arXiv subjects

Qin Wu

Publications and source records attributed to Qin Wu.

29 records · Page 2Linked to original sources

arXiv2024

Statistical properties and cosmological applications of fast radio bursts

Fast radio burst (FRB) is a type of extragalactic radio signal characterized by millisecond duration, extremely high brightness temperature, and large dispersion measure. It remains a mystery in the universe. Advancements in instrumentation have led to the discovery of 816 FRB sources and 7622 bursts from 67 repeating FRBs now. This field is undergoing rapid development, rapidly advancing our understanding of the physics of FRBs as new observational data accumulates. The accumulation of data has also promoted our exploration of our universe. In this review, we summarize the statistical analysis and cosmological applications using large samples of FRBs, including the energy functions, the waiting time distributions of repeating FRBs, the probe of "missing baryons" and circumgalactic medium in the universe, measurements of cosmological parameters, exploration of the epoch of reionization history, and study of the gravitational lensing of FRBs.

astro-ph.HE↗

arXiv2023

Tidal capture of an asteroid by a magnetar: FRB-like bursts, glitch and anti-glitch

Recently, remarkable anti-glitch and glitch accompanied by bright radio bursts of the Galactic magnetar SGR J1935+2154 were discovered. These two infrequent temporal coincidences between the glitch/anti-glitch and the fast radio burst (FRB)-like bursts reveal their physical connection of them. Here we propose that the anti-glitch/glitch and FRB-like bursts can be well understood by an asteroid tidally captured by a magnetar. In this model, an asteroid is tidally captured and disrupted by a magnetar. Then, the disrupted asteroid will transfer the angular momentum to the magnetar producing a sudden change in the magnetar rotational frequency at the magnetosphere radius. If the orbital angular momentum of the asteroid is parallel (or anti-parallel) to that of the spinning magnetar, a glitch (or anti-glitch) will occur. Subsequently, the bound asteroid materials fall back to the pericenter and eventually are accreted to the surface of the magnetar. Massive fragments of the asteroid cross magnetic field lines and produce bright radio bursts through coherent curvature radiation. Our model can explain the sudden magnetar spin changes and FRB-like bursts in a unified way.

astro-ph.HE↗

arXiv2023

Temperature and Pressure-Induced Atomic Structure Evolution During Solidification of Zr50Nb50 Metallic Melt via Molecular Dynamics Simulation

In this report, the evolution of the local atomic structure of the Zr50Nb50 melt was investigated by applying temperature (2600 to 300 K) and pressure (0 to 50 Gpa) using classical molecular dynamics simulations. To gain clear insight into the structural evolution during quenching, we used various methods of structural analysis such as the radial distribution function g(r), coordination number, bond angle distribution, and Voronoi tessellation. We found that the icosahedral motifs (which are the signature of the short-range ordering) and distorted BCC-like clusters dominate in the liquid and glass region under 0 and 5 Gpa external pressure. A first-order phase transition to a crystal-like structure was observed at 10, 15, and 20 Gpa external pressure at 1400, 1500, and 1600 K, respectively. Before the first-order phase transition, the system was dominated by icosahedral and distorted BCC-like clusters. When the temperature is lowered further below the glass transition at 10,15, and 20 Gpa external pressure, all structural analyses show that the solidified system consists mainly of body-centered cubic-like clusters in the case of our specific cooling rate of 1012 K/s.

cond-mat.mtrl-sci↗

arXiv2023

Striving for Authentic and Sustained Technology Use In the Classroom: Lessons Learned from a Longitudinal Evaluation of a Sensor-based Science Education Platform

Technology integration in educational settings has led to the development of novel sensor-based tools that enable students to measure and interact with their environment. Although reports from using such tools can be positive, evaluations are often conducted under controlled conditions and short timeframes. There is a need for longitudinal data collected in realistic classroom settings. However, sustained and authentic classroom use requires technology platforms to be seen by teachers as both easy to use and of value. We describe our development of a sensor-based platform to support science teaching that followed a 14-month user-centered design process. We share insights from this design and development approach, and report findings from a 6-month large-scale evaluation involving 35 schools and 1245 students. We share lessons learnt, including that technology integration is not an educational goal per se and that technology should be a transparent tool to enable students to achieve their learning goals.

cs.HC↗

arXiv2022

Convolutional Neural Networks for Searching Superflares from Pixel-level Data of the Transiting Exoplanet Survey Satellite

In this work, six convolutional neural networks (CNNs) have been trained based on %different feature images and arrays from the database including 15,638 superflare candidates on solar-type stars, which are collected from the three-years observations of Transiting Exoplanet Survey Satellite ({\em TESS}). These networks are used to replace the artificially visual inspection, which was a direct way to search for superflares, and exclude false positive events in recent years. Unlike other methods, which only used stellar light curves to search superflare signals, we try to identify superflares through {\em TESS} pixel-level data with lower risks of mixing false positive events, and give more reliable identification results for statistical analysis. The evaluated accuracy of each network is around 95.57\%. After applying ensemble learning to these networks, stacking method promotes accuracy to 97.62\% with 100\% classification rate, and voting method promotes accuracy to 99.42\% with relatively lower classification rate at 92.19\%. We find that superflare candidates with short duration and low peak amplitude have lower identification precision, as their superflare-features are hard to be identified. The database including 71,732 solar-type stars and 15,638 superflare candidates from {\em TESS} with corresponding feature images and arrays, and trained CNNs in this work are public available.

astro-ph.SR↗

arXiv2021

Geometric quantum adiabatic methods for quantum chemistry

Existing quantum algorithms for quantum chemistry work well near the equilibrium geometry of molecules, but the results can become unstable when the chemical bonds are broken at large atomic distances. For any adiabatic approach, this usually leads to serious problems, such as level crossing and/or energy gap closing along the adiabatic evolution path. In this work, we propose a quantum algorithm based on adiabatic evolution to obtain molecular eigenstates and eigenenergies in quantum chemistry, which exploits a smooth geometric deformation by changing bond lengths and bond angles. Even with a simple uniform stretching of chemical bonds, this algorithm performs more stably and achieves better accuracy than our previous adiabatic method [Phys. Rev. Research 3, 013104 (2021)]. It solves the problems related to energy gap closing and level crossing along the adiabatic evolution path at large atomic distances. We demonstrate its utility in several examples, including H${}_2$O, CH${}_2$, and a chemical reaction of H${}_2$+D${}_2\rightarrow$ 2HD. Furthermore, our fidelity analysis demonstrates that even with finite bond length changes, our algorithm still achieves high fidelity with the ground state.

physics.chem-ph↗

arXiv2020

Tunable Optomechanically Induced Sideband Comb

Cavity optomechanical system can exhibit higher-order sideband comb effect when it is driven by a control field $ω_{c}$ and a probe field $ω_{p}$, and works in the non-perturbative regime, as was shown in a previous work [Xiong et al., Opt. Lett. 38, 353 (2013)]. The repetition frequency of such a comb is equal to the mechanical frequency $ω_{b}$ and is untunable, which limits the precision of the comb. Here we address this problem by driving the system with an additional strong probe field $ω_{f}$, and the detuning between $ω_{f}$ and $ω_{c}$ is equal to $ω_{b}/n$ (here $n$ is an integer), i.e., this detuning is a fraction of the mechanical frequency. In this case, we obtain some interesting results. We find that not only the integer-order (higher-order) sidebands, but also the fraction-order sidebands, and the sum and difference sidebands between the integer- and fraction-order sidebands, will appear in the output spectrum. The generated nonlinear sidebands constitute an optomechanically induced sideband comb (OMISC). The frequency range and the repetition frequency of the OMISC are proportional to the sideband cutoff-order number and the sideband interval, respectively. We show that we can extend the frequency range of the OMISC by increasing the intensity of the probe field $ω_{p}$. More importantly, we can decrease the repetition frequency, and consequently, improve the precision of the OMISC by increasing $n$ and the intensity of the probe field $ω_{f}$.

quant-ph↗

arXiv2019

Effects of Lorentz breaking on the accretion onto a Schwarzschild-like black hole

We formulate and solve the problem of spherically symmetric, steady state, adiabatic accretion onto a Schwarzschild-like black hole obtained recently. We derive the general analytic expressions for the critical points, the critical velocity, the critical speed of sound, and subsequently the mass accretion rate. The case for polytropic gas is discussed in detail. We find the parameter characterizing the breaking of Lorentz symmetry will slow down the mass accretion rate, while has no effect on the gas compression and the temperature profile below the critical radius and at the event horizon.

gr-qc↗

arXiv2015

Tunable multi-channel inverse optomechanically induced transparency

In contrast to the optomechanically induced transparency (OMIT) defined conventionally, the inverse OMIT behaves as coherent absorption of the input lights in the optomechanical systems. We characterize a feasible inverse OMIT in a multi-channel fashion with a double-sided optomechanical cavity system coupled to a nearby charged nanomechanical resonator via Coulomb interaction, where two counter-propagating probe lights can be absorbed via one of the channels or even via three channels simultaneously with the assistance of a strong pump light. Under realistic conditions, we demonstrate the experimental feasibility of our model using two slightly different nanomechanical resonators and the possibility of detecting the energy dissipation of the system. In particular, we find that our model turns to be an unilateral inverse OMIT once the two probe lights are different with a relative phase, and in this case we show the possibility to measure the relative phase precisely.

quant-ph↗

arXiv2011

Molecular Binding Energies from Partition Density Functional Theory

Approximate molecular calculations via standard Kohn-Sham Density Functional Theory are exactly reproduced by performing self-consistent calculations on isolated fragments via Partition Density Functional Theory [Phys. Rev. A 82, 024501 (2010)]. We illustrate this with the binding curves of small diatomic molecules. We find that partition energies are in all cases qualitatively similar and numerically close to actual binding energies. We discuss qualitative features of the associated partition potentials.

cond-mat.other↗

arXiv2006

Extinction theorem and propagation of electromagnetic waves between two semi-infinite anisotropic magnetoelectric materials

Based on molecular optics we investigate the reflection and refraction of an electromagnetic wave between two semi-infinite anisotropic magnetoelectric materials. In terms of Hertz vectors and the principle of superposition, we generalize the extinction theorem and derive the propagation characteristics of wave. Using these results we can easily explain the physical origin of Brewster effect. Our results extend the extinction theorem to the propagation of wave between two arbitrary anisotropic materials and the methods used can be applied to other problems of wave propagation in materials, such as scattering of light.

physics.optics↗