arXiv subjects

Xin Zhang

Publications and source records attributed to Xin Zhang.

At least 1,081 records · Page 60Linked to original sources

arXiv2018

Inducing Super-Approximation

Let $Γ_2\subseteq Γ_1$ be finitely generated subgroups of ${\rm GL}_{n_0}(\mathbb{Z}[1/q_0])$. For $i=1$ or $2$, let $\mathbb{G}_i$ be the Zariski-closure of $Γ_i$ in $({\rm GL}_{n_0})_{\mathbb{Q}}$, $\mathbb{G}_i^{\circ}$ be the Zariski-connected component of $\mathbb{G}_i$, and let $G_i$ be the closure of $Γ_i$ in $\prod_{p\nmid q_0}{\rm GL}_{n_0}(\mathbb{Z}_p)$. In this article we prove that, if $\mathbb{G}_1^{\circ}$ is the smallest closed normal subgroup of $\mathbb{G}_1^{\circ}$ which contains $\mathbb{G}_2^{\circ}$ and $Γ_2\curvearrowright G_2$ has spectral gap, then $Γ_1\curvearrowright G_1$ has spectral gap.

math.GR↗

arXiv2018

Effect of different filling tendencies on the spatial quantum Zeno effect

The quantum Zeno effect is deeply related to the quantum measurement process and thus studies of it may help shed light on the hitherto mysterious measurement process in quantum mechanics. Recently, the spatial quantum Zeno effect is observed in a Bose-Einstein condensate depleted by an electron beam. We theoretically investigate how different intrinsic tendencies of filling affect the quantum Zeno effect in this system by changing the impinging point of the electron beam along the inhomogeneous condensate. Surprisingly, we find no visible effect on the critical dissipation intensity at which the quantum Zeno effect appear. Our finding shows the recent capability of combining the Bose-Einstein condensate with an electron beam offers a great opportunity for studying the spatial quantum Zeno effect, and more generally the dynamics of a quantum many-body system out of equilibrium.

quant-ph↗

arXiv2018

Sputtered Gold Nanoparticles Enhanced Quantum Dot Light-emitting Diodes

Surface plasmonic effects of metallic particles have been known to be an effective method to improve the performance light emitting didoes. In this work, we reported the sputtered Au nanoparticles enhanced electroluminescence in inverted quantum dot light emitting diodes (ITO/Au NPs/ZnMgO/QDs/TFB/PEDOT:PSS/Al). By combining the time-resolved photoluminescence, transient electroluminescence and ultraviolet photoelectron spectrometer measurements, the enhancement can be explained to the internal field enhanced exciton coupling to surface plasmons and the increased electron injection rate with Au nanoparticles incorporation. Phenomenological numerical calculations indicated that the electron mobility of the electron transport layer was increased from 1.39x10-5 to 1.91x10-5 cm2/V.s for Au NPs modified devices. As a result, the maximum device luminescence is enhanced by 1.41 folds from 14,600 to 20,720 cd/cm2 and maximum current efficiency is improved by 1.29 folds from 3.12 to 4.02 cd/A.

cond-mat.mtrl-sci↗

arXiv2018

Probing the sign-changeable interaction between dark energy and dark matter with current observations

We consider the models of vacuum energy interacting with cold dark matter in this study, in which the coupling can change sigh during the cosmological evolution. We parameterize the running coupling $b$ by the form $b(a)=b_0a+b_e(1-a)$, where at the early-time the coupling is given by a constant $b_{e}$ and today the coupling is described by another constant $b_{0}$. We explore six specific models with (i) $Q(a)=b(a)H_0ρ_0$, (ii) $Q(a)=b(a)H_0ρ_{\rm de}$, (iii) $Q(a)=b(a)H_0ρ_{\rm c}$, (iv) $Q(a)=b(a)Hρ_0$, (v) $Q(a)=b(a)Hρ_{\rm de}$, and (vi) $Q(a)=b(a)Hρ_{\rm c}$. The current observational data sets we use to constrain the models include the JLA compilation of type Ia supernova data, the Planck 2015 distance priors data of cosmic microwave background observation, the baryon acoustic oscillations measurements, and the Hubble constant direct measurement. We find that, for all the models, we have $b_0<0$ and $b_e>0$ at around the 1$σ$ level, and $b_0$ and $b_e$ are in extremely strong anti-correlation. Our results show that the coupling changes sign during the evolution at about the 1$σ$ level, i.e., the energy transfer is from dark matter to dark energy when dark matter dominates the universe and the energy transfer is from dark energy to dark matter when dark energy dominates the universe.

astro-ph.CO↗

arXiv2017

Coulomb-coupled quantum-dot thermal transistors

A quantum-dot thermal transistor consisting of three Coulomb-coupled quantum dots coupled to respective electronic reservoirs by tunnel contacts is established. The heat flows through the collector and emitter can be controlled by the temperature of the base. It is found that a small change in the base heat flow can induce a large heat flow change in the collector and emitter. The huge amplification factor can be obtained by optimizing the Coulomb interaction between the collector and the emitter or by decreasing the energy-dependent tunneling rate at the base. The proposed quantum-dot thermal transistor may open up potential applications in low-temperature solid-state thermal circuits at the nanoscale.

physics.app-ph↗

arXiv2017

Code Generation Techniques for Raw Data Processing

The motivation of the current study was to design an algorithm that can speed up the processing of a query. The important feature is generating code dynamically for a specific query. We present the technique of code generation that is applied to query processing on a raw file. The idea was to customize a query program with a given query and generate a machine- and query-specific source code. The generated code is compiled by GCC, Clang or any other C/C++ compiler, and the compiled file is dynamically linked to the main program for further processing. Code generation reduces the cost of generalizing query processing. It also avoids the overhead of the conventional interpretation during achieve high performance. Database Management Systems (DBMSs) perform excellent jobs in many aspects of big data, such as storage, indexing, and analysis. DBMSs typically format entire data and load them into their storage layer. They increase data-to-query time, which is the cost time it takes to convert data into a specific schema and persist them in a disk. Ideally, DBMSs should adapt to the input data and extract one/some of columns, not the entire data, that is/are associated with a given query. Therefore, the query engine on a raw file can reduce the cost of conventional general operators and avoid some unnecessary procedures, such as fully scanning, tokenizing and paring the whole data. In the current study, we introduce our code-generation approach for in-situ processing of raw files, which is based on the template approach and the hype approach. The approach minimizes the data-to-query time and achieves a high performance for query processing. There are some benefits from our work: reducing branches and instructions, unrolling loops, eliminating unnecessary data type checks and optimizing the binary code with a compiler on a local machine.

cs.DB↗

arXiv2017

Electromechanically Tunable Metasurface Transmission Waveplate at Terahertz Frequencies

Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to materials characterization and identification. We present a reconfigurable terahertz metasurface quarter-waveplate consisting of electromechanically actuated micro-cantilever arrays. Our anisotropic metasurface enables tunable polarization conversion cantilever actuation. Specifically, voltage-based actuation provides mode selective control of the resonance frequency, enabling real-time tuning of the polarization state of the transmitted light. The polarization tunable metasurface has been fabricated using surface micromachining and characterized using terahertz time domain spectroscopy. We observe a ~230 GHz cantilever actuated frequency shift of the resonance mode, sufficient to modulate the transmitted wave from pure circular polarization to linear polarization. Our CMOS-compatible tunable quarter-waveplate enriches the library of terahertz optical components, thereby facilitating practical applications of terahertz technologies.

physics.app-ph↗

arXiv2017

Reexploration of interacting holographic dark energy model: Cases of interaction term excluding the Hubble parameter

In this paper, we make a deep analysis for the five typical interacting holographic dark energy models with the interaction terms $Q=3βH_{0}ρ_{\rm{de}}$, $Q=3βH_{0}ρ_{\rm{c}}$, $Q=3βH_{0}(ρ_{\rm{de}}+ρ_{\rm c})$, $Q=3βH_{0}\sqrt{ρ_{\rm{de}}ρ_{\rm c}}$, and $Q=3βH_{0}\frac{ρ_{\rm{de}}ρ_{c}}{ρ_{\rm{de}}+ρ_{\rm c}}$, respectively. We obtain observational constraints on these models by using the type Ia supernova data (the Joint Light-curve Analysis sample), the cosmic microwave background data (Planck 2015 distance priors), the baryon acoustic oscillations data, and the direct measurement of the Hubble constant. We find that the values of $χ_{\rm min}^2$ for all the five models are almost equal (around~699), indicating that the current observational data equally favor these IHDE models. In addition, a comparison with the cases of interaction term involving the Hubble parameter $H$ is also made.

astro-ph.CO↗

arXiv2017

Sleep Stage Classification Based on Multi-level Feature Learning and Recurrent Neural Networks via Wearable Device

This paper proposes a practical approach for automatic sleep stage classification based on a multi-level feature learning framework and Recurrent Neural Network (RNN) classifier using heart rate and wrist actigraphy derived from a wearable device. The feature learning framework is designed to extract low- and mid-level features. Low-level features capture temporal and frequency domain properties and mid-level features learn compositions and structural information of signals. Since sleep staging is a sequential problem with long-term dependencies, we take advantage of RNNs with Bidirectional Long Short-Term Memory (BLSTM) architectures for sequence data learning. To simulate the actual situation of daily sleep, experiments are conducted with a resting group in which sleep is recorded in resting state, and a comprehensive group in which both resting sleep and non-resting sleep are included.We evaluate the algorithm based on an eight-fold cross validation to classify five sleep stages (W, N1, N2, N3, and REM). The proposed algorithm achieves weighted precision, recall and F1 score of 58.0%, 60.3%, and 58.2% in the resting group and 58.5%, 61.1%, and 58.5% in the comprehensive group, respectively. Various comparison experiments demonstrate the effectiveness of feature learning and BLSTM. We further explore the influence of depth and width of RNNs on performance. Our method is specially proposed for wearable devices and is expected to be applicable for long-term sleep monitoring at home. Without using too much prior domain knowledge, our method has the potential to generalize sleep disorder detection.

stat.ML↗

arXiv2017

Vibration Damping of Carbon Nanotube Assembly Materials

Vibration reduction is of great importance in various engineering applications, and a material that exhibits good vibration damping along with high strength and modulus has become more and more vital. Owing to the superior mechanical property of carbon nanotube (CNT), new types of vibration damping material can be developed. This paper presents recent advancements, including our progresses, in the development of high-damping macroscopic CNT assembly materials, such as forests, gels, films, and fibers. In these assemblies, structural deformation of CNTs, zipping and unzipping at CNT connection nodes, strengthening and welding of the nodes, and sliding between CNTs or CNT bundles are playing important roles in determining the viscoelasticity, and elasticity as well. Towards the damping enhancement, strategies for micro-structure and interface design are also discussed.

cond-mat.mtrl-sci↗

arXiv2017

Constraints on inflation revisited: An analysis including the latest local measurement of the Hubble constant

We revisit the constraints on inflation models by using the current cosmological observations involving the latest local measurement of the Hubble constant ($H_{0} = 73.00\pm 1.75$ km s $^{-1}$ Mpc$^{-1}$). We constrain the primordial power spectra of both scalar and tensor perturbations with the observational data including the Planck 2015 CMB full data, the BICEP2 and Keck Array CMB B-mode data, the BAO data, and the direct measurement of $H_0$. In order to relieve the tension between the local determination of the Hubble constant and the other astrophysical observations, we consider the additional parameter $N_{\rm eff}$ in the cosmological model. We find that, for the $Λ$CDM+$r$+$N_{\rm eff}$ model, the scale invariance is only excluded at the 3.3$σ$ level, and $ΔN_{\rm eff}>0$ is favored at the 1.6$σ$ level. Comparing the obtained 1$σ$ and 2$σ$ contours of $(n_s,r)$ with the theoretical predictions of selected inflation models, we find that both the convex and concave potentials are favored at 2$σ$ level, the natural inflation model is excluded at more than 2$σ$ level, the Starobinsky $R^2$ inflation model is only favored at around 2$σ$ level, and the spontaneously broken SUSY inflation model is now the most favored model.

astro-ph.CO↗

arXiv2017

Model-free Envelope Dimension Selection

An envelope is a targeted dimension reduction subspace for simultaneously achieving dimension reduction and improving parameter estimation efficiency. While many envelope methods have been proposed in recent years, all envelope methods hinge on the knowledge of a key hyperparameter, the structural dimension of the envelope. How to estimate the envelope dimension consistently is of substantial interest from both theoretical and practical aspects. Moreover, very recent advances in the literature have generalized envelope as a model-free method, which makes selecting the envelope dimension even more challenging. Likelihood-based approaches such as information criteria and likelihood-ratio tests either cannot be directly applied or have no theoretical justification. To address this critical issue of dimension selection, we propose two unified approaches -- called FG and 1D selections -- for determining the envelope dimension that can be applied to any envelope models and methods. The two model-free selection approaches are based on the two different envelope optimization procedures: the full Grassmannian (FG) optimization and the 1D algorithm (Cook and Zhang, 2016), and are shown to be capable of correctly identifying the structural dimension with a probability tending to 1 under mild moment conditions as the sample size increases. While the FG selection unifies and generalizes the BIC and modified BIC approaches that existing in the literature, and hence provides the theoretical justification of them under weak moment condition and model-free context, the 1D selection is computationally more stable and efficient in finite sample. Extensive simulations and a real data analysis demonstrate the superb performance of our proposals.

stat.ME↗

arXiv2017

Statistical Regularity of Apollonian Gaskets

Apollonian gaskets are formed by repeatedly filling the gaps between three mutually tangent circles with further tangent circles. In this paper we give explicit formulas for the the limiting pair correlation and the limiting nearest neighbor spacing of centers of circles from a fixed Apollonian gasket. These are corollaries of the convergence of moments that we prove. The input from ergodic theory is an extension of Mohammadi-Oh's Theorem on the equidisribution of expanding horospheres in infinite volume hyperbolic spaces.

math.DS↗

arXiv2017

Learning Multi-level Features For Sensor-based Human Action Recognition

This paper proposes a multi-level feature learning framework for human action recognition using a single body-worn inertial sensor. The framework consists of three phases, respectively designed to analyze signal-based (low-level), components (mid-level) and semantic (high-level) information. Low-level features capture the time and frequency domain property while mid-level representations learn the composition of the action. The Max-margin Latent Pattern Learning (MLPL) method is proposed to learn high-level semantic descriptions of latent action patterns as the output of our framework. The proposed method achieves the state-of-the-art performances, 88.7%, 98.8% and 72.6% (weighted F1 score) respectively, on Skoda, WISDM and OPP datasets.

cs.CV↗

arXiv2017

Search for sterile neutrinos in holographic dark energy cosmology: Reconciling Planck observation with the local measurement of the Hubble constant

We search for sterile neutrinos in the holographic dark energy cosmology by using the latest observational data. To perform the analysis, we employ the current cosmological observations, including the cosmic microwave background temperature power spectrum data from the Planck mission, the baryon acoustic oscillation measurements, the type Ia supernova data, the redshift space distortion measurements, the shear data of weak lensing observation, the Planck lensing measurement, and the latest direct measurement of $H_0$ as well. We show that, compared to the $Λ$CDM cosmology, the holographic dark energy cosmology with sterile neutrinos can relieve the tension between the Planck observation and the direct measurement of $H_0$ much better. Once we include the $H_0$ measurement in the global fit, we find that the hint of the existence of sterile neutrinos in the holographic dark energy cosmology can be given. Under the constraint of the all-data combination, we obtain $N_{\rm eff}= 3.76\pm0.26$ and $m_{ν,\rm sterile}^{\rm eff}< 0.215\,\rm eV$, indicating that the detection of $ΔN_{\rm eff}>0$ in the holographic dark energy cosmology is at the $2.75σ$ level and the massless or very light sterile neutrino is favored by the current observations.

astro-ph.CO↗

arXiv2017

$K^0-\bar{K}^0$ mixing in the minimal flavor-violating two-Higgs-doublet models

The two-Higgs-doublet model (2HDM), as one of the simplest extensions of the Standard Model (SM), is obtained by adding another scalar doublet to the SM, and is featured by a pair of charged scalars, which could affect many low-energy processes. In the "Higgs basis" for a generic 2HDM, only one scalar doublet gets a nonzero vacuum expectation value and, under the criterion of minimal flavor violation, the other one is fixed to be either color-singlet or color-octet, which are named as the type-III and the type-C 2HDM, respectively. In this paper, we study the charged-scalar effects of these two models on the $K^0-\bar{K}^0$ mixing, an ideal process to probe New Physics (NP) beyond the SM. Firstly, we perform a complete one-loop computation of the box diagrams relevant to the $K^0-\bar{K}^0$ mixing, keeping the mass and momentum of the external strange quark up to the second order. Together with the up-to-date theoretical inputs, we then give a detailed phenomenological analysis, in the cases of both real and complex Yukawa couplings of the charged scalars to quarks. The parameter spaces allowed by the current experimental data on the mass difference $Δm_K$ and the CP-violating parameter $ε_K$ are obtained and the differences between these two 2HDMs are investigated, which are helpful to distinguish them from each other from a phenomenological point of view.

hep-ph↗

arXiv2017

A note on the vertex arboricity of signed graphs

A signed tree-coloring of a signed graph $(G,σ)$ is a vertex coloring $c$ so that $G^{c}(i,\pm)$ is a forest for every $i\in c(u)$ and $u\in V(G)$, where $G^{c}(i,\pm)$ is the subgraph of $(G,σ)$ whose vertex set is the set of vertices colored by $i$ or $-i$ and edge set is the set of positive edges with two end-vertices colored both by $i$ or both by $-i$, along with the set of negative edges with one end-vertex colored by $i$ and the other colored by $-i$. If $c$ is a function from $V(G)$ to $M_n$, where $M_n$ is $\{\pm 1,\pm 2,\ldots,\pm k\}$ if $n=2k$, and $\{0,\pm 1,\pm 2,\ldots,\pm k\}$ if $n=2k+1$, then $c$ a signed tree-$n$-coloring of $(G,σ)$. The minimum integer $n$ such that $(G,σ)$ admits a signed tree-$n$-coloring is the signed vertex arboricity of $(G,σ)$, denoted by $va(G,σ)$. In this paper, we first show that two switching equivalent signed graphs have the same signed vertex arboricity, and then prove that $va(G,σ)\leq 3$ for every balanced signed triangulation and for every edge-maximal $K_5$-minor-free graph with balanced signature. This generalizes the well-known result that the vertex arboricity of every planar graph is at most 3.

math.CO↗

arXiv2017

Local-global principles in circle packings

We generalize work of Bourgain-Kontorovich and Zhang, proving an almost local-to-global property for the curvatures of certain circle packings, to a large class of Kleinian groups. Specifically, we associate in a natural way an infinite family of integral packings of circles to any Kleinian group $\mathcal A\leq\textrm{PSL}_2(K)$ satisfying certain conditions, where $K$ is an imaginary quadratic field, and show that the curvatures of the circles in any such packing satisfy an almost local-to-global principle. A key ingredient in the proof of this is that $\mathcal A$ possesses a spectral gap property, which we prove for any infinite-covolume, geometrically finite, Zariski dense Kleinian group in $\textrm{PSL}_2(\mathcal{O}_K)$ containing a Zariski dense subgroup of $\textrm{PSL}_2(\mathbb{Z})$.

math.NT↗