arXiv subjects

Cong Wang

Publications and source records attributed to Cong Wang.

At least 433 records · Page 24Linked to original sources

arXiv2016

Soliton repetition rate in a silicon-nitride microresonator

The repetition rate of a Kerr comb comprising a single soliton in an anomalous dispersion silicon nitride microcavity is measured as a function of pump frequency tuning. The contributions from the Raman soliton self-frequency shift (SSFS) and from thermal effects are evaluated both experimentally and theoretically; the SSFS is found to dominate the changes in repetition rate. The relationship between the changes in repetition rate and pump frequency detuning is found to be independent of the nonlinearity coefficient and dispersion of the cavity. Modeling of the repetition rate change by using the generalized Lugiato-Lefever equation is discussed; the Kerr shock is found to have only a minor effect on repetition rate for cavity solitons with duration down to ~50 fs.

physics.optics↗

arXiv2016

Can NDN Perform Better than OLSR in Wireless Ad Hoc Networks?

The emerging paradigm of Information-Centric Networking is an exciting field of research, opening opportunities in many areas, such as forwarding strategies, caching placement policies, applications (e.g. video streaming and instant messaging), to name a few. In this paper, we address the mobility aspect of ICN, as well as how it performs in tactical wireless ad hoc environments. In this paper, we present results from a simulation study that investigates the performance of Named Data Networking, an instantiation of ICN, in such environments. We perform a series of simulations based on ndnSIM studying different mobility scenarios. Our simulations show that even in the short-term absence of the producer, consumers can still achieve better file retrieval when caches are used. As an effort to increase the cache diversity and have a better utilization of the Content Store we study probabilistic LRU. Furthermore, we compare the performance of our NDN network with a TCP based approach, using OLSR routing protocol, discussing advantages and disadvantages of each approach.

cs.NI↗

arXiv2016

Frequency Comb Generation in 300 nm Thick SiN Concentric-Racetrack-Resonators: Overcoming the Material Dispersion Limit

Kerr nonlinearity based frequency combs and solitons have been generated from on-chip optical microresonators with high quality factors and global or local anomalous dispersion. However, fabrication of such resonators usually requires materials and/or processes that are not standard in semiconductor manufacturing facilities. Moreover, in certain frequency regimes such as visible and ultra-violet, the large normal material dispersion makes it extremely difficult to achieve anomalous dispersion. Here we present a concentric racetrack-shaped resonator that achieves anomalous dispersion in a 300 nm thick silicon nitride film, suitable for semiconductor manufacturing but previously thought to result only in waveguides with high normal dispersion, a high intrinsic Q of 1.5 million, and a novel mode-selective coupling scheme that allows coherent combs to be generated. We also provide evidence suggestive of soliton-like pulse formation in the generated comb. Our method can achieve anomalous dispersion over moderately broad bandwidth for resonators at almost any wavelength while still maintaining material and process compatibility with high-volume semiconductor manufacturing.

physics.optics↗

arXiv2016

$γγ\rightarrow M^{+}M^{-}(M=π, K)$ processes with twist-3 corrections in QCD

We study the $γγ\rightarrow M^{+}M^{-}(M=π, K)$ processes with the contributions from the two-particle twist-2 and twist-3 distribution amplitudes of pion and kaon mesons on BHL prescription in the standard hard-scattering approach. The results show that the contributions from twist-3 parts are actually not power suppressed comparing with the leading-twist contributions and the cross sections agree well with the experimental data in the two-photon center-of-mass energy $W>2.8$ GeV. We also predict the cross section ratio $σ_{0}(π^{+}π^{-})/σ_{0}(K^{+}K^{-})$, which is compatible with the experimental data from TPC and Belle.

hep-ph↗

arXiv2016

Explicit Conditions on Existence and Uniqueness of Load-Flow Solutions in Distribution Networks

We present explicit sufficient conditions that guarantee the existence and uniqueness of the feasible load-flow solution for distribution networks with a generic topology (radial or meshed) modeled with positive sequence equivalents. In the problem, we also account for the presence of shunt elements. The conditions have low computational complexity and thus can be efficiently verified in a real system. Once the conditions are satisfied, the unique load-flow solution can be reached by a given fixed point iteration method of approximately linear complexity. Therefore, the proposed approach is of particular interest for modern active distribution network (ADN) setup in the context of real-time control. The theory has been confirmed through numerical experiments.

math.OC↗

arXiv2015

A Fokker-Planck approach to graded information propagation in pulse-gated feedforward neuronal networks

Information transmission is a key element for information processing in the brain. A number of mechanisms have been proposed for transferring volleys of spikes between layers of a feedforward neural circuit. Many of these mechanisms use synchronous activity to provide windows in time when spikes may be transferred more easily from layer to layer. Recently, we have demonstrated that a pulse-gating mechanism can transfer graded information between layers in a feedforward neuronal network. Our transfer mechanism resulted in a time-translationally invariant firing rate and synaptic current waveforms of arbitrary amplitude, thus providing exact, graded information transfer between layers. In this paper, we apply a Fokker-Planck approach to understand how this translational invariance manifests itself in a high-dimensional, non-linear feedforward integrate-and-fire network. We show that there is good correspondence in spiking probabilities between the Fokker-Planck solutions and our previous mean-field solutions. We identify an approximate line attractor in state space as the essential structure underlying the time-translational invariance of our solutions. This approach has enabled us to better understand the role played by the synaptic coupling, gating pulse and the membrane potential probability density function in information transfer.

q-bio.NC↗

arXiv2014

Thermodynamics of the $α$-$γ$ transition in cerium studied by an LDA + Gutzwiller method

The $α$-$γ$ transition in cerium has been studied in both zero and finite temperature by Gutzwiller density functional theory. We find that the first order transition between $α$ and $γ$ phases persists to the zero temperature with negative pressure. By further including the entropy contributed by both electronic quasi-particles and lattice vibration, we obtain the total free energy at given volume and temperature, from which we obtain the $α$-$γ$ transition from the first principle calculation. We also computed the phase diagram and pressure versus volume isotherms of cerium at finite temperature and pressure, finding excellent agreement with the experiments. Our calculation indicate that both the electronic entropy and lattice vibration entropy plays important role in the $α$-$γ$ transition.

cond-mat.str-el↗

arXiv2014

Transient excitons at metal surfaces

Excitons, electron-hole pairs bound by the Coulomb potential, are fundamental quasiparticles of coherent light-matter interaction energizing processes from photosynthesis to optoelectronics. Excitons are observed in semiconductors, and their existence is implicit in the quantum theory of metals, yet their appearance is tenuous due to the screening of the Coulomb interaction on few femtosecond timescale. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full screening of the Coulomb interaction, in the course of a three-photon photoemission process with <15 femtosecond laser pulses. Electron-hole pair interaction through the excitonic response introduces coherent quasiparticle correlations beyond the single-particle description of the optics of metals, which dominate the multi-photon photoemission process.

cond-mat.mes-hall↗

arXiv2014

Electrical and optical properties of fluid iron from compressed to expanded regime

Using quantum molecular dynamics simulations, we show that the electrical and optical properties of fluid iron change drastically from compressed to expanded regime. The simulation results reproduce the main trends of the electrical resistivity along isochores and are found to be in good agreement with experimental data. The transition of expanded fluid iron into a nonmetallic state takes place close to the density at which the constant volume derivative of the electrical resistivity on internal energy becomes negative. The study of the optical conductivity, absorption coefficient, and Rosseland mean opacity shows that, quantum molecular dynamics combined with the Kubo-Greenwood formulation provides a powerful tool to calculate and benchmark the electrical and optical properties of iron from expanded fluid to warm dense region.

physics.plasm-ph↗

arXiv2013

Thermophysical properties of hydrogen-helium mixtures: Re-examination of the mixing rules via quantum molecular dynamics simulations

Thermophysical properties of hydrogen, helium, and hydrogen-helium mixtures have been investigated in the warm dense matter regime at electron number densities ranging from $6.02\times10^{29}\sim2.41\times10^{30}$/m$^{3}$ and temperatures from 4000 to 20000 K via quantum molecular dynamics simulations. We focus on the dynamical properties such as the equation of states, diffusion coefficients, and viscosity. Mixing rules (density matching, pressure matching, and binary ionic mixing rules) have been validated by checking composite properties of pure species against that of the fully interacting mixture derived from QMD simulations. These mixing rules reproduce pressures within 10% accuracy, while it is 75% and 50% for the diffusion and viscosity, respectively. Binary ionic mixing rule moves the results into better agreement. Predictions from one component plasma model are also provided and discussed.

cond-mat.mtrl-sci↗

arXiv2013

Equation of states and transport properties of warm dense beryllium: A quantum molecular dynamics study

We have calculated the equation of states, the viscosity and self-diffusion coefficients, and electronic transport coefficients of beryllium in the warm dense regime for densities from 4.0 to 6.0 g/cm$^{3}$ and temperatures from 1.0 to 10.0 eV by using quantum molecular dynamics simulations. The principal Hugoniot is accordant with underground nuclear explosive and high power laser experimental results up to $\sim$ 20 Mbar. The calculated viscosity and self-diffusion coefficients are compared with the one-component plasma model, using effective charges given by the average-atom model. The Stokes-Einstein relationship, which presents the relationship between the viscosity and self-diffusion coefficients, is found to hold fairly well in the strong coupling regime. The Lorenz number, which is the ratio between thermal and electrical conductivities, is computed via Kubo-Greenwood formula and compared to the well-known Wiedemann-Franz law in the warm dense region.

cond-mat.mtrl-sci↗

arXiv2013

Wide range equation of state for fluid hydrogen within density functional theory

Wide range equation of state (EOS) for liquid hydrogen is ultimately built by combining two kinds of density functional theory (DFT) molecular dynamics simulations, namely, first-principles molecular dynamics simulations and orbital-free molecular dynamics simulations. Specially, the present introduction of short cutoff radius pseudopotentials enables the hydrogen EOS to be available in the range $9.82\times10^{-4}$ to $1.347\times10^{3}$ g/cm$^{3}$ and up to $5\times10^{7}$ K. By comprehensively comparing with various attainable experimental and theoretical data, we derive the conclusion that our DFT-EOS can be readily and reliably conducted to hydrodynamic simulations of the inertial confinement fusion.

cond-mat.mtrl-sci↗

arXiv2012

Quantum molecular dynamics simulations of thermophysical properties of fluid ethane

We have performed first-principles molecular-dynamics simulations based on density-functional theory to study the thermophysical properties of ethane under extreme conditions. We present new results for the equation of state of fluid ethane in the warm dense region. The optical conductivity is calculated via the Kubo-Greenwood formula from which the dc conductivity and optical reflectivity are derived. The close correlation between the nonmetal-metal transition of ethane and its decomposition, that ethane dissociates significantly into molecular and/or atomic hydrogen and some long alkane chains, has been systematically studied by analyzing the optical conductivity spectra, pair correlation functions, electronic density of states and charge density distribution of fluid ethane.

cond-mat.mtrl-sci↗

arXiv2012

Inferring Attitude in Online Social Networks Based On Quadratic Correlation

The structure of an online social network in most cases cannot be described just by links between its members. We study online social networks, in which members may have certain attitude, positive or negative toward each other, and so the network consists of a mixture of both positive and negative relationships. Our goal is to predict the sign of a given relationship based on the evidences provided in the current snapshot of the network. More precisely, using machine learning techniques we develop a model that after being trained on a particular network predicts the sign of an unknown or hidden link. The model uses relationships and influences from peers as evidences for the guess, however, the set of peers used is not predefined but rather learned during the training process. We use quadratic correlation between peer members to train the predictor. The model is tested on popular online datasets such as Epinions, Slashdot, and Wikipedia. In many cases it shows almost perfect prediction accuracy. Moreover, our model can also be efficiently updated as the underlaying social network evolves.

cs.SI↗

arXiv2012

Transport properties of dense deuterium-tritium plasmas

Consistent descriptions of the equation of states, and information about transport coefficients of deuterium-tritium mixture are demonstrated through quantum molecular dynamic (QMD) simulations (up to a density of 600 g/cm$^{3}$ and a temperature of $10^{4}$ eV). Diffusion coefficients and viscosity are compared with one component plasma model in different regimes from the strong coupled to the kinetic one. Electronic and radiative transport coefficients, which are compared with models currently used in hydrodynamic simulations of inertial confinement fusion, are evaluated up to 800 eV. The Lorentz number is also discussed from the highly degenerate to the intermediate region.

cond-mat.mtrl-sci↗

arXiv2011

Quantum molecular dynamic simulations of warm dense carbon monoxide

Using quantum molecular dynamic simulations, we have studied the thermophysical properties of warm dense carbon monoxide under extreme conditions. The principal Hugoniot, which is derived from the equation of state, shows excellent agreement with available experimental data up to 67 GPa. The chemical decomposition of carbon monoxide has been predicted at 8 GPa by means of pair correlation function. Based on Kubo-Greenwood formula, the dc electrical conductivity and the optical reflectivity are determined, and the nonmetal-metal transition for shock compressed carbon monoxide is observed around 43 GPa.

cond-mat.mtrl-sci↗

arXiv2011

Ab Initio Simulations of Dense Helium Plasmas

We study the thermophysical properties of dense helium plasmas by using quantum molecular dynamics and orbital-free molecular dynamics simulations, where densities are considered from 400 to 800 g/cm$^{3}$ and temperatures up to 800 eV. Results are presented for the equation of state. From the Kubo-Greenwood formula, we derive the electrical conductivity and electronic thermal conductivity. In particular, with the increase in temperature, we discuss the change in the Lorenz number, which indicates a transition from strong coupling and degenerate state to moderate coupling and partial degeneracy regime for dense helium.

cond-mat.mtrl-sci↗

arXiv2011

Thermophysical properties for shock compressed polystyrene

We have performed quantum molecular dynamic simulations for warm dense polystyrene at high pressures. The principal Hugoniot up to 790 GPa is derived from wide range equation of states, where contributions from atomic ionizations are semiclassically determined. The optical conductivity is calculated via the Kubo-Greenwood formula, from which the dc electrical conductivity and optical reflectivity are determined. The nonmetal-to-metal transition is identified by gradual decomposition of the polymer. Our results show good agreement with recent high precision laser-driven experiments.

cond-mat.mtrl-sci↗