arXiv subjects

Xiaoyi Xie

Publications and source records attributed to Xiaoyi Xie.

13 recordsLinked to original sources

arXiv2024

Bridging relativistic jets from black hole scales to long-term electromagnetic radiation distances: A moving-mesh general relativistic hydrodynamics code with the HLLC Riemann solver

Relativistic jets accompany the collapse of massive stars, the merger of compact objects, or the accretion of gas in active galactic nuclei. They carry information about the central engine and generate electromagnetic radiation. No self-consistent simulations have been able to follow these jets from their birth at the black hole scale to the Newtonian dissipation phase, making the inference of central engine property through astronomical observations undetermined. We present the general relativistic moving-mesh framework to achieve the continuity of jet simulations throughout space and time. We implement the general relativistic extension for the moving-mesh relativistic hydrodynamic code, JET, and develop a tetrad formulation to utilize the Harten-Lax-van Leer Contact (HLLC) Riemann solver in the general relativistic moving-mesh code. The new framework is able to trace the radial movement of relativistic jets from central regions where strong gravity holds all the way to distances of jet dissipation.

astro-ph.HE↗

arXiv2021

Van der Waals Ferromagnetic Josephson Junctions

Superconductor-ferromagnet (S-F) interfaces in two-dimensional (2D) heterostructures present a unique opportunity to study the interplay between superconductivity and ferromagnetism. The realization of such nanoscale heterostructures in van der Waals (vdW) crystals remains largely unexplored due to the challenge of making an atomically-sharp interface from their layered structures. Here, we build a vdW ferromagnetic Josephson junction (JJ) by inserting a few-layer ferromagnetic insulator Cr2Ge2Te6 into two layers of superconductor NbSe2. Owing to the remanent magnetic moment of the barrier, the critical current and the corresponding junction resistance exhibit a hysteretic and oscillatory behavior against in-plane magnetic fields, manifesting itself as a strong Josephson coupling state. Through the control of this hysteresis, we can effectively trace the magnetic properties of atomic Cr2Ge2Te6 in response to the external magnetic field. Also, we observe a central minimum of critical current in some thick JJ devices, evidencing the coexistence of 0 and π phase coupling in the junction region. Our study paves the way to exploring the sensitive probes of weak magnetism and multifunctional building blocks for phase-related superconducting circuits with the use of vdW heterostructures.

cond-mat.mes-hall↗

arXiv2020

The Discovery of Tunable Universality Class in Superconducting $β$-W Thin Films

The interplay between quenched disorder and critical behavior in quantum phase transitions is conceptually fascinating and of fundamental importance for understanding phase transitions. However, it is still unclear whether or not the quenched disorder influences the universality class of quantum phase transitions. More crucially, the absence of superconducting-metal transitions under in-plane magnetic fields in 2D superconductors imposes constraints on the universality of quantum criticality. Here, we discover the tunable universality class of superconductor-metal transition by changing the disorder strength in $β$-W films with varying thickness. The finite-size scaling uncovers the switch of universality class: quantum Griffiths singularity to multiple quantum criticality at a critical thickness of $t_{c \perp 1}\sim 8 nm$ and then from multiple quantum criticality to single criticality at $t_{c\perp 2}\sim 16 nm$. Moreover, the superconducting-metal transition is observed for the first time under in-plane magnetic fields and the universality class is changed at $t_{c \parallel }\sim 8 nm$. The discovery of tunable universality class under both out-of-plane and in-plane magnetic fields provides broad information for the disorder effect on superconducting-metal transitions and quantum criticality.

cond-mat.supr-con↗

arXiv2020

Edge superconductivity in Multilayer WTe2 Josephson junction

WTe2, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. These features have recently attracted wide attention in condensed matter physics. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge superconductivity by superconducting quantum interference measurements in multilayer WTe2 Josephson junctions and we directly map the localized supercurrent. In thick WTe2 (~60 nm), the supercurrent is uniformly distributed by bulk states with symmetric Josephson effect ($\left|I_c^+(B)\right|=\left|I_c^-(B)\right|$). In thin WTe2 (10 nm), however, the supercurrent becomes confined to the edge and its width reaches up to 1.4 um and exhibits non-symmetric behavior $\left|I_c^+(B)\right|\neq \left|I_c^-(B)\right|$. The ability to tune the edge domination by changing thickness and the edge superconductivity establishes WTe2 as a promising topological system with exotic quantum phases and a rich physics.

cond-mat.supr-con↗

arXiv2020

Instabilities in neutron-star postmerger remnants

Using nonlinear, fully relativistic, simulations we investigate the dynamics and gravitational-wave signature associated with instabilities in neutron star postmerger remnants. For simplified models of the remnant we establish the presence of an instability in stars with moderate $T/|W|$, the ratio between the kinetic and the gravitational potential energies. Detailed analysis of the density oscillation pattern reveals a \emph{local} instability in the inner region of the more realistic differential rotation profile. We apply Rayleigh's inflection theorem and Fjørtoft's theorem to analyze the stability criteria concluding that this \emph{inner local} instability originates from a shear instability close to the peak of the angular velocity profile, and that it later evolves into a fast-rotating m=2 oscillation pattern. We discuss the importance of the presence of a corotation point in the fluid, its connection with the shear instability, and comparisons to the Rossby wave and Papaloizou Pringle instabilities considered in the wider literature.

astro-ph.HE↗

arXiv2019

Off-axis synchrotron light curves from full-time-domain moving-mesh simulations of jets from massive stars

We present full-time-domain, moving-mesh, relativistic hydrodynamic simulations of jets launched from the center of a massive progenitor star and compute the resulting synchrotron light curves for observers at a range of viewing angles. We follow jet evolution from ignition inside the stellar center, propagation in the stellar envelope and breakout from the stellar surface, then through the coasting and deceleration phases. The jet compresses into a thin shell, sweeps up the circumstellar medium, and eventually enters the Newtonian phase. The jets naturally develop angular and radial structure due to hydro-dynamical interaction with surrounding gas. The calculated synchrotron light curves cover the observed temporal range of prompt to late afterglow phases of long gamma-ray bursts (LGRBs). The on-axis light curves exhibit an early emission pulse originating in shock-heated stellar material, followed by a shallow decay and a later steeper decay. The off-axis light curves rise earlier than previously expected for top-hat jet models -- on a time scale of seconds to minutes after jet breakout, and decay afterwards. Sometimes the off-axis light curves have later re-brightening components that can be contemporaneous with SNe Ic-bl emission. Our calculations may shed light on the structure of GRB outflows in the afterglow stage. The off-axis light curves from full-time-domain simulations advocate new light curve templates for the search of off-axis/orphan afterglows.

astro-ph.HE↗

arXiv2018

Radio sky maps of the GRB 170817A afterglow from simulations

We present synthetic radio images of the GRB170817A afterglow, computed from moving-mesh hydrodynamic simulations of binary neutron star merger outflows. Having expanded for nearly a year, the merger remnant is expected to subtend roughly 5 milli-arcseconds (mas) on the sky, potentially resolvable by very long baseline radio imaging techniques. Any observations revealing the radio centroid to be offset from the line-of-site to the merger would be the smoking gun of a jetted outflow. However, our results indicate that a measurement of the centroid position alone cannot independently determine whether that jet escaped successfully from the merger debris cloud, or was "choked," yielding a quasi-spherical explosion. We find that in both scenarios, the centroid exhibits superluminal proper motion away from the merger site at roughly 4 - 10 mas per day for at least the first 300 days. We argue that a successful strategy for differentiating among the explosion models will need to include multiple observations over the coming months - years. In particular, we find the time at which the centroid attains its maximum offset, and begins heading back toward the merger site, is considerably later if the jet was choked. Detecting a reversal of the centroid trajectory earlier than 600 days would uniquely identify a successful jet. Our results indicate the source might be resolved using VLBI radio observing techniques with roughly 1 mas resolution starting at roughly 400 days post-merger, and that the the angular extent of a successful jet is significantly smaller than that of a choked jet (4.5 versus 7 mas respectively).

astro-ph.HE↗

arXiv2018

A Strange EUV Emission: Scattered Continuum in the Lymann Limit Absorption Edge toward the Quasar SDSS J125903.26+621211.5?

We identified a peculiar proximate sub-damped Lya absorption system (sub-DLA) at z=3.234 with a neutral hydrogen column density of $N_{H} =10^{19.70\pm0.15}~\rm cm^{-2}$ toward the quasar SDSS J125903.26+621211.5 in two epoch optical spectra of the Sloan Digital Sky Survey. We detected Lya residue in the proximate sub-DLA trough at $> 8σ$ level. To our surprise, extreme ultraviolet (EUV) continuum emission is significantly ($> 4σ$) detected in the corresponding Lymann limit absorption edge at both of the FUV and NUV bands by the Galaxy Evolution Explorer. The high neutral hydrogen column density should allow a negligible transmission of both the Lya line photons and EUV continuum photons due to the high optical depth of the gas. The possible scenarios of foreground galaxy contamination, partial coverage, emission from the quasar host galaxy, and extended Lya emission are excluded in turn, and we speculate that the residual Lya and EUV emission is due to photons scattering (broad Lya and the continuum emission) of electrons residing at a spatial scale larger than that of the proximate sub-DLA. Electron scattering is wavelength-independent, the scattered light is therefore a copy of the incident spectrum that might originate in the accretion disk. With the assistances from the neutral hydrogen absorbers as the "natural coronagraph" and the scatterers as the "natural mirror", we yielded a very hard EUV spectral index of $α= 0.65\pm 0.25$ ($F_ν\propto ν^α$), consistent with the standard picture of the locally heated accretion disk in the inner EUV-emitting radii, as well as in the outer near-infrared-emitting radii suggested by Kishimoto et al. (2008).

astro-ph.GA↗

arXiv2018

Numerical simulations of the jet dynamics and synchrotron radiation of binary neutron star merger event GW170817/GRB170817A

We present numerical simulations of energetic flows propagating through the debris cloud of a binary neutron star (BNS) merger. Starting from the scale of the central engine, we use a moving-mesh hydrodynamics code to simulate the complete dynamical evolution of the produced relativistic jets. We compute synchrotron emission directly from the simulations and present multi-band light curves of the early (sub-day) through late (weeks to years) afterglow stages. Our work systematically compares two distinct models for the central engine, referred to as the narrow and wide engine scenario, which is associated with a successful structured jet and a quasi-isotropic explosion respectively. Both engine models naturally evolve angular and radial structure through hydrodynamical interaction with the merger debris cloud. They both also result in a relativistic blast wave capable of producing the observed multi-band afterglow data. However, we find that the narrow and wide engine scenario might be differentiated by a new emission component that we refer to as a merger flash. This component is a consequence of applying the synchrotron radiation model to the shocked optically thin merger cloud. Such modeling is appropriate if injection of non-thermal electrons is sustained in the breakout relativistic shell, for example by internal shocks or magnetic reconnection. The rapidly declining signature may be detectable for future BNS mergers during the first minutes to day following the GW chirp. Furthermore, its non-detection for the GRB170817A event may disfavor the wide, quasi-isotropic explosion model.

astro-ph.HE↗

arXiv2018

An Intercomparison Study of Two Proximate Damped Ly$α$ Systems with Residual Flux upon the Ly$α$ Absorption Trough toward Quasars

In this paper, we present an intercomparison study of two quasars, SDSS J145618.32+340037.2 and SDSS J215331.50-025514.1, which have proximate damped Lya systems (PDLAs) with residual flux upon the Lya absorption trough. Though they both have residual flux as luminous as $10^{43}erg/s$, their PDLAs are quite different in, e.g., HI column density, metal line absorption strength, high-ionization absorption lines as well as residual flux strength. For J1456, the HI column density is $20.6\pm0.2$, with za=2.3138, nearly identical to the quasar redshift (z=2.3142) determined from the [OIII] emission line. The metallicity of this system is typical of DLAs and there is high ionization therein, suggesting that the PDLA system is multiphase, putting it in the quasar environment. For J2153, we measure the HI column density to be $21.5\pm0.1$ at za=3.511, slightly redshifted with respect to the quasar (z=3.490) measured from CIII]. The metallicity of this system is quite low and there is a lack of significant high-ionization absorption lines therein, suggesting that the system is beyond the quasar host galaxy. The residual flux is wide (1000 km/s) in J1456, with a significance of $8σ$, while also wide (1500 km/s) but with a smaller significance of $3σ$ in J2153. Among many explanations, we find that Lya fuzz or resonant scattering can be used to explain the residual flux in the two sources while partial coverage cannot be excluded for J1456. By comparing these two cases, together with similar cases reported previously, we suggest that the strength of the residual flux is related to properties such as metallicity and high-ionization absorption lines of PDLAs. The residual flux recorded upon the PDLA absorption trough opens a window for us to see the physical conditions and processes of the quasar environment, and their profile and strength further remind us of their spatial scales.

astro-ph.GA↗

arXiv2016

The luminosity dependence of quasar UV continuum slope: dust extinction scenario

We investigate the UV continuum slope $α$ of a large quasar sample from SDSS DR7. By using specific continuum windows, we build two samples at lower ($0.71<z<1.19$) and higher ($1.90<z<3.15$) redshifts, which correspond to the continuum slopes at longer (NUV) and shorter (FUV) rest wavelength ranges respectively. Overall, the average continuum slopes are $-0.36$ and $-0.51$ for $α_{\rm NUV}$ and $α_{\rm FUV}$ with similar dispersions $σ_α \sim 0.5$. For both samples, we confirm the luminosity dependence of the continuum slope, i.e., fainter quasars have redder spectra. We further find that both $α_{\rm NUV}$ and $α_{\rm FUV}$ have a common upper limit ($\sim 1/3$) which is almost independent of the quasar luminosity $L_{\rm bol}$. This finding implies that the intrinsic quasar continuum (or the bluest quasar), at any luminosity, obey the standard thin disk model. We propose that the other quasars with redder $α$ are caused by the reddening from the dust {\it locally}. With this assumption, we employ the dust extinction scenario to model the observed $L_{\rm bol}-α$ relation. We find that, a typical value of $E(B-V)\sim0.1$ to $0.3$ mag (depending on the types of extinction curve) of the quasar {\it local} dust is enough to explain the discrepancy of $α$ between the observation ($\sim-0.5$) and the standard accretion disk model prediction ($\sim 1/3$).

astro-ph.GA↗

arXiv2015

An Apparent Redshift Dependence of Quasar Continuum: Implication for Cosmic Dust Extinction?

We investigate the luminosity and redshift dependence of the quasar continuum by means of composite spectrum using a large non-BAL radio-quiet quasar sample drawn from the Sloan Digital Sky Survey. Quasar continuum slopes in the UV-Opt band are measured at two different wavelength ranges, i.e., $α_{\nu12}$ ($1000\sim 2000 \rmÅ$) and $α_{\nu24}$ ($2000 \sim 4000 \rmÅ$) derived from power law fitting. Generally, the UV spectra slope becomes harder (higher $α_ν$) towards higher bolometric luminosity. On the other hand, when quasars are further grouped into luminosity bins, we find both $α_{\nu12}$ and $α_{\nu24}$ show significant anti-correlation with redshift (i.e., quasar continuum becomes redder towards higher redshift). We suggest that the cosmic dust extinction is very likely the cause of this observed $α_ν-z$ relation. We build a simple cosmic dust extinction model to quantify the observed reddening tendency and find an effective dust density $nσ_v \sim 10^{-5}h~\rm Mpc^{-1}$ at $z<1.5$. The other possibilities that could produce such a reddening effect have also been discussed.

astro-ph.CO↗

arXiv2013

Finite size analysis of zero-temperature jamming transition under applied shear stress

By finding local minima of an enthalpy-like energy, we can generate jammed packings of frictionless spheres under constant shear stress $σ$ and obtain the yield stress $σ_y$ by sampling the potential energy landscape. For three-dimensional systems with harmonic repulsion, $σ_y$ satisfies the finite size scaling with the limiting scaling relation $σ_y\simϕ- ϕ_{_{c,\infty}}$, where $ϕ_{_{c,\infty}}$ is the critical volume fraction of the jamming transition at $σ=0$ in the thermodynamic limit. The width or uncertainty of the yield stress decreases with decreasing $ϕ$ and decays to zero in the thermodynamic limit. The finite size scaling implies a length $ξ\sim (ϕ-ϕ_{_{c,\infty}})^{-ν}$ with $ν=0.81\pm 0.05$, which turns out to be a robust and universal length scale exhibited as well in the finite size scaling of multiple quantities measured without shear and independent of particle interaction. Moreover, comparison between our new approach and quasi-static shear reveals that quasi-static shear tends to explore low-energy states.

cond-mat.soft↗