arXiv subjects

Xi Chen

Publications and source records attributed to Xi Chen.

At least 1,189 records · Page 66Linked to original sources

arXiv2016

Competitive analysis of the top-K ranking problem

Motivated by applications in recommender systems, web search, social choice and crowdsourcing, we consider the problem of identifying the set of top $K$ items from noisy pairwise comparisons. In our setting, we are non-actively given $r$ pairwise comparisons between each pair of $n$ items, where each comparison has noise constrained by a very general noise model called the strong stochastic transitivity (SST) model. We analyze the competitive ratio of algorithms for the top-$K$ problem. In particular, we present a linear time algorithm for the top-$K$ problem which has a competitive ratio of $\tilde{O}(\sqrt{n})$; i.e. to solve any instance of top-$K$, our algorithm needs at most $\tilde{O}(\sqrt{n})$ times as many samples needed as the best possible algorithm for that instance (in contrast, all previous known algorithms for the top-$K$ problem have competitive ratios of $\tildeΩ(n)$ or worse). We further show that this is tight: any algorithm for the top-$K$ problem has competitive ratio at least $\tildeΩ(\sqrt{n})$.

cs.DS↗

arXiv2016

TMRT Observations of Carbon-chain molecules in Serpens South 1A

We report Shanghai Tian Ma Radio Telescope detections of several long carbon-chain molecules at C and Ku band, including HC3N, HC5N, HC7N, HC9N, C3S, C6H and C8H toward the starless cloud Serpens South 1a. We detected some transitions (HC9N J=13-12 F=12-11 and F=14-13, H13CCCN J=2-1 F=1-0 and F=1-1, HC13CCN J=2-1 F=2-2, F=1-0 and F=1-1, HCC13CN J=2-1 F=1-0 and F=1-1) and resolved some hyperfine components (HC5N J=6-5 F=5-4, H13CCCN J=2-1 F=2-1) for the first time in the interstellar medium. The column densities of these carbon-chain molecules in a range of 10^{12}-10^{13} cm^{-2} are comparable to two carbon-chain molecule rich sources, TMC-1 and Lupus-1A. The abundance ratios are 1.00:(1.11\pm0.15):(1.47\pm0.18) for [H13CCCN]:[HC13CCN]:[HCC13CN]. This result implies that the 13C isotope is also concentrated in the carbon atom adjacent to the nitrogen atom in HC3N in Serpens south 1a, which is similar to TMC-1. The [HC3N]/[H13CCCN] ratio of 78\pm9, the [HC3N]/[HC13CCN] ratio of 70\pm8, and the [HC3N]/[HCC13CN] ratio of 53\pm4 are also comparable to those in TMC-1. In any case, Serpens South 1a proves a testing ground for understanding carbon-chain chemistry.

astro-ph.GA↗

arXiv2016

Visualizing the elongated vortices in $γ$-Ga nanostrips

We study the magnetic response of superconducting $γ$-Ga via low temperature scanning tunneling microscopy and spectroscopy. The magnetic vortex cores rely substantially on the Ga geometry, and exhibit an unexpectedly-large axial elongation with aspect ratio up to 40 in rectangular Ga nano-strips (width $l$ $<$ 100 nm). This is in stark contrast with the isotropic circular vortex core in a larger round-shaped Ga island. We suggest that the unusual elongated vortices in Ga nanostrips originate from geometric confinement effect probably via the strong repulsive interaction between the vortices and Meissner screening currents at the sample edge. Our finding provides novel conceptual insights into the geometrical confinement effect on magnetic vortices and forms the basis for the technological applications of superconductors.

cond-mat.supr-con↗

arXiv2016

Extracting the differential phase in dual atom interferometers by modulating magnetic fields

We present a new scheme for measuring the differential phase in dual atom interferometers. The magnetic field is modulated in one interferometer, and the differential phase can be extracted without measuring the amplitude of the magnetic field by combining the ellipse and linear fitting methods. The gravity gradient measurements are discussed based on dual atom interferometers. Numerical simulation shows that the systematic error of the differential phase measurement is largely decreased when the duration of the magnetic field is symmetrically modulated. This combined fitting scheme has a high accuracy for measuring an arbitrary differential phase in dual atom interferometers.

physics.atom-ph↗

arXiv2016

Comparison of three Statistical Classification Techniques for Maser Identification

We applied three statistical classification techniques - linear discriminant analysis (LDA), logistic regression and random forests - to three astronomical datasets associated with searches for interstellar masers. We compared the performance of these methods in identifying whether specific mid-infrared or millimetre continuum sources are likely to have associated interstellar masers. We also discuss the ease, or otherwise, with which the results of each classification technique can be interpreted. Non-parametric methods have the potential to make accurate predictions when there are complex relationships between critical parameters. We found that for the small datasets the parametric methods logistic regression and LDA performed best, for the largest dataset the non-parametric method of random forests performed with comparable accuracy to parametric techniques, rather than any significant improvement. This suggests that at least for the specific examples investigated here accuracy of the predictions obtained is not being limited by the use of parametric models. We also found that for LDA, transformation of the data to match a normal distribution in the input parameters led to big improvements in accuracy. The different classification techniques had significant overlap in their predictions, further astronomical observations will enable the accuracy of these predictions to be tested.

astro-ph.IM↗

arXiv2016

Prediction of a Two-dimensional Phosphorus Nitride Monolayer

Today, 2D semiconductor materials have been extended into the nitrogen group: phosphorene, arsenene, antimonene and even nitrogene. Motivated by them, based upon first-principles density functional calculations, we propose a new two-dimensional phosphorus nitride (PN) structure that is stable well above the room temperature, due to its extremely high cohesive energy. Unlike phosphorene, PN structure is resistant to high temperature oxidation. The structure is predicted to be a semiconductor with a wide, indirect band gap of 2.64 eV. More interestingly, the phosphorus nitride monolayer experiences an indirect-to-direct band-gap transition at a relatively small tensile strain. Such dramatic transformation in the electronic structure combined with structural stability and oxidation resistance at high temperature could pave the way for exciting innovations in high-speed ultrathin transistors, power electronic modules, ultra-high efficiency LEDs and semiconductor lasers.

cond-mat.mes-hall↗

arXiv2016

Zero-energy modes and valley asymmetry in the Hofstadter spectrum of bilayer graphene van der Waals heterostructures with hBN

We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moire potential. Due to inversion-symmetry breaking characteristic for the moire potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of secondary Dirac points in the valence band, which we confirm by numerical simulations. Then, we analyze how single-particle gaps in the fractal energy spectrum produce a sequence of incompressible states observable under a variation of carrier density and magnetic field.

cond-mat.mes-hall↗

arXiv2016

Bound exciton and free exciton states in GaSe thin slab

The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10K. An obvious energy difference of about 34meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak are attributed to recombination of bound exciton at 10K. This strong bound exciton effect is stable up to 50K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with active energy extracted as 0.5meV, dominates PL emission.

cond-mat.mes-hall↗

arXiv2016

The bandmerged Planck Early Release Compact Source Catalogue: Probing sub-structure in the molecular gas at high Galactic latitude

The Planck Early Release Compact Source Catalogue (ERCSC) includes nine lists of highly reliable sources, individually extracted at each of the nine Planck frequency channels. To facilitate the study of the Planck sources, especially their spectral behaviour across the radio/infrared frequencies, we provide a "bandmerged" catalogue of the ERCSC sources. This catalogue consists of 15191 entries, with 79 sources detected in all nine frequency channels of Planck and 6818 sources detected in only one channel. We describe the bandmerging algorithm, including the various steps used to disentangle sources in confused regions. The multi-frequency matching allows us to develop spectral energy distributions of sources between 30 and 857 GHz, in particular across the 100 GHz band, where the energetically important CO J=1->0 line enters the Planck bandpass. We find ~3-5sigma evidence for contribution to the 100 GHz intensity from foreground CO along the line of sight to 147 sources with |b|>30 deg. The median excess contribution is 4.5+/-0.9 percent of their measured 100 GHz flux density which cannot be explained by calibration or beam uncertainties. This translates to 0.5+/-0.1 K km s^{-1} of CO which must be clumped on the scale of the Planck 100 GHz beam, i.e., ~10 arcmin. If this is due to a population of low mass (~15 Msun) molecular gas clumps, the total mass in these clumps may be more than 2000 Msun. Further, high-spatial-resolution, ground-based observations of the high-latitude sky will help shed light on the origin of this diffuse, clumpy CO emission.

astro-ph.GA↗

arXiv2016

Optimal shortcuts for atomic transport in anharmonic traps

We design optimal trajectories to transport cold atoms in anharmonic traps, combining invariant-based inverse engineering, perturbation theory, and optimal control theory. The anharmonic perturbation energy is minimized constraining the maximally allowed relative displacement between the trap center and the atom.

quant-ph↗

arXiv2016

Generating Discriminative Object Proposals via Submodular Ranking

A multi-scale greedy-based object proposal generation approach is presented. Based on the multi-scale nature of objects in images, our approach is built on top of a hierarchical segmentation. We first identify the representative and diverse exemplar clusters within each scale by using a diversity ranking algorithm. Object proposals are obtained by selecting a subset from the multi-scale segment pool via maximizing a submodular objective function, which consists of a weighted coverage term, a single-scale diversity term and a multi-scale reward term. The weighted coverage term forces the selected set of object proposals to be representative and compact; the single-scale diversity term encourages choosing segments from different exemplar clusters so that they will cover as many object patterns as possible; the multi-scale reward term encourages the selected proposals to be discriminative and selected from multiple layers generated by the hierarchical image segmentation. The experimental results on the Berkeley Segmentation Dataset and PASCAL VOC2012 segmentation dataset demonstrate the accuracy and efficiency of our object proposal model. Additionally, we validate our object proposals in simultaneous segmentation and detection and outperform the state-of-art performance.

cs.CV↗

arXiv2016

$\mathbb{A}^1$ curves on log K3 surfaces

In this paper, we study $\mathbb{A}^1$ curves on log K3 surfaces. We classify all genuine log K3 surfaces of type II which admits countably infinite $\mathbb{A}^1$ curves.

math.AG↗

arXiv2016

Unsupervised Network Pretraining via Encoding Human Design

Over the years, computer vision researchers have spent an immense amount of effort on designing image features for the visual object recognition task. We propose to incorporate this valuable experience to guide the task of training deep neural networks. Our idea is to pretrain the network through the task of replicating the process of hand-designed feature extraction. By learning to replicate the process, the neural network integrates previous research knowledge and learns to model visual objects in a way similar to the hand-designed features. In the succeeding finetuning step, it further learns object-specific representations from labeled data and this boosts its classification power. We pretrain two convolutional neural networks where one replicates the process of histogram of oriented gradients feature extraction, and the other replicates the process of region covariance feature extraction. After finetuning, we achieve substantially better performance than the baseline methods.

cs.CV↗

arXiv2016

Total positivity of Riordan arrays

We present sufficient conditions for total positivity of Riordan arrays. As applications we show that many well-known combinatorial triangles are totally positive and many famous combinatorial numbers are log-convex in a unified approach.

math.CO↗

arXiv2016

Total positivity of recursive matrices

Let $A=[a_{n,k}]_{n,k\ge 0}$ be an infinite lower triangular matrix defined by the recurrence $$a_{0,0}=1,\quad a_{n+1,k}=r_{k}a_{n,k-1}+s_{k}a_{n,k}+t_{k+1}a_{n,k+1},$$ where $a_{n,k}=0$ unless $n\ge k\ge 0$ and $r_k,s_k,t_k$ are all nonnegative. Many well-known combinatorial triangles are such matrices, including the Pascal triangle, the Stirling triangle (of the second kind), the Bell triangle, the Catalan triangles of Aigner and Shapiro. We present some sufficient conditions such that the recursive matrix $A$ is totally positive. As applications we give the total positivity of the above mentioned combinatorial triangles in a unified approach.

math.CO↗

arXiv2016

Effects of intrinsic strain on the structural stability and mechanical properties of phosphorene nanotubes

Using molecular dynamics (MD) simulations, we explore the structural stability and mechanical integrity of phosphorene nanotubes (PNTs), where the intrinsic strain in the tubular PNT structure plays an important role. It is proposed that the atomic structure of larger-diameter armchair PNTs (armPNTs) can remain stable at higher temperature, but the high intrinsic strain in the hoop direction renders zigzag PNTs (zigPNTs) less favorable. The mechanical properties of PNTs, including the Young's modulus and fracture strength, are sensitive to the diameter, showing a size dependence. A simple model is proposed to express the Young's modulus as a function of the intrinsic axial strain which in turns depends on the diameter of PNTs. In addition, the compressive buckling of armPNTs is length-dependent, whose instability modes transit from column buckling to shell buckling are observed as the ratio of diameter/length increases.

cond-mat.mtrl-sci↗

arXiv2015

Hierarchy of gaps and magnetic minibands in graphene in the presence of the Abrikosov vortex lattice

We determine the structure of band and gaps in graphene encapsulated in hexagonal boron nitride and subjected to magnetic field of Abrikosov lattice of vortices in the underlying superconducting film. The spectrum features one non-dispersive magnetic miniband at zero energy, separated by the largest gaps in the miniband spectrum from a pair of minibands resembling slightly broadened first Landau levels in graphene, suggesting the persistence of $ν= \pm 2$ quantum Hall effect states. Also, we identify occasional merging point of magnetic minibands which feature Dirac-type dispersion at the consecutive miniband edges.

cond-mat.mes-hall↗

arXiv2015

Tight Bounds for the Distribution-Free Testing of Monotone Conjunctions

We improve both upper and lower bounds for the distribution-free testing of monotone conjunctions. Given oracle access to an unknown Boolean function $f:\{0,1\}^n \rightarrow \{0,1\}$ and sampling oracle access to an unknown distribution $\mathcal{D}$ over $\{0,1\}^n$, we present an $\tilde{O}(n^{1/3}/ε^5)$-query algorithm that tests whether $f$ is a monotone conjunction versus $ε$-far from any monotone conjunction with respect to $\mathcal{D}$. This improves the previous best upper bound of $\tilde{O}(n^{1/2}/ε)$ by Dolev and Ron when $1/ε$ is small compared to $n$. For some constant $ε_0>0$, we also prove a lower bound of $\tildeΩ(n^{1/3})$ for the query complexity, improving the previous best lower bound of $\tildeΩ(n^{1/5})$ by Glasner and Servedio. Our upper and lower bounds are tight, up to a poly-logarithmic factor, when the distance parameter $ε$ is a constant. Furthermore, the same upper and lower bounds can be extended to the distribution-free testing of general conjunctions, and the lower bound can be extended to that of decision lists and linear threshold functions.

cs.DM↗