arXiv subjects

N. Zhang

Publications and source records attributed to N. Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Variational Dual-path Attention Network for CSI-Based Gesture Recognition

Wi-Fi gesture recognition based on Channel State Information (CSI) is challenged by high-dimensional noise and resource constraints on edge devices. Prevailing end-to-end models tightly couple feature extraction with classification, overlooking the inherent time-frequency sparsity of CSI and leading to redundancy and poor generalization. To address this, this paper proposes a lightweight feature preprocessing module--the Variational Dual-path Attention Network (VDAN). It performs structured feature refinement through frequency-domain filtering and temporal detection. Variational inference is introduced to model the uncertainty in attention weights, thereby enhancing robustness to noise. The design principles of the module are explained from the perspectives of the information bottleneck and regularization. Experiments on a public dataset demonstrate that the learned attention weights align with the physical sparse characteristics of CSI, verifying its interpretability. This work provides an efficient and explainable front-end processing solution for resource-constrained wireless sensing systems.

cs.NI↗

arXiv2025

Twinning Relationships in Coexisting Cubic and Tetragonal Phases of Ferroelectrics

Many ferroelectric materials undergo thermally driven transitions between cubic and tetragonal phases. Despite being well-known for decades, such transitions still pose unresolved questions, particularly about the role of domains in shaping the transition pathway. It is understood that tetragonal domain assemblages are crucial for mismatch-free coexistence of both tetragonal and cubic phases, as described by the Weschler-Lieberman-Read (WLR) crystallographic theory. However, direct experimental characterization of domain patterns during the phase transition remains challenging. Here, we enhance the single-crystal X-ray diffraction to investigate domain microstructures during cubic-tetragonal phase transition, leveraging the bulk-penetrating and non-destructive character of this method. We extend the WLR model by the analytical expressions for the orientation relationships between the real and reciprocal bases of the cubic phase and tetragonal domains. We systematically catalogue the Bragg peak separations for 24 possible coexistence variants and validate these predictions using 3D reciprocal space maps from a piezo-/ferroelectric PMN-35PT crystal at phase coexistence temperatures. Our results provide a framework for interpreting domain configurations during the phase transition and offer insights applicable to other systems involving orientation relationships in coexisting phases.

cond-mat.mtrl-sci↗

arXiv2024

Robust and Sparse Portfolio Selection: Quantitative Insights and Efficient Algorithms

We extend the classical mean-variance (MV) framework and propose a robust and sparse portfolio selection model incorporating an ellipsoidal uncertainty set to reduce the impact of estimation errors and fixed transaction costs to penalize over-diversification. In the literature, the MV model under fixed transaction costs is referred to as the sparse or cardinality-constrained MV optimization, which is a mixed integer problem and is challenging to solve when the number of assets is large. We develop an efficient semismooth Newton-based proximal difference-of-convex algorithm to solve the proposed model and prove its convergence to at least a local minimizer with a locally linear convergence rate. We explore properties of the robust and sparse portfolio both analytically and numerically. In particular, we show that the MV optimization is indeed a robust procedure as long as an investor makes the proper choice on the risk-aversion coefficient. We contribute to the literature by proving that there is a one-to-one correspondence between the risk-aversion coefficient and the level of robustness. Moreover, we characterize how the number of traded assets changes with respect to the interaction between the level of uncertainty on model parameters and the magnitude of transaction cost.

q-fin.PM↗

arXiv2023

The Lobster Eye Imager for Astronomy Onboard the SATech-01 Satellite

The Lobster Eye Imager for Astronomy (LEIA), a pathfinder of the Wide-field X-ray Telescope of the Einstein Probe (EP) mission, was successfully launched onboard the SATech-01 satellite of the Chinese Academy of Sciences on 27 July 2022. In this paper, we introduce the design and on-ground test results of the LEIA instrument. Using state-of-the-art Micro-Pore Optics (MPO), a wide field-of-view (FoV) of 346 square degrees (18.6 degrees * 18.6 degrees) of the X-ray imager is realized. An optical assembly composed of 36 MPO chips is used to focus incident X-ray photons, and four large-format complementary metal-oxide semiconductor (CMOS) sensors, each of 6 cm * 6 cm, are used as the focal plane detectors. The instrument has an angular resolution of 4 - 8 arcmin (in FWHM) for the central focal spot of the point spread function, and an effective area of 2 - 3 cm2 at 1 keV in essentially all the directions within the field of view. The detection passband is 0.5 - 4 keV in the soft X-rays and the sensitivity is 2 - 3 * 10-11 erg s-1 cm-2 (about 1 mini-Crab) at 1,000 second observation. The total weight of LEIA is 56 kg and the power is 85 W. The satellite, with a design lifetime of 2 years, operates in a Sun-synchronous orbit of 500 km with an orbital period of 95 minutes. LEIA is paving the way for future missions by verifying in flight the technologies of both novel focusing imaging optics and CMOS sensors for X-ray observation, and by optimizing the working setups of the instrumental parameters. In addition, LEIA is able to carry out scientific observations to find new transients and to monitor known sources in the soft X-ray band, albeit limited useful observing time available.

astro-ph.IM↗

arXiv2023

pH-dependent water permeability switching and its memory in 1T' MoS$_2$ membranes

Intelligent transport of molecular species across different barriers is critical for various biological functions and is achieved through the unique properties of biological membranes. An essential feature of intelligent transport is the ability to adapt to different external and internal conditions and also the ability to memorise the previous state. In biological systems, the most common form of such intelligence is expressed as hysteresis. Despite numerous advances made over previous decades on smart membranes, it is still a challenge for a synthetic membrane to display stable hysteretic behaviour for molecular transport. Here we show the memory effects and stimuli regulated transport of molecules through an intelligent phase changing MoS$_2$ membrane in response to external pH. We show that water and ion permeation through 1T' MoS$_2$ membranes follows a pH dependent hysteresis with a permeation rate that switches by a few orders of magnitude. We demonstrate that this phenomenon is unique to the 1T' phase of MoS$_2$ due to the presence of surface charge and exchangeable ions on the surface. We further demonstrate the potential application of this phenomenon in autonomous wound infection monitoring and pH-dependent nanofiltration. Our work significantly deepens understanding of the mechanism of water transport at the nanoscale and opens an avenue for developing neuromorphic applications, smart drug delivery systems, point-of-care diagnostics, smart sensors, and intelligent filtration devices.

cond-mat.soft↗

arXiv2022

First wide field-of-view X-ray observations by a lobster eye focusing telescope in orbit

As a novel X-ray focusing technology, lobster eye micro-pore optics (MPO) feature both a wide observing field of view and true imaging capability, promising sky monitoring with significantly improved sensitivity and spatial resolution in soft X-rays. Since first proposed by Angel (1979), the optics have been extensively studied, developed and trialed over the past decades. In this Letter, we report on the first-light results from a flight experiment of the Lobster Eye Imager for Astronomy ($LEIA$), a pathfinder of the wide-field X-ray telescope of the Einstein Probe mission. The piggyback imager, launched in July 2022, has a mostly un-vignetted field of view of $18.6^\circ \times 18.6^\circ $. Its spatial resolution is in the range of 4$-$7 arcmin in FWHM and the focal spot effective area is 2$-$3 cm$^2$, both showing only mild fluctuations across the field of view. We present images of the Galactic center region, Sco X-1 and the diffuse Cygnus Loop nebular taken in snapshot observations over 0.5$-$4 keV. These are truly wide-field X-ray images of celestial bodies observed, for the first time, by a focusing imaging telescope. Initial analyses of the in-flight data show excellent agreement between the observed images and the on-ground calibration and simulations. The instrument and its characterization are briefly described, as well as the flight experiment. The results provide a solid basis for the development of the present and proposed wide-field X-ray missions using lobster eye MPO.

astro-ph.HE↗

arXiv2022

The origin of Earth's mantle nitrogen: primordial or early biogeochemical cycling?

Earth's mantle nitrogen (N) content is comparable to that found in its N-rich atmosphere. Mantle N has been proposed to be primordial or sourced by later subduction, yet its origin has not been elucidated. Here we model N partitioning during the magma ocean stage following planet formation and the subsequent cycling between the surface and mantle over Earth history using argon (Ar) and N isotopes as tracers. The partitioning model, constrained by Ar, shows that only about 10% of the total N content can be trapped in the solidified mantle due to N's low solubility in magma and low partitioning coefficients in minerals in oxidized conditions supported from geophysical and geochemical studies. A possible solution for the primordial origin is that Earth had about 10 times more N at the time of magma ocean solidification. We show that the excess N could be removed by impact erosion during late accretion. The cycling model, constrained by N isotopes, shows that mantle N can originate from efficient N subduction, if the sedimentary N burial rate on early Earth is comparable to that of modern Earth. Such a high N burial rate requires biotic processing. Finally, our model provides a methodology to distinguish the two possible origins with future analysis of the surface and mantle N isotope record.

physics.geo-ph↗

arXiv2020

Convex Floating Bodies of Equilibrium

We study a long standing open problem by Ulam, which is whether the Euclidean ball is the unique body of uniform density which will float in equilibrium in any direction. We answer this problem in the class of origin symmetric n-dimensional convex bodies whose relative density to water is 1/2. For n=3, this result is due to Falconer.

math.MG↗

arXiv2020

ATiO$_{3}$/TiO (A=Pb, Sn) superlattice: bridging ferroelectricity and conductivity

We propose to insert TiO layers to perovskite ATiO$_{3}$ to form a superlattice and use first-principles calculations to investigate its basic properties. Our computational analysis shows that the structure, which consists of repeated ATiO_{3} and TiO layers, has strong anisotropic conductivity. The structure immediately suggests a possible control of its conductivity by ion displacements related to its intrinsic ferroelectricity. In addition, we have obtained the structural information of its low-energy phases with the aid of phonon calculation and examined their evolution with epitaxial strain. Since the number of possible combinations is huge, we have therefore suggested an approach to mix perovskites and simpler metal-oxides to build materials with novel properties.

cond-mat.mtrl-sci↗

arXiv2019

Generation of low-symmetry perovskite structures for ab initio computation

Ion displacements are the cause of the ferroelectricity in perovskites. By properly shifting ions, ab initio computations have been extensively used to investigate properties of perovsites in various structural phases. In addition to the relatively simple ion displacements, perovskites have another type of structural distortion known as antiferrodistortion or oxygen octahedron tilting. The interplay between these two types of distortions have generated abundant structural phases that can be tedious to prepare for ab initio computation, especially for large supercells. Here, we design and implement a computer program to facilitate the generation of distorted perovskite structures, which can be readily used for ab initio computation to gain further insight into the perovskite of a given structural phase.

cond-mat.mtrl-sci↗

arXiv2018

Understanding doped perovskite ferroelectrics with defective dipole model

While doping is widely used for tuning physical properties of perovskites in experiments, it remains a challenge to exactly know how doping achieves the desired effects. Here, we propose an empirical and computationally tractable model to understand the effects of doping with Fe-doped BaTiO$_{3}$ as an example. This model assumes that the lattice sites occupied by Fe ion and its nearest six neighbors lose their ability to polarize, giving rise to a small cluster of defective dipoles. Employing this model in Monte-Carlo simulations, many important features like reduced polarization and the convergence of phase transition temperatures, which have been observed experimentally in acceptor doped systems, are successfully obtained. Based on microscopic information of dipole configurations, we provide insights into the driving forces behind doping effects and propose that active dipoles, which exist in proximity to the defective dipoles, can account for experimentally observed phenomena. Close attention to these dipoles are necessary to understand and predict doping effects.

cond-mat.mtrl-sci↗

arXiv2018

Epitaxially strained SnTiO$_{3}$ at finite temperatures

Combining effective Hamiltonian and direct $ab$ $initio$ computation, we obtain the phase diagram of SnTiO$_{3}$ with respect to epitaxial strain and temperature, demonstrating the complex features of the phase diagram and providing insight into the epitaxially strained SnTiO$_{3}$, a presumably simple perovskite. In the phase diagram, two triple points are found, which may be exploited to to achieve high-performance piezoelectric effects. On the other hand, despite the inclusion of the degree of freedoms related to oxygen octahedron tilting, it is found that ferroelectric displacements dominate the structural phases over the whole strain misfit range. Finally, it is shown that the SnTiO$_{3}$ can be converted from hard to soft ferroelectrics with epitaxial strain.

cond-mat.mtrl-sci↗

arXiv2018

Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites

A detailed understanding of the carrier dynamics and emission characteristics of organic-inorganic lead halide perovskites is critical for their optoelectronic and energy harvesting applications. In this work, we reveal the impact of the crystal lattice disorder on the photo-generated electron-hole pairs through low-temperature photoluminescence measurements. We provide strong evidence that the intrinsic disorder forms a sub-bandgap tail density of states, which determines the emission properties at low temperature. The PL spectra indicate that the disorder evolves with increasing temperature, changing its character from static to dynamic. This change is accompanied by a rapid drop of the PL efficiency, originating from the increased mobility of excitons/polarons, which enables them to reach deep non-radiative recombination centers more easily.

cond-mat.mtrl-sci↗

arXiv2018

Simulation Study on Local Influence Diagnosis for Poisson Mixed-Effect Linear Model

Given that hierarchical count data in many fields are not Normally-distributed and include random effects, this paper extends the Generalized Linear Mixed Models (GLMMs) into Poisson Mixed-Effect Linear Model (PMELM) and do numerical simulation experiments to verify the approach proposed by Rakhmawati et al. (2016) in detecting outliers. This paper produces random data based on epilepsy longitudinal data in Thall and Vail (1990), use six ways to contaminate it and try to use code mentioned in supplementary materials in previous research to detect the man-made outlier. Output shows that this method is effective sometimes but does not always work, this is probably because of the limitation of coding or some other reasons. Even though the data set and local influence method has been researched and analyzed extensively in previous papers, this paper makes contributions in data visualization. Figures in this paper show the effect of each influencial component, which are clearer than the original output in R and SAS.

stat.ME↗

arXiv2017

Probing the inter-layer exciton physics in a MoS$_2$/MoSe$_2$/MoS$_2$ van der Waals heterostructure

Stacking atomic monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of TMD heterostructures should result in the formation of inter-layer excitons with long lifetimes and robust valley polarization. However, these features have been observed simultaneously only in MoSe$_2$/WSe$_2$ heterostructures. Here we report on the observation of long lived inter-layer exciton emission in a MoS$_2$/MoSe$_2$/MoS$_2$ trilayer van der Waals heterostructure. The inter-layer nature of the observed transition is confirmed by photoluminescence spectroscopy, as well as by analyzing the temporal, excitation power and temperature dependence of the inter-layer emission peak. The observed complex photoluminescence dynamics suggests the presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We show that circularly polarized optical pumping results in long lived valley polarization of inter-layer exciton. Intriguingly, the inter-layer exciton photoluminescence has helicity opposite to the excitation. Our results show that through a careful choice of the TMDs forming the van der Waals heterostructure it is possible to control the circular polarization of the inter-layer exciton emission.

cond-mat.mes-hall↗

arXiv2016

Electrostriction coefficient of ferroelectric materials from ab initio computation

Electrostriction is an important material property that characterizes how strain changes with the development of polarization inside a material. We show that \textit{ab initio }techniques developed in recent years can be exploited to compute and understand electrostriction of ferroelectric materials. Here, electrostriction coefficients of ferroelectric BaTiO$_{3}$, PbTiO$_{3}$, as well as dielectric BaZrO$_{3}$, are obtained and analyzed. Possible causes of the difference between experimental and numerical results are discussed. We also identified that relative displacements between certain ions at a given polarization could be a good indicator of a material's electrostriction property.

cond-mat.mtrl-sci↗

arXiv2005

Generalization of Classical Statistical Mechanics to Quantum Mechanics and Stable Property of Condensed Matter

Classical statistical average values are generally generalized to average values of quantum mechanics, it is discovered that quantum mechanics is direct generalization of classical statistical mechanics, and we generally deduce both a new general continuous eigenvalue equation and a general discrete eigenvalue equation in quantum mechanics, and discover that a eigenvalue of quantum mechanics is just an extreme value of an operator in possibility distribution, the eigenvalue f is just classical observable quantity. A general classical statistical uncertain relation is further given, the general classical statistical uncertain relation is generally generalized to quantum uncertainty principle, the two lost conditions in classical uncertain relation and quantum uncertainty principle, respectively, are found. We generally expound the relations among uncertainty principle, singularity and condensed matter stability, discover that quantum uncertainty principle prevents from the appearance of singularity of the electromagnetic potential between nucleus and electrons, and give the failure conditions of quantum uncertainty principle. Finally, we discover that the classical limit of quantum mechanics is classical statistical mechanics, the classical statistical mechanics may further be degenerated to classical mechanics, and we discover that only saying that the classical limit of quantum mechanics is classical mechanics is mistake. As application examples, we deduce both Shrodinger equation and state superposition principle, deduce that there exist decoherent factor from a general mathematical representation of state superposition principle, and the consistent difficulty between statistical interpretation of quantum mechanics and determinant property of classical mechanics is overcome.

quant-ph↗

arXiv1999

BeppoSAX observations of Mkn 3: Piercing through the torus of a Seyfert 2 galaxy

A new BeppoSAX broad-band (0.6-150 keV) spectrum of the Seyfert 2 galaxy Mkn 3 is presented. The spectrum provides a direct measurement of a large, neutral column of gas with Nh~10**24 cm**-2 in the source direction. The source, as bright as 3C 273 above 10 keV, has a steep (Gamma~1.8) spectrum without any evidence of a high-energy cutoff up to at least 150 keV. At lower energies, the data are best modeled with the addition of an unabsorbed reflection component. Combining these data with previous Ginga and ASCA observations, the Fe K$_α$ and reflection continuum indicate that the reprocessed emission is responding slower than the intrinsic continuum variations suggesting a size of the reprocessor >= 2 pc. Identifying such a reprocessor with a (close to edge-on) obscuring torus, the overall result fits well into unified models since, presumably, one can interpret the strong absorption as due to transmission through the rim of the torus and the unabsorbed (directly viewed) reflection component as due to reprocessing from the torus inner surface.

astro-ph↗