arXiv subjects

H. Xiao

Publications and source records attributed to H. Xiao.

At least 19 recordsLinked to original sources

arXiv2026

The type Ia supernova 2023vjh: a peculiar 1991bg-like SN with unusually faint light curves

We present observations of the 1991bg-like type Ia supernova (SN) 2023vjh, associated with the elliptical galaxy MCG+04-10-013. Its light-curve shape parameters ($\Delta m_{15}(B)=1.89 \pm 0.01$ mag and $s_{BV} = 0.45 \pm 0.03$), together with its spectroscopic evolution, place it firmly within the class of fast-declining, subluminous SNe Ia. The near-peak spectra show prominent features of Si II, Ca II, and Ti II, consistent with a cool photosphere, and place SN 2023vjh within the "cool" and extreme cool regions on the classification diagrams. In addition, the three late-phase near-infrared (NIR) spectra display the Ca II NIR triplet, Fe II, and Co II absorptions, but no obvious $H$-band break. Although SN 2023vjh falls in the same regions of the classification diagrams as other well-studied 91bg-like events, it shows some deviation within this class. In particular, it is systematically fainter than predicted by explosion models and fainter than other well-studied 91bg-like SNe. Light-curve fitting and color-based analyses indicate a relatively large reddening ($E(B-V)_{host}\sim$ 0.2 - 0.35 mag), which is unusual for 91bg-like SNe. Yet, the lack of detectable Na I D absorption in its spectra, along with its large projected distance from the center of its passive host galaxy (6.8 kpc), suggests that interstellar extinction along the line of sight is minimal. SN 2023vjh appears to be fainter than typical 91bg-like SNe, and it could be affected by circumstellar material (CSM). Comparisons with explosion models and alternative extinction prescriptions show that including CSM-like extinction improves agreement in the blue bands, but residual discrepancies in the $i$-band may reflect limitations in the model.

astro-ph.HE↗

arXiv2025

The nebular phase of SN 2024ggi: a low-mass progenitor with no signs of interaction

Context: SN 2024ggi is a Type II supernova (SN) discovered in the nearby galaxy NGC 3621 (D $\approx6.7\pm0.d$ Mpc) on 2024 April 03.21 UT. Its proximity enabled a detailed investigation of the SN's properties and its progenitor star. This work focuses on the optical evolution of SN 2024ggi at the nebular phase. Aims: We investigate the progenitor properties and possible asymmetries in the ejecta by studying the nebular phase evolution between days 287 and 400 after the explosion. Methods: We present optical photometry and spectroscopy of SN 2024ggi during the nebular phase, obtained with the Las Campanas and Gemini South Observatories. Four nebular spectra were taken at 287, 288, 360, and 396 days post-explosion, supplemented by late-time $uBVgri$-band photometry spanning $320-400$ days. The analysis of the nebular emission features is performed to probe ejecta asymmetries. Based on the [O I] flux and [O I]/[Ca II] ratio, and comparisons with spectra models from the literature, we arrive to an estimate of the progenitor mass. Additionally, we construct the bolometric light curve from optical photometry and near-infrared data to derive the synthesized nickel mass. Results: Our analysis suggests a progenitor zero-age-main-sequence mass between $12-15 M_\odot$. The late-time bolometric light curve is consistent with a synthesized $^{56}$Ni mass of $0.05-0.06 M_\odot$. The line profiles exhibit only minor changes over the observed period and suggest a roughly symmetrical ejecta, with a possible clump of oxygen-rich material moving towards the observer. No signatures of circumstellar material interaction are detected up to 400 days after the explosion.

astro-ph.SR↗

arXiv2025

ASASSN-24fw: An 8-month long, 4.1 mag, optically achromatic and polarized dimming event

We discuss ASASSN-24fw, a 13th-magnitude star that optically faded by $\Delta g = 4.12 \pm 0.02$ mag starting in September 2024 after over a decade of quiescence in ASAS-SN. The dimmimg lasted $\sim$8 months before returning to quiescence in late May 2025. The spectral energy distribution (SED) before the event is that of a pre-main sequence or a modestly evolved F star with some warm dust emission. The shape of the optical SED during the dim phase is unchanged and the optical and near-infrared spectra are those of an F star. The SED and the dilution of some of the F star infrared absorption features near minimum suggest the presence of a $\sim$0.25$M_\odot$ M dwarf binary companion. The 43.8 year period proposed by Nair & Denisenko (2024) appears correct and is probably half the precession period of a circumbinary disk. The optical eclipse is nearly achromatic, although slightly deeper in bluer filters, $\Delta (g-z)=0.31\pm0.15$ mag, and the $V$ band emission is polarized by up to 4%. The materials most able to produce such small optical color changes and a high polarization are big ($\sim$20 $\mu$m) carbonaceous or water ice grains. Particle distributions dominated by big grains are seen in protoplanetary disks, Saturn-like ring systems and evolved debris disks. We also carry out a survey of occultation events, finding 46 additional systems, of which only 7 (4) closely match $\varepsilon$ Aurigae (KH 15D), the two archetypes of stars with long and deep eclipses. The full sample is widely distributed in an optical color-magnitude diagram, but roughly half show a mid-IR excess. It is likely many of the others have cooler dust since it seems essential to produce the events.

astro-ph.SR↗

arXiv2024

The solar cycle 25 multi-spacecraft solar energetic particle event catalog of the SERPENTINE project

The Solar energetic particle analysis platform for the inner heliosphere (SERPENTINE) project presents it's new multi-spacecraft SEP event catalog for events observed in solar cycle 25. Observations from five different viewpoints are utilized, provided by Solar Orbiter, Parker Solar Probe, STEREO A, BepiColombo, and the near-Earth spacecraft Wind and SOHO. The catalog contains key SEP parameters for 25-40 MeV protons, 1 MeV electrons, and 100 keV electrons. Furthermore, basic parameters of the associated flare and type-II radio burst are listed, as well as the coordinates of the observer and solar source locations. SEP onset times are determined using the Poisson-CUSUM method. SEP peak times and intensities refer to the global intensity maximum. If different viewing directions are available, we use the one with the earliest onset for the onset determination and the one with the highest peak intensity for the peak identification. Associated flares are identified using observations from near Earth and Solar Orbiter. Associated type II radio bursts are determined from ground-based observations in the metric frequency range and from spacecraft observations in the decametric range. The current version of the catalog contains 45 multi-spacecraft events observed in the period from Nov 2020 until May 2023, of which 13 were widespread events and four were classified as narrow-spread events. Using X-ray observations by GOES/XRS and Solar Orbiter/STIX, we were able to identify the associated flare in all but four events. Using ground-based and space-borne radio observations, we found an associated type-II radio burst for 40 events. In total, the catalog contains 142 single event observations, of which 20 (45) have been observed at radial distances below 0.6 AU (0.8 AU).

physics.space-ph↗

arXiv2023

Multi-point study of the energy release and impulsive CME dynamics in an eruptive C7 flare

We combine observations from different vantage points to perform a detailed study of a long duration eruptive C7 class flare that occurred on 17 April 2021 and was partially occulted from Earth view. The dynamics and thermal properties of the flare-related plasma flows, the flaring arcade, and the energy releases and particle acceleration are studied together with the kinematic evolution of the associated CME in order to place this long duration event in context of previous eruptive flare studies. The flare showed hard X-ray (HXR) bursts over the duration of an hour in two phases lasting from 16:04 UT to 17:05 UT. During the first phase, a strong increase in emission from hot plasma and impulsive acceleration of the CME was observed. The CME acceleration profile shows a three-part evolution of slow rise, acceleration, and propagation in line with the first STIX HXR burst phase, which is triggered by a rising hot (14 MK) plasmoid. During the CME acceleration phase, we find signatures of ongoing magnetic reconnection behind the erupting structure, in agreement with the standard eruptive flare scenario. The subsequent HXR bursts that occur about 30 minutes after the primary CME acceleration show a spectral hardening (from $δ\approx $ 7 to $δ\approx $ 4) but do not correspond to further CME acceleration and chromospheric evaporation. Therefore, the CME-flare feedback relationship may only be of significance within the first 25 minutes of the event under study, as thereafter the flare and the CME eruption evolve independently of each other.

astro-ph.SR↗

arXiv2023

The out-of-plane magnetoresistance in a Van der Waals thin film of WTe2

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a Van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes.At low temperatures, we found a large unsaturated in-plane magnetoresistance and a saturated out-of-plane magnetoresistance when the external magnetic fields are applied perpendicular to the plane. By analysis of Shubnikov-de Haas oscillations, one oscillation peak is found in the out-of-plane magnetoresistance, in contrast to four oscillation peaks in the in-plane magnetoresistance.Our work provides new insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.

cond-mat.str-el↗

arXiv2023

pH-dependent water permeability switching and its memory in 1T' MoS$_2$ membranes

Intelligent transport of molecular species across different barriers is critical for various biological functions and is achieved through the unique properties of biological membranes. An essential feature of intelligent transport is the ability to adapt to different external and internal conditions and also the ability to memorise the previous state. In biological systems, the most common form of such intelligence is expressed as hysteresis. Despite numerous advances made over previous decades on smart membranes, it is still a challenge for a synthetic membrane to display stable hysteretic behaviour for molecular transport. Here we show the memory effects and stimuli regulated transport of molecules through an intelligent phase changing MoS$_2$ membrane in response to external pH. We show that water and ion permeation through 1T' MoS$_2$ membranes follows a pH dependent hysteresis with a permeation rate that switches by a few orders of magnitude. We demonstrate that this phenomenon is unique to the 1T' phase of MoS$_2$ due to the presence of surface charge and exchangeable ions on the surface. We further demonstrate the potential application of this phenomenon in autonomous wound infection monitoring and pH-dependent nanofiltration. Our work significantly deepens understanding of the mechanism of water transport at the nanoscale and opens an avenue for developing neuromorphic applications, smart drug delivery systems, point-of-care diagnostics, smart sensors, and intelligent filtration devices.

cond-mat.soft↗

arXiv2022

Multi-instrument STIX microflare study

During its commissioning phase in 2020, the Spectrometer/Telescope for Imaging X-rays (STIX) on board the Solar Orbiter spacecraft observed 69 microflares. The two most significant events from this set (of GOES class B2 and B6) were observed on-disk from the spacecraft as well as from Earth and analysed in terms of the spatial, temporal, and spectral characteristics. We complement the observations from the STIX instrument with EUV imagery from SDO/AIA and GOES soft X-ray data by adding imaging and plasma diagnostics over different temperature ranges for a detailed microflare case study that is focussed on energy release and transport. Spectral fitting of the STIX data shows clear nonthermal emission for both microflares studied here. The deduced plasma parameters from DEM reconstruction as well as spectral fitting roughly agree with the values in the literature for microflares as do the nonthermal fit parameters from STIX. The observed Neupert effects and impulsive and gradual phases indicate that both events covered in this study are consistent with the standard chromospheric evaporation flare scenario. For the B6 event on 7 June 2020, this interpretation is further supported by the temporal evolution seen in the DEM profiles of the flare ribbons and loops. For this event, we also find that accelerated electrons can roughly account for the required thermal energy. The 6 June 2020 event demonstrates that STIX can detect nonthermal emission for GOES class B2 events that is nonetheless smaller than the background rate level. We demonstrate for the first time how detailed multi-instrument studies of solar flares can be performed with STIX.

astro-ph.SR↗

arXiv2021

Numerical study of COVID-19 spatial-temporal spreading in London

Recent study reported that an aerosolised virus (COVID-19) can survive in the air for a few hours. It is highly possible that people get infected with the disease by breathing and contact with items contaminated by the aerosolised virus. However, the aerosolised virus transmission and trajectories in various meteorological environments remain unclear. This paper has investigated the movement of aerosolised viruses from a high concentration source across a dense urban area. The case study looks at the highly air polluted areas of London: University College Hospital (UCH) and King Cross and St Pancras International Station (KCSPI). We explored the spread and decay of COVID-19 released from the hospital and railway stations with the prescribed meteorological conditions. The study has three key findings: the primary result is that it is possible for the virus to travel from meters up to hundred meters from the source location. The secondary finding shows viruses released into the atmosphere from entry and exit points at KCSPI remain trapped within a small radial distance of < 50m. This strengthens the case for the use of face coverings to reduce the infection rate. The final finding shows that there are different levels of risk at various door locations for UCH, depending on which door is used there can be a higher concentration of COVID-19. Although our results are based on London, since the fundamental knowledge processes are the same, our study can be further extended to other locations (especially the highly air polluted areas) in the world.

physics.med-ph↗

arXiv2019

Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr$_\mathbf{0.67}$Na$_\mathbf{0.33}$Fe$_\mathbf{2}$As$_\mathbf{2}$

We report the temperature-dependent optical conductivity and ARPES studies of the iron-based superconductor (SC) Sr$_{0.67}$Na$_{0.33}$Fe$_2$As$_2$ in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at $T_{\rm N}\simeq$125 K in the orthorhombic spin-density-wave (SDW)-like phase, and $T_r\simeq$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the SC transition at $T_c\simeq$10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below $T_{\rm N}$ and $T_r$, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below $T_r$. Below $T_c$ the dramatic decrease in the low-frequency optical conductivity signals the formation of a SC energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron- and hole-like bands at the corners. Below $T_{\rm N}$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below $T_r$ the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at ~20 meV. The magnetic states, with resulting SDW and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the SC in this material.

cond-mat.str-el↗

arXiv2018

Evidence for unconventional superconductivity in Half-Heusler YPdBi and TbPdBi compounds revealed by London penetration depth measurements

The half-Heusler compounds YPdBi and TbPdBi, while having similar band structure, exhibit different magnetic properties. YPdBi is a diamagnet, while TbPdBi shows antiferromagnetic order below 5.5 K. Both are superconductors with T${_c}\approx$1 K for YPdBi and T${_c}\approx$1.75 K for TbPdBi. Such a contrast in properties between these two compounds opens a question about the effects of band structure or magnetic correlations on superconductivity. Using the combination of a tunnel diode oscillator (TDO) and a commercial dilution refrigerator, we measured temperature dependent magnetic penetration depth $Δλ(T)$ in single crystals of YPdBi and TbPdBi, down to temperatures as low as 0.1K. We found that penetration depths of both compounds do not show exponential temperature dependence and saturation at low temperatures, as expected for conventional BCS superconductors. Instead, in both compounds, the penetration depth can be described by a power law $Δλ(T) = A\times T^{n}$. The coefficient A was found to be about 50$\%$ smaller in TbPdBi, but the exponents are very similar, $n = 2.76\pm 0.04$ in YPdBi and $n = 2.6\pm 0.3$ in TbPdBi, respectively. Our results suggest unconventional superconductivity in both YPdBi and TbPdBi.

cond-mat.supr-con↗

arXiv2018

Superconductivity in half-Heusler compound TbPdBi

We have studied the half-Heusler compound TbPdBi through resistivity, magnetization, Hall effect and heat capacity measurements. A semimetal behavior is observed in its normal state transport properties, which is characterized by a large negative magnetoresistance below 100 K. Notably, we find the coexistence of superconductivity and antiferromagnetism in this compound. The superconducting transition appears at 1.7 K, while the antiferromagnetic phase transition takes place at 5.5 K. The upper critical field $H_{c2}$ shows an unusual linear temperature dependence, implying unconventional superconductivity. Moreover, when the superconductivity is suppressed by magnetic field, its resistivity shows plateau behavior, a signature often seen in topological insulators/semimetals. These findings establish TbPdBi as a platform for study of the interplay between superconductivity, magnetism and non-trivial band topology.

cond-mat.supr-con↗

arXiv2018

Pressure tuned superconductivity and normal state behavior in Ba(Fe$_{0.943}$Co$_{0.057}$)$_2$As$_2$ near the antiferromagnetic boundary

Superconductivity in iron pnictides is unconventional and pairing may be mediated by magnetic fluctuations in the Fe-sublattice. Pressure is a clean method to explore superconductivity in iron based superconductors by tuning the ground state continuously without introducing disorder. Here we present a systematic high pressure transport study in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals with $x=$ 0.057, which is near the antiferromagnetic instability. Resistivity $ρ=ρ_0+AT^{n}$ was studied under applied pressure up to 7.90 GPa. The parameter $n$ approaches a minimum value of $n\approx 1$ at a critical pressure $P_c =$ 3.65 GPa. Near $P_c$, the superconducting transition temperature $T_c$ reaches a maximum value of 25.8 K. In addition, the superconducting diamagnetism at 2 K shows a sudden change around the same critical pressure. These results may be associated with a possible quantum critical point hidden inside the superconducting dome, near optimum $T_c$.

cond-mat.supr-con↗

arXiv2018

Optical study of Dirac fermions and related phonon anomalies in antiferromagnetic compound CaFeAsF

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at $T_s$ = 121 K and a magnetic one to a spin density wave (SDW) state at $T_N$ = 110 K. In the low temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm$^{-1}$ that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe$_2$As$_2$ system [1]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in-between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm$^{-1}$) develops a strongly asymmetric line shape in the SDW state and note that this behaviour can be explained in terms of a strong coupling with the Dirac fermions.

cond-mat.supr-con↗

arXiv2017

Infrared probe of the gap evolution across the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

We measured the optical conductivity of superconducting single crystals of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with $x$ ranging from 0.40 (optimal doping, $T_c = 39$ K) down to 0.20 (underdoped, $T_c = 16$ K), where a magnetic order coexists with superconductivity. In the normal state, the low-frequency optical conductivity can be described by an incoherent broad Drude component and a coherent narrow Drude component: the broad one is doping-independent, while the narrow one shows strong scattering in the heavily underdoped compound. In the superconducting state, the formation of the condensate leads to a low-frequency suppression of the optical conductivity spectral weight. In the heavily underdoped region, the superfluid density is significantly suppressed, and the weight of unpaired carriers rapidly increases. We attribute these results to changes in the superconducting gap across the phase diagram, which could show a nodal-to-nodeless transition due to the strong interplay between magnetism and superconductivity in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$.

cond-mat.supr-con↗

arXiv2017

Relationship between critical current and flux-flow resistivity in the mixed state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed-state of unconventional superconducting alloys Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.044 \leq x \leq 0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity $ρ_{\rm ff}$: $ρ_{\rm ff}/ρ_{\rm n} = (1- j_{c}/j_{0})^{-1}$, where $ρ_{\rm n}=E_0/j_0$ is the normal-state resistivity just above the superconducting transition. In addition, $E_0$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity $ρ_{\rm ff}$: it increases with decreasing magnetic field. In contrast, $E_0$ is negative for the conventional superconductor Nb since, as expected, $ρ_{\rm ff}$ decreases with decreasing magnetic field. Furthermore, in the under-doped and over-doped single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, the parameter $E_0$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($ 0.060\leq x\leq 0.072 $). This result points to the co-existence of superconductivity with some other phase around optimal doping.

cond-mat.supr-con↗

arXiv2016

Quantifying and Reducing Model-Form Uncertainties in Reynolds-Averaged Navier-Stokes Simulations: A Data-Driven, Physics-Based Bayesian Approach

Despite their well-known limitations, Reynolds-Averaged Navier-Stokes (RANS) models are still the workhorse tools for turbulent flow simulations in today's engineering application. For many practical flows, the turbulence models are by far the largest source of uncertainty. In this work we develop an open-box, physics-informed Bayesian framework for quantifying model-form uncertainties in RANS simulations. Uncertainties are introduced directly to the Reynolds stresses and are represented with compact parameterization accounting for empirical prior knowledge and physical constraints (e.g., realizability, smoothness, and symmetry). An iterative ensemble Kalman method is used to assimilate the prior knowledge and observation data in a Bayesian framework, and to propagate them to posterior distributions of velocities and other Quantities of Interest (QoIs). We use two representative cases, the flow over periodic hills and the flow in a square duct, to evaluate the performance of the proposed framework. Simulation results suggest that, even with very sparse observations, the posterior mean velocities and other QoIs have significantly better agreement with the benchmark data compared to the baseline results. At most locations the posterior distribution adequately captures the true model error within the developed model form uncertainty bounds. The framework is a major improvement over existing black-box, physics-neutral methods for model-form uncertainty quantification, where prior knowledge and details of the models are not exploited. This approach has potential implications in many fields in which the governing equations are well understood but the model uncertainty comes from unresolved physical processes.

physics.comp-ph↗

arXiv2016

Universality and unconventional enhancement of flux-flow resistivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 \leq x \leq 0.1$. An unconventional increase in the flux-flow resistivity $ρ_{\rm ff}$ with decreasing magnetic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of $ρ_{\rm ff}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn$_5$. A significantly enhanced $ρ_{\rm ff}$ was found for the x=0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized $ρ_{\rm ff}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.

cond-mat.supr-con↗