arXiv subjects

Jun Yan

Publications and source records attributed to Jun Yan.

At least 325 records · Page 18Linked to original sources

arXiv2012

Goodness-of-fit testing based on a weighted bootstrap: A fast large-sample alternative to the parametric bootstrap

The process comparing the empirical cumulative distribution function of the sample with a parametric estimate of the cumulative distribution function is known as the empirical process with estimated parameters and has been extensively employed in the literature for goodness-of-fit testing. The simplest way to carry out such goodness-of-fit tests, especially in a multivariate setting, is to use a parametric bootstrap. Although very easy to implement, the parametric bootstrap can become very computationally expensive as the sample size, the number of parameters, or the dimension of the data increase. An alternative resampling technique based on a fast weighted bootstrap is proposed in this paper, and is studied both theoretically and empirically. The outcome of this work is a generic and computationally efficient multiplier goodness-of-fit procedure that can be used as a large-sample alternative to the parametric bootstrap. In order to approximately determine how large the sample size needs to be for the parametric and weighted bootstraps to have roughly equivalent powers, extensive Monte Carlo experiments are carried out in dimension one, two and three, and for models containing up to nine parameters. The computational gains resulting from the use of the proposed multiplier goodness-of-fit procedure are illustrated on trivariate financial data. A by-product of this work is a fast large-sample goodness-of-fit procedure for the bivariate and trivariate t distribution whose degrees of freedom are fixed.

stat.ME↗

arXiv2011

The rate of convergence of new Lax-Oleinik type operators for time-periodic positive definite Lagrangian systems

Assume that the Aubry set of the time-periodic positive definite Lagrangian $L$ consists of one hyperbolic 1-periodic orbit. We provide an upper bound estimate of the rate of convergence of the family of new Lax-Oleinik type operators associated with $L$ introduced by the authors in \cite{W-Y}. In addition, we construct an example where the Aubry set of a time-independent positive definite Lagrangian system consists of one hyperbolic periodic orbit and the rate of convergence of the Lax-Oleinik semigroup cannot be better than $O(\frac{1}{t})$.

math.DS↗

arXiv2011

Rapid collapse of spin waves in non-uniform phases of the second Landau level

The spin degree of freedom in quantum phases of the second Landau level is probed by resonant light scattering. The long wavelength spin wave, which monitors the degree of spin polarization, is at the Zeeman energy in the fully spin-polarized state at $ν$=3. At lower filling factors the intensity of the Zeeman mode collapses indicating loss of polarization. A novel continuum of low-lying excitations emerges that dominates near $ν$=8/3 and $ν$=5/2. Resonant Rayleigh scattering reveals that quantum fluids for $ν<3$ break up into robust domain structures. While the state at $ν$=5/2 is considered to be fully polarized, these results reveal unprecedented roles for spin degrees of freedom.

cond-mat.str-el↗

arXiv2011

Correlated charged impurity scattering in graphene

Understanding disorder in graphene is essential for electronic applications; in contrast to conventional materials, the extraordinarily low electron-phonon scattering1, 2 in graphene implies that disorder3-7 dominates its resistivity even at room temperature. Charged impurities5, 8-10 have been identified as an important disorder type in graphene on SiO2 substrates11, 12, giving a nearly linear carrier-density-dependent conductivity σ(n), and producing electron and hole puddles13-15 which determine the magnitude of graphene's minimum conductivity σmin10. Correlations of charged impurities are known to be essential in achieving the highest mobilities in remotely-doped semiconductor heterostructures16-18, and are present to some degree in any impurity system at finite temperature. Here we show that even modest correlations in the position of charged impurities, realized by annealing potassium on graphene, can increase the mobility by more than a factor of four. The results are well understood theoretically19 considering an impurity correlation length which is temperature dependent but independent of impurity density. Impurity correlations also naturally explain the sub-linear σ(n) commonly observed in substrate-bound graphene devices2, 11, 12, 20.

cond-mat.mes-hall↗

arXiv2011

Large-sample tests of extreme-value dependence for multivariate copulas

Starting from the characterization of extreme-value copulas based on max-stability, large-sample tests of extreme-value dependence for multivariate copulas are studied. The two key ingredients of the proposed tests are the empirical copula of the data and a multiplier technique for obtaining approximate p-values for the derived statistics. The asymptotic validity of the multiplier approach is established, and the finite-sample performance of a large number of candidate test statistics is studied through extensive Monte Carlo experiments for data sets of dimension two to five. In the bivariate case, the rejection rates of the best versions of the tests are compared with those of the test of Ghoudi, Khoudraji and Rivest (1998) recently revisited by Ben Ghorbal, Genest and Neslehova (2009). The proposed procedures are illustrated on bivariate financial data and trivariate geological data.

stat.ME↗

arXiv2011

Linear density response function in the projector-augmented wave method: Applications to solids, surfaces, and interfaces

We present an implementation of the linear density response function within the projector-augmented wave (PAW) method with applications to the linear optical and dielectric properties of both solids, surfaces, and interfaces. The response function is represented in plane waves while the single-particle eigenstates can be expanded on a real space grid or in atomic orbital basis for increased efficiency. The exchange-correlation kernel is treated at the level of the adiabatic local density approximation (ALDA) and crystal local field effects are included. The calculated static and dynamical dielectric functions of Si, C, SiC, AlP and GaAs compare well with previous calculations. While optical properties of semiconductors, in particular excitonic effects, are generally not well described by ALDA, we obtain excellent agreement with experiments for the surface loss function of the Mg(0001) surface with plasmon energies deviating by less than 0.2 eV. Finally, we apply the method to study the influence of substrates on the plasmon excitations in graphene. On SiC(0001), the long wavelength $π$ plasmons are significantly damped although their energies remain almost unaltered. On Al(111) the $π$ plasmon is completely quenched due to the coupling to the metal surface plasmon.

cond-mat.mtrl-sci↗

arXiv2011

Quantum molecular dynamics simulations for the nonmetal-metal transition in shocked methane

We have performed quantum molecular-dynamics simulations for methane under shock compressions up to 80 GPa. We obtain good agreement with available experimental data for the principal Hugoniot, derived from the equation of state. A systematic study of the optical conductivity spectra, one-particle density of states, and the distributions of the electronic charge over supercell at Hugoniot points shows that the transition of shocked methane to a metallic state takes place close to the density at which methane dissociates significantly into molecular hydrogen and some long alkane chains. Through analyzing the pair correlation function, we predict the chemical picture of the shocked methane. In contrast to usual assumptions used for high pressure modeling of methane, we find that no diamond-like configurations occurs for the whole density-temperature range studied.

cond-mat.mtrl-sci↗

arXiv2011

A goodness-of-fit test for bivariate extreme-value copulas

It is often reasonable to assume that the dependence structure of a bivariate continuous distribution belongs to the class of extreme-value copulas. The latter are characterized by their Pickands dependence function. In this paper, a procedure is proposed for testing whether this function belongs to a given parametric family. The test is based on a Cramér--von Mises statistic measuring the distance between an estimate of the parametric Pickands dependence function and either one of two nonparametric estimators thereof studied by Genest and Segers [Ann. Statist. 37 (2009) 2990--3022]. As the limiting distribution of the test statistic depends on unknown parameters, it must be estimated via a parametric bootstrap procedure, the validity of which is established. Monte Carlo simulations are used to assess the power of the test and an extension to dependence structures that are left-tail decreasing in both variables is considered.

math.ST↗

arXiv2011

The sky brightness and transparency in i-band at Dome A, Antarctica

The i-band observing conditions at Dome A on the Antarctic plateau have been investigated using data acquired during 2008 with the Chinese Small Telescope ARray. The sky brightness, variations in atmospheric transparency, cloud cover, and the presence of aurorae are obtained from these images. The median sky brightness of moonless clear nights is 20.5 mag arcsec^{-2} in the SDSS $i$ band at the South Celestial Pole (which includes a contribution of about 0.06 mag from diffuse Galactic light). The median over all Moon phases in the Antarctic winter is about 19.8 mag arcsec^{-2}. There were no thick clouds in 2008. We model contributions of the Sun and the Moon to the sky background to obtain the relationship between the sky brightness and transparency. Aurorae are identified by comparing the observed sky brightness to the sky brightness expected from this model. About 2% of the images are affected by relatively strong aurorae.

astro-ph.IM↗

arXiv2010

A new kind of Lax-Oleinik type operator with parameters for time-periodic positive definite Lagrangian systems

In this paper we introduce a new kind of Lax-Oleinik type operator with parameters associated with positive definite Lagrangian systems for both the time-periodic case and the time-independent case. On one hand, the new family of Lax-Oleinik type operators with an arbitrary $u\in C(M,\mathbb{R}^1)$ as initial condition converges to a backward weak KAM solution in the time-periodic case, while it was shown by Fathi and Mather that there is no such convergence of the Lax-Oleinik semigroup. On the other hand, the new family of Lax-Oleinik type operators with an arbitrary $u\in C(M,\mathbb{R}^1)$ as initial condition converges to a backward weak KAM solution faster than the Lax-Oleinik semigroup in the time-independent case.

math.DS↗

arXiv2010

Quantum molecular dynamics simulations of lithium melting using Z-method

We performed first-principles molecular dynamics calculations for lithium using the projector augmented waves method and the generalized gradient approximation as exchange-correlation energy. The melting curve of lithium was computed using the \textit{Z}-method technique for pressures up to 30 GPa, which agrees well with the experimental and two-phase simulated results. The change of the melting line slope from positive to negative was predicted by the characteristic shape inversion of the \textit{Z} curve at about 8.2 GPa. Through analyzing the static properties, we conclude that no liquid-liquid phase transition accompanies the occurrence of the melting line maximum, which is caused by the higher compressibility of the liquid phase compared to the solid phase. In addition, we systematically studied the dynamic and optical properties of lithium near melting curve at critical superheating and melting temperatures. It was suggested that spectra difference at critical superheating and melting temperature may be able to diagnose the homogeneous melting.

cond-mat.mtrl-sci↗

arXiv2010

Charge transport in dual gated bilayer graphene with Corbino geometry

The resistance of dual-gated bilayer graphene is measured as a function of temperature and gating electric fields in the Corbino geometry which precludes edge transport. The temperature-dependent resistance is quantitatively described by a two channel conductance model including parallel thermal activation and variable range hopping channels, which gives the electric-field-dependent band gap whose magnitude is found to be in good agreement with infrared absorption experiments. Low temperature transport is similar to previous studies of dual-gated bilayer graphene with edges, suggesting that edge transport does not play an important role.

cond-mat.mes-hall↗

arXiv2010

The First Release of the CSTAR Point Source Catalog from Dome A, Antarctica

In 2008 January the 24th Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to DomeA, the highest point on the Antarctic plateau. CSTAR consists of four 14.5cm optical telescopes, each with a different filter (g, r, i and open) and has a 4.5degree x 4.5degree field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every 30 seconds throughout the year whenever it is dark. During 2008, CSTAR #1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hours during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to 16 mag could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the South Celestial Pole (Dec =-90 degree), all the sources within the FOV of CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts and minor planets.

astro-ph.GA↗

arXiv2010

Testing and Data Reduction of the Chinese Small Telescope Array (CSTAR) for Dome A, Antarctica

The Chinese Small Telescope ARray (hereinafter CSTAR) is the first Chinese astronomical instrument on the Antarctic ice cap. The low temperature and low pressure testing of the data acquisition system was carried out in a laboratory refrigerator and on the 4500m Pamirs high plateau, respectively. The results from the final four nights of test observations demonstrated that CSTAR was ready for operation at Dome A, Antarctica. In this paper we present a description of CSTAR and the performance derived from the test observations.

astro-ph.IM↗

arXiv2010

Anomalous optical and electronic properties of dense sodium

Based on ab initio density-functional-theory using generalized gradient approximation, we systematically study the optical and electronic properties of the insulating dense sodium phase (Na-hp4) reported recently [Ma \textit{et al.}, Nature \textbf{458}, 182 (2009)]. The structure is found optically anisotropic and transparent to visible light, which can be well interpreted using its electronic band structure and angular moment decomposed density of states. Through the bader analysis of Na-hp4 at different pressures, we conclude that ionicity exists in the structure and becomes stronger with increasing pressure. In addition, the absorption spectra in the energy range from 1.4 to 2.4 eV are compared with recent experimental results and found good agreement. It is found that the deep-lying valence electrons participate in the interband transition.

cond-mat.mtrl-sci↗

arXiv2007

Observation of Anomalous Phonon Softening in Bilayer Graphene

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena. The Kohn anomaly is a renowned example where electron-phonon coupling leads to non-analytic phonon dispersion at specific momentum nesting the Fermi surface. Here we report evidence of another type of phonon anomaly discovered by low temperature Raman spectroscopy in bilayer graphene where the charge density is modulated by the electric field effect. This anomaly, arising from charge-tunable modulations of particle-hole pairs that are resonantly coupled to lattice vibrations, is predicted to exhibit a logarithmic divergence in the long-wavelength optical-phonon energy. In a non-uniform bilayer of graphene, the logarithmic divergence is abated by charge density inhomogeneity leaving as a vestige an anomalous phonon softening. The observed softening marks the first confirmation of the phonon anomaly as a key signature of the resonant deformation-potential electron-phonon coupling. The high sensitivity of the phonon softening to charge density non-uniformity creates significant venues to explore the interplay between fundamental interactions and disorder in the atomic layers.

cond-mat.mes-hall↗

arXiv2006

Electric Field Effect Tuning of Electron-Phonon Coupling in Graphene

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long wavelength optical phonons of graphene. The EFE in this two dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and line-width demonstrate optically the particle-hole symmetry about the charge-neutral Dirac-point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of mass-less Dirac fermions.

cond-mat.mes-hall↗

arXiv2002

The loop homology algebra of spheres and projective spaces

Chas and Sullivan recently defined an intersection product on the homology $H_*(LM)$ of the space of smooth loops in a closed, oriented manifold $M$. In this paper we will use the homotopy theoretic realization of this product described by the first two authors to construct a second quadrant spectral sequence of algebras converging to the loop homology multiplicatively, when $M$ is simply connected. The $E_2$ term of this spectral sequence is $H^*(M;H_*(ΩM))$ where the product is given by the cup product on the cohomology of the manifold $H^* (M)$ with coefficients in the Pontryagin ring structure on the homology of its based loop space $H_*(ΩM)$. We then use this spectral sequence to compute the ring structures of $H_* (LS^n)$ and $H_* (L\bcp^n)$.

math.AT↗