arXiv subjects

S. Dong

Publications and source records attributed to S. Dong.

At least 19 recordsLinked to original sources

arXiv2026

Transition path theory insights into hurricane rapid intensification

We explore hurricane and ocean reanalysis data to understand how rapid intensification (RI) of tropical cyclones is impacted by the upper ocean density structure, with an emphasis on barrier layer (BL) thickness and thermocline depth in the eastern Caribbean Sea and adjacent western tropical North Atlantic. This analysis leverages transition path theory (TPT), supported by basic statistical methods. In TPT, Markov chains are constructed by discretizing data series related to weather system intensity, changes in intensity, translational speed, and BL thickness and thermocline depth. These series are viewed as trajectories in abstract state spaces, following a memoryless stochastic process. RI imminence is rigorously framed using a newly derived TPT statistic, which gives the time distribution to first reach a target -- the RI state -- from a source -- for instance, the state determined by a certain BL range and system intensity -- conditional on connecting paths exhibiting minimal detours. Increased RI frequency is observed in the eastern Caribbean and nearby Atlantic, influenced by river runoff, primarily in tropical storms and category 2 hurricanes. RI frequently correlates with a well-developed BL; however, increased translational speed is necessary for RI. TPT shows a stronger connection between RI and thermocline depth than BL presence, with RI likelihood rising for hurricanes with a thin BL, especially category 1. Across all strength categories, a deep thermocline consistently elevates RI probability, a factor missed by basic statistical analysis. Furthermore, translational speed is crucial, with faster, stronger hurricanes more susceptible to RI, while slower systems are less so.

physics.ao-ph↗

arXiv2025

SN 2018is: a low-luminosity Type IIP supernova with narrow hydrogen emission lines at early phases

We present a comprehensive photometric and spectroscopic study of the Type IIP SN 2018is. The $V$-band luminosity and the expansion velocity at 50 days post-explosion are $-$15.1$\pm$0.2 mag (corrected for A$_V$=1.34 mag) and 1400 km s$^{-1}$, classifying it as a low-luminosity SN II. The recombination phase in the $V$-band is shorter, lasting around 110 days, and exhibits a steeper decline (1.0 mag per 100 days) compared to most other low-luminosity SNe II. Additionally, the optical and near-infrared spectra display hydrogen emission lines that are strikingly narrow, even for this class. The Fe II and Sc II line velocities are at the lower end of the typical range for low-luminosity SNe II. Semi-analytical modelling of the bolometric light curve suggests an ejecta mass of $\sim$8 M$_\odot$, corresponding to a pre-supernova mass of $\sim$9.5 M$_\odot$, and an explosion energy of $\sim$0.40 $\times$ 10$^{51}$ erg. Hydrodynamical modelling further indicates that the progenitor had a zero-age main sequence mass of 9 M$_\odot$, coupled with a low explosion energy of 0.19 $\times$ 10$^{51}$ erg. The nebular spectrum reveals weak [O I] $\lambda\lambda$6300,6364 lines, consistent with a moderate-mass progenitor, while features typical of Fe core-collapse events, such as He I, [C I], and [Fe I], are indiscernible. However, the redder colours and low ratio of Ni to Fe abundance do not support an electron-capture scenario either. As a low-luminosity SN II with an atypically steep decline during the photospheric phase and remarkably narrow emission lines, SN 2018is contributes to the diversity observed within this population.

astro-ph.HE↗

arXiv2024

Observations of Microlensed Images with Dual-field Interferometry: On-sky Demonstration and Prospects

Interferometric observations of gravitational microlensing events offer an opportunity for precise, efficient, and direct mass and distance measurements of lensing objects, especially those of isolated neutron stars and black holes. However, such observations have previously been possible for only a handful of extremely bright events. The recent development of a dual-field interferometer, GRAVITY Wide, has made it possible to reach out to significantly fainter objects and increase the pool of microlensing events amenable to interferometric observations by two orders of magnitude. Here, we present the first successful observation of a microlensing event with GRAVITY Wide and the resolution of microlensed images in the event OGLE-2023-BLG-0061/KMT-2023-BLG-0496. We measure the angular Einstein radius of the lens with subpercent precision, $\theta_{\rm E} = 1.280 \pm 0.009$ mas. Combined with the microlensing parallax detected from the event light curve, the mass and distance to the lens are found to be $0.472 \pm 0.012\,M_{\odot}$ and $1.81 \pm 0.05$ kpc, respectively. We present the procedure for the selection of targets for interferometric observations and discuss possible systematic effects affecting GRAVITY Wide data. This detection demonstrates the capabilities of the new instrument, and it opens up completely new possibilities for the follow-up of microlensing events and future routine discoveries of isolated neutron stars and black holes.

astro-ph.IM↗

arXiv2024

Element-specific ultrafast lattice dynamics in monolayer WSe2

We study monolayer WSe2 using ultrafast electron diffraction. We introduce an approach to quantitatively extract atomic-site-specific information, providing an element-specific view of incoherent atomic vibrations following femtosecond excitation. Via differences between W and Se vibrations, we identify stages in the nonthermal evolution of the lattice. Combined with a calculated phonon dispersion, this element specificity enables us to identify a long-lasting overpopulation of specific optical phonons, and to interpret the stages as energy transfer processes between specific phonon groups. These results demonstrate the appeal of resolving element-specific vibrational information in the ultrafast time domain.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic structure and magnetoelectric coupling in antiferromagnet Co5(TeO3)4Cl2

The van der Waals (vdW) layered multiferroics, which host simultaneous ferroelectric and magnetic orders, have attracted attention not only for their potentials to be utilized in nanoelectric devices and spintronics, but also offer alternative opportunities for emergent physical phenomena. To date, the vdW layered multiferroic materials are still very rare. In this work, we have investigated the magnetic structure and magnetoelectric effects in Co5(TeO3)4Cl2, a promising new multiferroic compound with antiferromagnetic (AFM) Neel point TN = 18 K. The neutron powder diffraction reveals the non-coplanar AFM state with preferred Neel vector along the c-axis, while a spin re-orientation occurring between 8 K and 15 K is identified, which results from the distinct temperature dependence of the non-equivalent Co sites moment in Co5(TeO3)4Cl2. What is more, it is found that Co5(TeO3)4Cl2 is one of the best vdW multiferroics studied so far in terms of the multiferroic performance. The measured linear ME coefficient exhibits the emergent oscillation dependence of the angle between magnetic field and electric field, and the maximal value is as big as 45 ps/m. It is suggested that Co5(TeO3)4Cl2 is an appreciated platform for exploring the emergent multiferroicity in vdW layered compounds.

cond-mat.mtrl-sci↗

arXiv2023

Strain tuned magnetotransport of Jeff=1/2 antiferromagnetic Sr2IrO4 thin films

In this work, we report observation of strain effect on physical properties of Sr2IrO4 thin films grown on SrTiO3 (001) and LaAlO3 (001) substrates. It is found that the film on LaAlO3 with compressive strain has a lower antiferromagnetic transition temperature (TN~210 K) than the film on SrTiO3 (TN~230 K) with tensile strain, which is probably caused by modified interlayer coupling. Interestingly, magnetoresistance due to pseudospin-flip of the film on LaAlO3 is much larger than that of tensile-strained film on SrTiO3, and robust anisotropic magnetoresistance is observed in the former, but H-driven reversal behavior is seen in the latter. By performing first principles calculations, it is revealed that epitaxial strain plays an efficient role in tuning the canting angle of Jeff=1/2 moments and thus net moment at every IrO2 layer, responsible for the difference in magnetoresistance between the films. The reversal of anisotropic magnetoresistance in the thin film on SrTiO3 can be ascribed to stabilization of a metastable stable with smaller bandgap as the Jeff=1/2 moments are aligned along the diagonal of basal plane by H. However, theoretical calculations reveal much higher magnetocrystalline anisotropy energy in the film on LaAlO3. This causes difficulties to drive the Jeff=1/2 moments to reach the diagonal and thereby the metastable state, explaining the distinct anisotropic magnetoresistance between two samples in a qualitative sense. Our findings indicate that strain can be a highly efficient mean to engineer the functionalities of Jeff=1/2 antiferromagnet Sr2IrO4.

cond-mat.str-el↗

arXiv2023

Supernova Rates and Luminosity Functions from ASAS-SN I: 2014--2017 Type Ia SNe and Their Subtypes

We present the volumetric rates and luminosity functions (LFs) of Type Ia supernovae (SNe Ia) from the $V$-band All-Sky Automated Survey for Supernovae (ASAS-SN) catalogues spanning discovery dates from UTC 2014-01-26 to UTC 2017-12-29. Our standard sample consists of 404 SNe Ia with $m_{V,\mathrm{peak}}<17$ mag and Galactic latitude $|b|>15^{\circ}$. Our results are both statistically more precise and systematically more robust than previous studies due to the large sample size and high spectroscopic completeness. We make completeness corrections based on both the apparent and absolute magnitudes by simulating the detection of SNe Ia in ASAS-SN light curves. We find a total volumetric rate for all subtypes of $R_{\mathrm{tot}}=2.28^{+0.20}_{-0.20}\,\times 10^{4}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$ for $M_{V,\mathrm{peak}}<-16.5$ mag ($R_{\mathrm{tot}}=1.91^{+0.12}_{-0.12}\,\times 10^{4}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$ for $M_{V,\mathrm{peak}}<-17.5$ mag) at the median redshift of our sample, $z_{\mathrm{med}}=0.024$. This is in agreement ($1\sigma$) with the local volumetric rates found by previous studies. We also compile luminosity functions (LFs) for the entire sample as well as for subtypes of SNe Ia for the first time. The major subtypes with more than one SN include Ia-91bg, Ia-91T, Ia-CSM, and Ia-03fg with total rates of $R_{\mathrm{Ia-91bg}}=1.4^{+0.5}_{-0.5} \times 10^{3}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$, $R_{\mathrm{Ia-91T}}=8.5^{+1.6}_{-1.7} \times 10^{2}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$, $R_{\mathrm{Ia-CSM}}=10^{+7}_{-7}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$, and $R_{\mathrm{Ia-03fg}}=30^{+20}_{-20}\,\mathrm{yr}^{-1}\,\mathrm{Gpc}^{-3}\,h^{3}_{70}$, respectively. We estimate a mean host extinction of $E(V-r)\approx 0.2$ mag based on the shift between our $V$-band and the ZTF $r$-band LFs.

astro-ph.HE↗

arXiv2022

Coherent Light Control of a Metastable Hidden Phase

Metastable phases present a promising route to expand the functionality of complex materials. Of particular interest are light-induced metastable phases that are inaccessible under equilibrium conditions, as they often host new, emergent properties switchable on ultrafast timescales. However, the processes governing the trajectories to such hidden phases remain largely unexplored. Here, using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast dynamics of the formation of a hidden quantum state in the layered dichalcogenide 1T-TaS$_2$ upon photoexcitation. Our results reveal the nonthermal character of the transition governed by a collective charge-density-wave excitation. Utilizing a double-pulse excitation of the structural mode, we show vibrational coherent control of the phase-transition efficiency. Our demonstration of exceptional control, switching speed, and stability of the hidden phase are key for device applications.

cond-mat.mtrl-sci↗

arXiv2021

A fast radio burst source at a complex magnetised site in a barred galaxy

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio bursts. Recent observations of a Galactic FRB suggest that at least some FRBs originate from magnetars, but the origin of cosmological FRBs is still not settled. Here we report the detection of 1863 bursts in 82 hr over 54 days from the repeating source FRB~20201124A. These observations show irregular short-time variation of the Faraday rotation measure (RM), which probes the density-weighted line-of-sight magnetic field strength, of individual bursts during the first 36 days, followed by a constant RM. We detected circular polarisation in more than half of the burst sample, including one burst reaching a high fractional circular polarisation of 75%. Oscillations in fractional linear and circular polarisations as well as polarisation angle as a function of wavelength were detected. All of these features provide evidence for a complicated, dynamically evolving, magnetised immediate environment within about an astronomical unit (au; Earth-Sun distance) of the source. Our optical observations of its Milky-Way-sized, metal-rich host galaxy reveal a barred spiral, with the FRB source residing in a low stellar density, interarm region at an intermediate galactocentric distance. This environment is inconsistent with a young magnetar engine formed during an extreme explosion of a massive star that resulted in a long gamma-ray burst or superluminous supernova.

astro-ph.HE↗

arXiv2021

Close, bright and boxy: the superluminous SN 2018hti

SN 2018hti was a very nearby (z=0.0614) superluminous supernova with an exceedingly bright absolute magnitude of -21.7 mag in r-band at maximum. The densely sampled pre-maximum light curves of SN 2018hti show a slow luminosity evolution and constrain the rise time to ~50 rest-frame days. We fitted synthetic light curves to the photometry to infer the physical parameters of the explosion of SN 2018hti for both the magnetar and the CSM-interaction scenarios. We conclude that one of two mechanisms could be powering the luminosity of SN 2018hti; interaction with ~10 Msun of circumstellar material or a magnetar with a magnetic field of B_p~1.3e13 G and initial period of P_spin~1.8 ms. From the nebular spectrum modelling we infer that SN 2018hti likely results from the explosion of a ~40 Msun progenitor star.

astro-ph.HE↗

arXiv2021

Ultrafast Momentum-resolved Hot Electron Dynamics in the Two-dimensional Topological Insulator Bismuthene

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically-protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than in \red{conventional} semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.

cond-mat.mes-hall↗

arXiv2021

Excited-state band structure mapping

Angle-resolved photoelectron spectroscopy is an extremely powerful probe of materials to access the occupied electronic structure with energy and momentum resolution. However, it remains blind to those dynamic states above the Fermi level that determine technologically relevant transport properties. In this work, we extend band structure mapping into the unoccupied states and across the entire Brillouin zone by using a state-of-the-art high repetition rate, extreme ultraviolet fem- tosecond light source to probe optically excited samples. The wide-ranging applicability and power of this approach are demonstrated by measurements on the 2D semiconductor WSe2, where the energy-momentum dispersion of valence and conduction bands are observed in a single experiment. This provides a direct momentum-resolved view not only on the complete out-of-equilibrium band gap but also on its renormalization induced by electron-hole interaction and screening. Our work establishes a new benchmark for measuring the band structure of materials, with direct access to the energy-momentum dispersion of the excited-state spectral function.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic structure and multiferroicity of Sc-substituted hexagonal YbFeO$_3$

Hexagonal rare-earth ferrite RFeO$_3$ family represents a unique class of multiferroics exhibiting weak ferromagnetism, and a strong coupling between magnetism and structural trimerization is predicted. However, the hexagonal structure for RFeO$_3$ remains metastable in conventional condition. We have succeeded in stabilizing the hexagonal structure of polycrystalline YbFeO$_3$ by partial Sc substitution of Yb. Using bulk magnetometry and neutron diffraction, we find that Yb$_{0.42}$Sc$_{0.58}$FeO$_3$ orders into a canted antiferromagnetic state with the Neel temperature $T_N$ ~ 165 K, below which the $Fe^{3+}$ moments form the triangular configuration in the $ab$-plane and their in-plane projections are parallel to the [100] axis, consistent with magnetic space group $P$6$_{3}$$c'm'$. It is determined that the spin-canting is aligned along the $c$-axis, giving rise to the weak ferromagnetism. Furthermore, the $Fe^{3+}$ moments reorient toward a new direction below reorientation temperature $T_R$ ~ 40 K, satisfying magnetic subgroup $P$6$_{3}$, while the $Yb^{3+}$ moments order independently and ferrimagnetically along the $c$-axis at the characteristic temperature $T_{Yb}$ ~ 15 K. Interestingly, reproducible modulation of electric polarization induced by magnetic field at low temperature is achieved, suggesting that the delicate structural distortion associated with two-up/one-down buckling of the Yb/Sc-planes and tilting of the FeO$_5$ bipyramids may mediate the coupling between ferroelectric and magnetic orders under magnetic field. The present work represents a substantial progress to search for high-temperature multiferroics in hexagonal ferrites and related materials.

cond-mat.mtrl-sci↗

arXiv2020

A quantitative comparison of time-of-flight momentum microscopes and hemispherical analyzers for time- and angle-resolved photoemission spectroscopy experiments

Time-of-flight-based momentum microscopy has a growing presence in photoemission studies, as it enables parallel energy- and momentum-resolved acquisition of the full photoelectron distribution. Here, we report table-top extreme ultraviolet (XUV) time- and angle-resolved photoemission spectroscopy (trARPES) featuring both a hemispherical analyzer and a momentum microscope within the same setup. We present a systematic comparison of the two detection schemes and quantify experimentally relevant parameters, including pump- and probe-induced space-charge effects, detection efficiency, photoelectron count rates, and depth of focus. We highlight the advantages and limitations of both instruments based on exemplary trARPES measurements of bulk WSe2. Our analysis demonstrates the complementary nature of the two spectrometers for time-resolved ARPES experiments. Their combination in a single experimental apparatus allows us to address a broad range of scientific questions with trARPES.

physics.ins-det↗

arXiv2020

Detector control system for the CBM-TOF

A high-performance time-of-flight (TOF) MRPC wall is being built for the CBM experiment at FAIR for charged hadron identification. The detector control system for the TOF system will be based on EPICS. All components like power supplies for low and high voltages, power distribution boxes, gas control and front-end electronics (FEE) are controlled and monitored. In a test, called mini-CBM, all these functionalities are implemented and tested. For monitoring the detector environment and the status of the front-end electronics, a slow control application is implemented based on IPbus, which is an FPGA-based slow control bus used for the TOF data acquisition system. In addition to the functions of control and monitoring, exception handling and data archiving services are implemented as well. This system has been fully verified in beam tests in 2019 at GSI.

physics.ins-det↗

arXiv2019

On a Simple and Effective Thermal Open Boundary Condition for Convective Heat Transfer Problems

We present an effective thermal open boundary condition for convective heat transfer problems on domains involving outflow/open boundaries. This boundary condition is energy-stable, and it ensures that the contribution of the open boundary will not cause an ``energy-like'' temperature functional to increase over time, irrespective of the state of flow on the open boundary. It is effective in coping with thermal open boundaries even in flow regimes where strong vortices or backflows are prevalent on such boundaries, and it is straightforward to implement. Extensive numerical simulations are presented to demonstrate the stability and effectiveness of our method for heat transfer problems with strong vortices and backflows occurring on the open boundaries. Simulation results are compared with previous works to demonstrate the accuracy of the presented method.

physics.flu-dyn↗

arXiv2019

An Energy-Stable Scheme for Incompressible Navier-Stokes Equations with Periodically Updated Coefficient Matrix

We present an energy-stable scheme for simulating the incompressible Navier-Stokes equations based on the generalized Positive Auxiliary Variable (gPAV) framework. In the gPAV-reformulated system the original nonlinear term is replaced by a linear term plus a correction term, where the correction term is put under control by an auxiliary variable. The proposed scheme incorporates a pressure-correction type strategy into the gPAV procedure, and it satisfies a discrete energy stability property. The scheme entails the computation of two copies of the velocity and pressure within a time step, by solving an individual de-coupled linear equation for each of these field variables. Upon discretization the pressure linear system involves a constant coefficient matrix that can be pre-computed, while the velocity linear system involves a coefficient matrix that is updated periodically, once every $k_0$ time steps in the current work, where $k_0$ is a user-specified integer. The auxiliary variable, being a scalar-valued number, is computed by a well-defined explicit formula, which guarantees the positivity of its computed values. It is observed that the current method can produce accurate simulation results at large (or fairly large) time step sizes for the incompressible Navier-Stokes equations. The impact of the periodic coefficient-matrix update on the overall cost of the method is observed to be small in typical numerical simulations. Several flow problems have been simulated to demonstrate the accuracy and performance of the method developed herein.

physics.comp-ph↗

arXiv2019

Investigation of two Fermi-LAT gamma-ray blazars coincident with high-energy neutrinos detected by IceCube

After the identification of the gamma-ray blazar TXS 0506+056 as the first compelling IceCube neutrino source candidate, we perform a systematic analysis of all high-energy neutrino events satisfying the IceCube realtime trigger criteria. We find one additional known gamma-ray source, the blazar GB6 J1040+0617, in spatial coincidence with a neutrino in this sample. The chance probability of this coincidence is 30% after trial correction. For the first time, we present a systematic study of the gamma-ray flux, spectral and optical variability, and multi-wavelength behavior of GB6 J1040+0617 and compare it to TXS 0506+056. We find that TXS 0506+056 shows strong flux variability in the Fermi-LAT gamma-ray band, being in an active state around the arrival of IceCube-170922A, but in a low state during the archival IceCube neutrino flare in 2014/15. In both cases the spectral shape is statistically compatible ($\leq 2σ$) with the average spectrum showing no indication of a significant relative increase of a high-energy component. While the association of GB6 J1040+0617 with the neutrino is consistent with background expectations, the source appears to be a plausible neutrino source candidate based on its energetics and multi-wavelength features, namely a bright optical flare and modestly increased gamma-ray activity. Finding one or two neutrinos originating from gamma-ray blazars in the given sample of high-energy neutrinos is consistent with previously derived limits of neutrino emission from gamma-ray blazars, indicating the sources of the majority of cosmic high-energy neutrinos remain unknown.

astro-ph.HE↗