rather scattered dataset of reported SHG nonlinear susceptibility. Towards streamlining the reliability and accuracy of this process, we have developed an open-source package called the Second Harmonic Analysis of Anisotropic Rotational Polarimetry (SHAARP) which derives analytical solutions and performs numerical simulations of reflection SHG from a single interface for homogeneous crystals. Five key generalizations in SHG modeling are implemented, including all crystal symmetries down to triclinic, any crystal orientation, complex dielectric tensor (refractive indices) with frequency dispersion, and general polarization states of the light. SHAARP enables accurate anisotropic modeling of SHG response for a broad range of materials systems. The method is extendible to multiple interfaces. The code is free to download from https://github.com/Rui-Zu/SHAARP

physics.optics↗

arXiv2026

On Explicit Super-Expressive Approximation for Neural Networks

In this work, we investigate the fixed-architecture neural network approximation with explicit parameter bounds and elementary activations. While prior work demonstrated super-expressive approximation using fixed-size networks, they lack quantitative and non-asymptotic characterizations of parameter magnitude with respect to the approximation error. We resolve this issue by introducing the Chinese Remainder Theorem as a constructive encoding mechanism. For Lipschitz continuous functions on $[0,1]^D$, we construct a width-$\max\{D,4\}$, depth-$5$ network with explicit parameter-error trade-offs. For Hölder-smooth functions in $C^{r,γ}_A\left([0,1]^D\right)$, our fixed network of width $\max\{2D,\ D+5N+1\}$ and depth $r + 9$ achieves the parameter magnitude $\mathcal{P}$ bounded by $\log_2 \mathcal{P}=\mathcal{O}\bigl(\varepsilon^{-2D/(r+γ)}\log(1/\varepsilon)\bigr)$. This is the dual result compared to those in the parameter-bounded and architecture-unbounded paradigm.

cs.LG↗

arXiv2019

A Fast and Robust TSVM for Pattern Classification

Twin support vector machine~(TSVM) is a powerful learning algorithm by solving a pair of smaller SVM-type problems. However, there are still some specific issues such as low efficiency and weak robustness when it is faced with some real applications. In this paper, we propose a Fast and Robust TSVM~(FR-TSVM) to deal with the above issues. In order to alleviate the effects of noisy inputs, we propose an effective fuzzy membership function and reformulate the TSVMs such that different input instances can make different contributions to the learning of the separating hyperplanes. To further speed up the training procedure, we develop an efficient coordinate descent algorithm with shirking to solve the involved a pair of quadratic programming problems (QPPs). Moreover, theoretical foundations of the proposed model are analyzed in details. The experimental results on several artificial and benchmark datasets indicate that the FR-TSVM not only obtains a fast learning speed but also shows a robust classification performance. Code has been made available at: https://github.com/gaobb/FR-TSVM.

cs.LG↗

arXiv2017

Mechanical Switching of Nanoscale Multiferroic Phase Boundaries

Tuning the lattice degree of freedom in nanoscale functional crystals is critical to exploit the emerging functionalities such as piezoelectricity, shape-memory effect, or piezomagnetism, which are attributed to the intrinsic lattice-polar or lattice-spin coupling. Here it is reported that a mechanical probe can be a dynamic tool to switch the ferroic orders at the nanoscale multiferroic phase boundaries in BiFeO 3 with a phase mixture, where the material can be reversibly transformed between the "soft" tetragonal-like and the "hard" rhombohedrallike structures. The microscopic origin of the nonvolatile mechanical switching of the multiferroic phase boundaries, coupled with a reversible 180° rotation of the in-plane ferroelectric polarization, is the nanoscale pressure-induced elastic deformation and reconstruction of the spontaneous strain gradient across the multiferroic phase boundaries. The reversible control of the room-temperature multiple ferroic orders using a pure mechanical stimulus may bring us a new pathway to achieve the potential energy conversion and sensing applications.

cond-mat.mtrl-sci↗

arXiv2007

The rare decays $B^+_u\toπ^+\ell^+\ell^-$, $ρ^+\ell^+\ell^-$ and $B^0_d\to \ell^+\ell^-$ in the R-parity violating supersymmetry

We study the rare decays $B^{+}_u\to π^{+}\ell^{+}\ell^{-}$, $ρ^{+}\ell^{+}\ell^{-}$ and $B^{0}_d \to \ell^{+} \ell^{-}(\ell=e,μ)$ in the R-parity violating supersymmetric standard model. From the latest upper limits of $\mathcal{B}(B^+_u\toπ^+\ell^+\ell^-)$ and $\mathcal{B}(B^0_d\to\ell^+\ell^-)$, we have derived new upper bounds on the relevant R-parity violating couplings products, which are stronger than the existing ones. Using the constrained parameter space, we present the R-parity violating effects on the branching ratios and the forward-backward asymmetries of these decays. We find that $\mathcal{B}(B^0_d\to \ell^+\ell^-)$ and $\mathcal{B}(B^+_u \toρ^+\ell^+\ell^-)$ could be enhanced several orders by the R-parity violating sneutrino and squark exchanges, respectively. The R-parity violating effects on the dilepton invariant mass spectra of $B^+_u\to π^+\ell^+\ell^-$, $ρ^+\ell^+\ell^-$ and the normalized forward-backward asymmetry $\mathcal{A}_{FB}(B^+_u\toπ^+\ell^+\ell^-)$ and $\mathcal{A}_{FB}(B^+_u \toρ^+\ell^+\ell^-)$ are studied in detail. Our results could be used to probe the R-parity violating effects and will correlate with searches for the direct R-parity violating signals at the future experiments.

hep-ph↗