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C. Gong

Publications and source records attributed to C. Gong.

At least 19 recordsLinked to original sources
arXiv2025

Micro-Mirror-Devices (MMDs): A New Family of MOEMS for the Habitable World Observatory

We present a new program aimed at developing a new generation of micromirror devices specifically tailored for astronomical applications, multi-slit spectroscopy in particular. We first overview the general characteristics of Multi-Object-Spectrographs based on the current Digital Micromirror Devices (DMDs), with particular focus on the newly deployed SAMOS instrument at the 4.1 m SOAR telescope on Cerro Pachon. We illustrate the operational advantages of DMD-based instruments and the technical limitations of the currently available devices, the DMDs produced by Texas Instruments (TI). We then introduce the baseline and target parameters of the new Micro-Mirror-Devices (MMDs) that we plan to develop with the goal of reaching TRL-5 by mid-2029 as required by the Habitable Worlds Observatory (HWO) timeline. We conclude with a brief illustration of the exciting potential of MMD-based spectrographs for an 8 m class space telescope like HWO.

astro-ph.IM↗
arXiv2024

Beyond Imitation: A Life-long Policy Learning Framework for Path Tracking Control of Autonomous Driving

Model-free learning-based control methods have recently shown significant advantages over traditional control methods in avoiding complex vehicle characteristic estimation and parameter tuning. As a primary policy learning method, imitation learning (IL) is capable of learning control policies directly from expert demonstrations. However, the performance of IL policies is highly dependent on the data sufficiency and quality of the demonstrations. To alleviate the above problems of IL-based policies, a lifelong policy learning (LLPL) framework is proposed in this paper, which extends the IL scheme with lifelong learning (LLL). First, a novel IL-based model-free control policy learning method for path tracking is introduced. Even with imperfect demonstration, the optimal control policy can be learned directly from historical driving data. Second, by using the LLL method, the pre-trained IL policy can be safely updated and fine-tuned with incremental execution knowledge. Third, a knowledge evaluation method for policy learning is introduced to avoid learning redundant or inferior knowledge, thus ensuring the performance improvement of online policy learning. Experiments are conducted using a high-fidelity vehicle dynamic model in various scenarios to evaluate the performance of the proposed method. The results show that the proposed LLPL framework can continuously improve the policy performance with collected incremental driving data, and achieves the best accuracy and control smoothness compared to other baseline methods after evolving on a 7 km curved road. Through learning and evaluation with noisy real-life data collected in an off-road environment, the proposed LLPL framework also demonstrates its applicability in learning and evolving in real-life scenarios.

cs.RO↗
arXiv2023

Backreaction effect and plasma oscillation in pair production for rapidly oscillating electric fields

The backreaction effect and plasma oscillation in pair production for rapidly oscillating electric fields are investigated by solving quantum Vlasov equation. Contrary to previously thought, it is found that the backreaction effect can be neglected in the pair production for a rapidly oscillating but weak electric field, particularly, for a subcritical external electric field with frequency chirp. In some cases the oscillation period of created electron-positron plasma can be described by a simple formula constructed based on the Langmiur oscillation frequency, but it is impossible in general case because the plasma oscillation period directly depend not only on the final number density of created particles, but also on the external electric field parameters. Moreover, it is found that the momentum spectrum presents complex interferences after considering the backreaction. These results give us the safety range of external electric fields for taking no account of the backreaction effect and deepen our understanding of the pair production with the backreaction effect.

hep-ph↗
arXiv2022

Effects of control fields on the pair creation and the vacuum information transmission

The effects of control fields on the energy spectra and the number of created pairs and the information transmission by the Dirac vacuum modes are investigated by employing computational quantum field theory approach. It is found that the oscillation structures on the energy spectra are sensitive to the direction, the width, and the oscillation frequency of control fields. The pair yield can have obvious changes for a small frequency and a very large frequency. Moreover, the information encoded in the control fields, such as the field direction, the laser frequency and the time interval between two laser pulses, can also embodied by the vacuum modes in the change of pair-creation rate with time. These results not only can deepen our understanding of the control of pair creation and the information transmission, but also can provide a theoretical reference to the related experiments in the future.

quant-ph↗
arXiv2022

Sub-monolayer Biolasers: Lower Gain, Higher Sensitivity

Biomarker detection is the key to identifying health risks. However, designing sensitive biosensors in a single-use mode for disease diagnosis remains a major challenge. Here, we report sub-monolayer biolasers with remarkable repeatability for ultrasensitive and disposable biomarker detection. The biolaser sensors are designed by employing the telecom optical fibers as distributed optical microcavities and pushing the gain molecules down to the sub-monolayer level. We observe a status transition from the monolayer biolaser to the sub-monolayer biolaser by tuning the specific conjugation. By reducing the fluorophores down to the threshold density (~ 3.2 x 10-13 mol/cm2), we demonstrate an ultimate sensitivity of sub-monolayer biolaser with six orders of magnitude enhancement compared with the monolayer biolasers. We further achieved ultrasensitive immunoassay for Parkinson's disease biomarker, alpha-synuclein, with a lower limit of detection of 0.32 pM in serum. This biosensor with massive fabrication capability at ultralow cost provides a general method for the ultrasensitive disposable biodetection of disease biomarkers.

physics.optics↗
arXiv2022

Tunable nodal kagome superconductivity in charge ordered RbV3Sb5

Unconventional superconductors often feature competing orders, small superfluid density, and nodal electronic pairing. While unusual superconductivity has been proposed in the kagome metals AV3Sb5, key spectroscopic evidence has remained elusive. Here we utilize pressure-tuned (up to 1.85 GPa) and ultra-low temperature (down to 18 mK) muon spin spectroscopy to uncover the unconventional nature of superconductivity in RbV3Sb5. At ambient pressure, we detect an enhancement of the width of the internal magnetic field distribution sensed by the muon ensemble, indicative of time-reversal symmetry breaking charge order. Remarkably, the superconducting state displays nodal energy gap and a reduced superfluid density, which can be attributed to the competition with the novel charge order. Upon applying pressure, the charge-order transitions are suppressed, the superfluid density increases, and the superconducting state progressively evolves from nodal to nodeless. Once charge order is eliminated, we find a superconducting pairing state that is not only fully gapped, but also spontaneously breaks time-reversal symmetry. Our results point to unprecedented tunable nodal kagome superconductivity competing with time-reversal symmetry-breaking charge order and offer unique insights into the nature of the pairing state.

cond-mat.supr-con↗
arXiv2021

Study of pair production in inhomogeneous two-color electric fields using the computational quantum field theory

We first demonstrate theoretically that the computational quantum field theory is equivalent to the quantum kinetic theory for pair creation in a spatially homogeneous and time-dependent electric field, then verify numerically their equivalence for pair creation in one-dimensional time-dependent electric fields, and finally investigate detailedly the effects of the field frequency, spatial width, pulse duration, and relative phase on dynamically assisted Schwinger pair production in an inhomogeneous two-color electric field. It is found that the enhancement effect of pair creation is very sensitive to the field frequency and generally very obvious for a shorter field width, a longer pulse duration, and a relative phase of maximizing the field strength. These results can provide a significant reference for the optimal control theory of pair creation which aims to maximize the created pair yield within a given scope of field parameters.

hep-ph↗
arXiv2021

Observation of Unconventional Charge Density Wave without Acoustic Phonon Anomaly in Kagome Superconductors AV3Sb5 (A=Rb,Cs)

The combination of non-trivial band topology and symmetry breaking phases gives rise to novel quantum states and phenomena such as topological superconductivity, quantum anomalous Hall effect and axion electrodynamics. Evidence of intertwined charge density wave (CDW) and superconducting order parameters has recently been observed in a novel kagome material AV3Sb5 (A=K,Rb,Cs) that features a Z2 topological invariant in the electronic structure. However, the origin of the CDW and its intricate interplay with topological state has yet to be determined. Here, using hard x-ray scattering, we demonstrate a three-dimensional (3D) CDW with 2*2*2 superstructure in (Rb,Cs)V3Sb5. Unexpectedly, we find that the CDW fails to induce acoustic phonon anomalies at the CDW wavevector but yields a novel Raman mode which quickly damps into a broad continuum below the CDW transition temperature. Our observations exclude strong electron-phonon coupling driven CDW in AV3Sb5 and point to an unconventional and electronic-driven mechanism that couples the CDW and the topological band structure.

cond-mat.supr-con↗
arXiv2019

Electron-positron pair production in frequency modulated laser fields

The momentum spectrum and the number density of created electron-positron pairs in a frequency modulated laser field are investigated using quantum kinetic equation. It is found that the momentum spectrum presents obvious interference pattern. This is an imprint of the frequency modulated field on the momentum spectrum, because the momentum peaks correspond to the pair production process by absorbing different frequency component photons. Moreover, the interference effect can also be understood qualitatively by analyzing turning point structures. The study of the pair number density shows that the number density is very sensitive to modulation parameters and can be enhanced by over two orders of magnitude for certain modulation parameters, which may provide a new way to increase the number of created electron-positron pairs in future experiments.

quant-ph↗
arXiv2019

Microscopic Cross-Correlations in the Finite-Size Kuramoto Model of Coupled Oscillators

Super-critical Kuramoto oscillators with distributed frequencies separate into two disjoint groups: an ordered one locked to the mean field, and a disordered one consisting of effectively decoupled oscillators -- at least so in the thermodynamic limit. In finite ensembles, in contrast, such clear separation fails: The mean field fluctuates due to finite-size effects and thereby induces order in the disordered group. To our best knowledge, this publication is the first to reveal such an effect, similar to noise-induced synchronization, in a purely deterministic system. We start by modeling the situation as a stationary mean field with additional white noise acting on a pair of unlocked Kuramoto oscillators. An analytical expression shows that the cross-correlation between the two increases with decreasing ratio of natural frequency difference and noise intensity. In a deterministic finite Kuramoto model, the strength of the mean field fluctuations is inextricably linked to the typical natural frequency difference. Therefore, we let a fluctuating mean field, generated by a finite ensemble of active oscillators, act on pairs of passive oscillators with a microscopic natural frequency difference between which we then measure the cross-correlation, at both super- and sub-critical coupling.

nlin.AO↗
arXiv2015

Measuring strain and rotation fields at the dislocation core in graphene

Strain fields, dislocations and defects may be used to control electronic properties of graphene. By using advanced imaging techniques with high-resolution transmission electron microscopes, we have measured the strain and rotation fields about dislocations in monolayer graphene with single-atom sensitivity. These fields differ qualitatively from those given by conventional linear elasticity. However, atom positions calculated from two dimensional (2D) discrete elasticity and three dimensional discrete periodized Föppl-von Kármán equations (dpFvKEs) yield fields close to experiments when determined by geometric phase analysis. 2D theories produce symmetric fields whereas those from experiments exhibit asymmetries. Numerical solutions of dpFvKEs provide strain and rotation fields of dislocation dipoles and pairs that also exhibit asymmetries and, compared with experiments, may yield information on out-of-plane displacements of atoms. While discrete theories need to be solved numerically, analytical formulas for strains and rotation about dislocations can be obtained from 2D Mindlin's hyperstress theory. These formulas are very useful for fitting experimental data and provide a template to ascertain the importance of nonlinear and nonplanar effects. Measuring the parameters of this theory, we find two characteristic lengths between three and four times the lattice spacings that control dilatation and rotation about a dislocation. At larger distances from the dislocation core, the elastic fields decay to those of conventional elasticity. Our results may be relevant for strain engineering in graphene and other 2D materials of current interest.

cond-mat.mes-hall↗
arXiv2014

Film structure of epitaxial graphene oxide on SiC: Insight on the relationship between interlayer spacing, water content, and intralayer structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter-layer structure of these ultra-thin oxide films. X-ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter-layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 Å. Density functional theory calculations show that the apparent contradiction of "little water but large interlayer spacing in the graphene oxide films" can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non-homogenous oxidation, where non-oxidized and highly oxidized nano-domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter-layer separations. This work sheds light on both the vertical and intra-layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter-layer spacing, water content, and structure of graphene/graphite oxide materials.

cond-mat.mtrl-sci↗
arXiv2013

First-Principles Calculation of Thermal Transport in the Metal/Graphene System

Thermal properties in the metal/graphene (Gr) systems are analyzed by using an atomistic phonon transport model based on Landauer formalism and first-principles calculations. The specific structures under investigation include chemisorbed Ni(111)/Gr, physisorbed Cu(111)/Gr and Au(111)/Gr, as well as Pd(111)/Gr with intermediate characteristics. Calculated results illustrate a strong dependence of thermal transfer on the details of interfacial microstructures. In particular, it is shown that the chemisorbed case provides a generally smaller interfacial thermal resistance than the physisorbed due to the stronger bonding. However, our calculation also indicates that the weakly chemisorbed interface of Pd/Gr may be an exception, with the largest thermal resistance among the considered. Further examination of the electrostatic potential and interatomic force constants reveal that the mixed bonding force between the Pd and C atoms results in incomplete hybridization of Pd and graphene orbital states at the junction, leading effectively to two phonon interfaces and a larger than expected thermal resistance. Comparison with available experimental data shows good agreement. The result clearly suggests the feasibility of phonon engineering for thermal property optimization at the interface.

cond-mat.mes-hall↗
arXiv2012

Laser-driven shock acceleration of monoenergetic ion beams

We show that monoenergetic ion beams can be accelerated by moderate Mach number collisionless, electrostatic shocks propagating in a long scale-length exponentially decaying plasma profile. Strong plasma heating and density steepening produced by an intense laser pulse near the critical density can launch such shocks that propagate in the extended plasma at high velocities. The generation of a monoenergetic ion beam is possible due to the small and constant sheath electric field associated with the slowly decreasing density profile. The conditions for the acceleration of high-quality, energetic ion beams are identified through theory and multidimensional particle-in-cell simulations. The scaling of the ion energy with laser intensity shows that it is possible to generate $\sim 200$ MeV proton beams with state-of-the-art 100 TW class laser systems.

physics.plasm-ph↗
arXiv2005

Hamiltonian Dynamics of Yang-Mills Fields on a Lattice

We review recent results from studies of the dynamics of classical Yang-Mills fields on a lattice. We discuss the numerical techniques employed in solving the classical lattice Yang-Mills equations in real time, and present results exhibiting the universal chaotic behavior of nonabelian gauge theories. The complete spectrum of Lyapunov exponents is determined for the gauge group SU(2). We survey results obtained for the SU(3) gauge theory and other nonlinear field theories. We also discuss the relevance of these results to the problem of thermalization in gauge theories.

nucl-th↗
arXiv1993

Lyapunov Spectra in SU(2) Lattice Gauge Theory

We develop a method for calculating the Lyapunov characteristic exponents of lattice gauge theories. The complete Lyapunov spectrum of SU(2) gauge theory is obtained and Kolmogorov-Sinai entropy is calculated. Rapid convergence with lattice size is found.

hep-lat↗