arXiv subjects

Y. Xue

Publications and source records attributed to Y. Xue.

11 recordsLinked to original sources

arXiv2024

A simple model for longitudinal electron transport during and after laser excitation: Emergence of electron resistive transport

Laser-driven electron transport across a sample has garnered enormous attentions over several decades, as it provides a much faster way to control electron dynamics. Light is an electromagnetic wave, so how and why an electron can acquire a longitudinal velocity remains unanswered. Here we show that it is the magnetic field that steers the electron to the light propagation direction. But, quantitatively, our free-electron model is still unable to reproduce the experimental velocities. Going beyond the free electron mode and assuming the system absorbs all the photon energy, the theoretical velocity matches the experimental observation. We introduce a concept of the resistive transport, where electrons deaccelerate under a constant resistance after laser excitation. This theory finally explains why the experimental distance-versus-time forms a down-concave curve, and unifies ballistic and superdiffusive transports into a single resistive transport. We expect that our finding will motivate further investigations.

cond-mat.mtrl-sci↗

arXiv2023

Electron transport under an ultrafast laser pulse: Implication for spin transport

Laser-driven electron transport across a sample has garnered enormous attentions over several decades, because it potentially allows one to control spin transports in spintronics. But light is a transverse electromagnetic wave, how an electron acquires a longitudinal velocity has been very puzzling. In this paper, we show a general mechanism is working. It is the magnetic field {\bf B} that steers the electron moving along the light propagation direction, while its strong transverse motion leads to local excitation. We employ the formalism put forth by Varga and Toroke to show that if we only include {\bf E}, the electron only moves transversely with a large velocity. Including both {\bf B} and {\bf E} and using real experimental laser parameters, we are able to demonstrate that a laser pulse can drive the electron along the axial direction by 20 to 262 $\rm Å$, consistent with the experiments. The key insight is that {\bf B} changes the direction of the electron and allows the electron to move along the Poynting vector of light. Our finding has an important consequence. Because a nonzero {\bf B} means a spatially dependent vector potential ${\bf A} (\br,t)$, ${\bf B}=\nabla \times {\bf A}(\br,t)$, this points out that the Coulomb gauge, that is, replacing ${\bf A}(\br,t)$ by a spatial independent ${\bf A}(t)$, is unable to describe electron and spin transport under laser excitation. Our finding is expected to have a potential impact on the ongoing investigation of laser-driven spin transport.

cond-mat.mtrl-sci↗

arXiv2021

The XMM-SERVS survey: XMM-Newton point-source catalogs for the W-CDF-S and ELAIS-S1 fields

We present the X-ray point-source catalogs in two of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) fields, W-CDF-S (4.6 deg$^2$) and ELAIS-S1 (3.2 deg$^2$), aiming to fill the gap between deep pencil-beam X-ray surveys and shallow X-ray surveys over large areas. The W-CDF-S and ELAIS-S1 regions were targeted with 2.3 Ms and 1.0 Ms of XMM-Newton observations, respectively; 1.8 Ms and 0.9 Ms exposures remain after flare filtering. The survey in W-CDF-S has a flux limit of 1.0 $\times$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ over 90% of its area in the 0.5-10 keV band; 4053 sources are detected in total. The survey in ELAIS-S1 has a flux limit of 1.3 $\times$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$ over 90% of its area in the 0.5-10 keV band; 2630 sources are detected in total. Reliable optical-to-IR multiwavelength counterpart candidates are identified for $\approx$ 89% of the sources in W-CDF-S and $\approx$ 87% of the sources in ELAIS-S1. 3186 sources in W-CDF-S and 1985 sources in ELAIS-S1 are classified as AGNs. We also provide photometric redshifts for X-ray sources; $\approx$ 84% of the 3319/2001 sources in W-CDF-S/ELAIS-S1 with optical-to-NIR forced photometry available have either spectroscopic redshifts or high-quality photometric redshifts. The completion of the XMM-Newton observations in the W-CDF-S and ELAIS-S1 fields marks the end of the XMM-SERVS survey data gathering. The $\approx$ 12,000 point-like X-ray sources detected in the whole $\approx$ 13 deg$^2$ XMM-SERVS survey will benefit future large-sample AGN studies.

astro-ph.GA↗

arXiv2018

Phase transitions and self-assemblies of lower diamondoids and derivatives

Applying ab initio calculation and molecular dynamics simulation methods, we have been calculating and predicting the essential self-assemblies and phase transitions of two lower diamondoids (adamantane and diamantane), three of their important derivatives (amantadine, memantine and rimantadine), and two organometallic molecules that are built by substituting one hydrogen ion with one sodium ion in both adamantane and diamantine molecules (ADM-Na and Optimized DIM-Na). To study their self-assembly and phase transition behaviors, we built seven different MD simulation systems, and each system consisting of 125 molecules. We obtained self-assembly structures and simulation trajectories for the seven molecules. Radial distribution function studies showed clear phase transitions for the seven molecules. Higher aggregation temperatures were observed for diamondoid derivatives. We also studied the density dependence of the phase transition which demonstrates that the higher the density - the higher the phase transition points.

cond-mat.soft↗

arXiv2018

Self-Assembly of Diamondoid Molecules and Derivatives (MD Simulations and DFT Calculations)

We report self-assembly and phase transition behavior of lower diamondoid molecules and their primary derivatives using molecular dynamic (MD) simulation and density functional theory (DFT) calculations. Two lower diamondoids (adamantane and diamantane), three adamantane derivatives (amantadine, memantine and rimantadine) and two artificial molecules (Adamantane+Na and Diamantane+Na) are studied separately in 125-molecule simulation systems. We performed DFT calculations to optimize their molecular geometries and obtain atomic electronic charges for the corresponding MD simulation, by which we obtained self-assembly structures and simulation trajectories for the seven molecules. Radial distribution functions and structure factors studies showed clear phase transitions for the seven molecules.

physics.chem-ph↗

arXiv2017

First-principles and model simulation of all-optical spin reversal

All-optical spin switching is a potential trailblazer for information storage and communication at an unprecedented fast rate and free of magnetic fields. However, the current wisdom is largely based on semiempirical models of effective magnetic fields and heat pulses, so it is difficult to provide high-speed design protocols for actual devices. Here, we carry out a massively parallel first-principles and model calculation for thirteen spin systems and magnetic layers, free of any effective field, to establish a simpler and alternative paradigm of laser-induced ultrafast spin reversal and to point out a path to a full-integrated photospintronic device. It is the interplay of the optical selection rule and sublattice spin orderings that underlines seemingly irreconcilable helicity-dependent/independent switchings. Using realistic experimental parameters, we predict that strong ferrimagnets, in particular, Laves phase C15 rare-earth alloys, meet the telecommunication energy requirement of 10 fJ, thus allowing a cost-effective subpicosecond laser to switch spin in the GHz region.

cond-mat.mtrl-sci↗

arXiv2017

A New, Faint Population of X-ray Transients

We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint ($m_{\rm R}=27.5$ mag, $z_{\rm ph}$$\sim$2.2) host in the CANDELS survey. The X-ray event is comprised of 115$^{+12}_{-11}$ net 0.3-7.0 keV counts, with a light curve characterised by a $\approx$100 s rise time, a peak 0.3-10 keV flux of $\approx$5$\times$10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, and a power-law decay time slope of $-1.53\pm0.27$. The average spectral slope is $Γ=1.43^{+0.23}_{-0.13}$, with no clear spectral variations. The \hbox{X-ray} and multi-wavelength properties effectively rule out the vast majority of previously observed high-energy transients. A few theoretical possibilities remain: an "orphan" X-ray afterglow from an off-axis short-duration Gamma-ray Burst (GRB) with weak optical emission; a low-luminosity GRB at high redshift with no prompt emission below $\sim$20 keV rest-frame; or a highly beamed Tidal Disruption Event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.

astro-ph.HE↗

arXiv2016

Long-Term X-ray Variability of Typical Active Galactic Nuclei in the Distant Universe

We perform long-term ($\approx 15$ yr, observed-frame) X-ray variability analyses of the 68 brightest radio-quiet active galactic nuclei (AGNs) in the 6 Ms $Chandra$ Deep Field-South (CDF-S) survey; the majority are in the redshift range of $0.6-3.1$, providing access to penetrating rest-frame X-rays up to $\approx 10-30$ keV. Twenty-four of the 68 sources are optical spectral type I AGNs, and the rest (44) are type II AGNs. The time scales probed in this work are among the longest for X-ray variability studies of distant AGNs. Photometric analyses reveal widespread photon-flux variability: $90\%$ of AGNs are variable above a 95% confidence level, including many X-ray obscured AGNs and several optically classified type II quasars. We characterize the intrinsic X-ray luminosity ($L_{\rm{X}}$) and absorption ($N_{\rm{H}}$) variability via spectral fitting. Most (74%) sources show $L_{\rm{X}}$ variability; the variability amplitudes are generally smaller for quasars. A Compton-thick candidate AGN shows variability of its high-energy X-ray flux, indicating the size of reflecting material to be $\lesssim 0.3$ pc. $L_{\rm{X}}$ variability is also detected in a broad absorption line (BAL) quasar. The $N_{\rm{H}}$ variability amplitude for our sample appears to rise as time separation increases. About 16% of sources show $N_{\rm{H}}$ variability. One source transitions from an X-ray unobscured to obscured state while its optical classification remains type I; this behavior indicates the X-ray eclipsing material is not large enough to obscure the whole broad-line region.

astro-ph.HE↗

arXiv2008

Superconductivity up to 37 K in (A1-xSrx)Fe2As2 with A=K and Cs

A new hight Tc Fe-based compound system, AFe2As2 with A = K, Cs, K/Sr and Cs/Sr has been found. Through electron-doping, Tc of the A = K and Cs compounds rises to ~37 K, and finally enter a spin-density-wave state (SDW) through further electron doping with Sr. The observation demonstrates the crucial role of the (FeAs)-layers in the superconductivity in the Fe-based layered system and the special feature of elemental A-layers in this complex chemical system may provide new avenues to superconductivity at a higher Tc.

cond-mat.supr-con↗

arXiv2006

Further Evidence for Variable Synchrotron Emission in XTE J1118+480 in Outburst

We present simultaneous multicolor infrared and optical photometry of the black hole X-ray transient XTE J1118+480 during its short 2005 January outburst, supported by simultaneous X-ray observations. The variability is dominated by short timescales, ~10s, although a weak superhump also appears to be present in the optical. The optical rapid variations, at least, are well correlated with those in X-rays. Infrared JHKs photometry, as in the previous outburst, exhibits especially large amplitude variability. The spectral energy distribution (SED) of the variable infrared component can be fitted with a power-law of slope alpha=-0.78 where Fnu is proportional to nu^alpha. There is no compelling evidence for evolution in the slope over five nights, during which time the source brightness decayed along almost the same track as seen in variations within the nights. We conclude that both short-term variability, and longer timescale fading, are dominated by a single component of constant spectral shape. We cannot fit the SED of the IR variability with a credible thermal component, either optically thick or thin. This IR SED is, however, approximately consistent with optically thin synchrotron emission from a jet. These observations therefore provide indirect evidence to support jet-dominated models for XTE J1118+480 and also provide a direct measurement of the slope of the optically thin emission which is impossible based on the average spectral energy distribution alone.

astro-ph↗

arXiv2005

Metal Abundances in the ICM as a Diagnostics of the Cluster History

Galaxy clusters with a dense cooling core exhibit a central increase in the metallicity of the intracluster medium. Recent XMM-Newton studies with detailed results on the relative abundances of several heavy elements show that the high central abundances are mostly due to the contribution from supernovae type Ia. The dominant source is the stellar population of the central cluster galaxy. With this identification of the origin of heavy elements and the observed rates of SN Ia in elliptical galaxies, the central abundance peak can be used as a diagnostic for the history of the cluster core region. We find for four nearby cooling core clusters that the enrichment times for the central peaks are larger than 6 - 10 Gyrs even for a higher SN Ia rate in the past. This points to an old age and a relatively quiet history of these cluster core regions. A detailed analysis of the element abundance ratios provides evidence that the SN Ia yields in the central cluster galaxies are more rich in intermediate mass elements, like Si and S, compared to the SN Ia models used to explain the heavy element enrichment in our Galaxy.

astro-ph↗