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Q. Zhang

Publications and source records attributed to Q. Zhang.

At least 109 records · Page 6Linked to original sources
arXiv2019

Analysis of Testbeam Data of the Highly Granular RPC-Steel CALICE Digital Hadron Calorimeter and Validation of Geant4 Monte Carlo Models

We present a study of the response of the highly granular Digital Hadronic Calorimeter with steel absorbers, the Fe-DHCAL, to positrons, muons, and pions with momenta ranging from 2 to 60 GeV/c. Developed in the context of the CALICE collaboration, this hadron calorimeter utilises Resistive Plate Chambers as active media, interspersed with steel absorber plates. With a transverse granularity of $1\,\times\,1\,$cm$^{2}$ and a longitudinal segmentation of 38 layers, the calorimeter counted 350,208 readout channels, each read out with single-bit resolution (digital readout). The data were recorded in the Fermilab test beam in 2010-11. The analysis includes measurements of the calorimeter response and the energy resolution to positrons and muons, as well as detailed studies of various shower shape quantities. The results are compared to simulations based on Geant4, which utilise different electromagnetic and hadronic physics lists.

physics.ins-det↗
arXiv2019

A Submillimeter Polarization Analysis of Frosty Leo

We present a polarimetric investigation of the protoplanetary nebula Frosty Leo performed with the Submillimeter Array. We were able to detect, in the low continuum level (peak at 14.4 mJy beam$^{-1}$), a marginal polarization at $\sim2.6σ$. The molecular line investigation based on the CO $J=3\rightarrow2$ emission shows a peak emission of 68.1 Jy beam$^{-1}$ km s$^{-1}$ and the polarization detection in this CO line is also marginal, with a peak at $\sim3.8σ$. In both cases, it was therefore not possible to use the electric vector maps (B-field) to accurately trace the magnetic field (B-field) within the PPN. The spatio-kinematic modelling realised with the different velocity channel maps indicates three main structures: a distorted torus accompanied by a bipolar outflow or jet aligned with its axis and a flattened spherical "cap". The comparison of the CO polarization segments with our model suggests that the polarized emission probably arises in the first two components.

astro-ph.GA↗
arXiv2019

Discovery of a sub-Keplerian disk with jet around a 20Msun young star. ALMA observations of G023.01-00.41

It is well established that Solar-mass stars gain mass via disk accretion, until the mass reservoir of the disk is exhausted and dispersed, or condenses into planetesimals. Accretion disks are intimately coupled with mass ejection via polar cavities, in the form of jets and less collimated winds, which allow mass accretion through the disk by removing a substantial fraction of its angular momentum. Whether disk accretion is the mechanism leading to the formation of stars with much higher masses is still unclear. Here, we are able to build a comprehensive picture for the formation of an O-type star, by directly imaging a molecular disk which rotates and undergoes infall around the central star, and drives a molecular jet which arises from the inner disk regions. The accretion disk is truncated between 2000-3000au, it has a mass of about a tenth of the central star mass, and is infalling towards the central star at a high rate (6x10^-4 Msun/yr), as to build up a very massive object. These findings, obtained with the Atacama Large Millimeter/submillimeter Array at 700au resolution, provide observational proof that young massive stars can form via disk accretion much like Solar-mass stars.

astro-ph.SR↗
arXiv2018

"$\mathbf{{\textit K^-}{\textit p}{\textit p}}$", a ${\overline{K}}$-Meson Nuclear Bound State, Observed in $^{3}{\rm He}({K^-}, Λ p)n$ Reactions

We observed a distinct peak in the $Λp$ invariant mass spectrum of $^{3}{\rm He}(K^-, \, Λp)n$, well below the mass threshold of $m_K + 2 m_p$. By selecting a relatively large momentum-transfer region $q = 350 \sim 650$ MeV/$c$, one can clearly separate the peak from the quasi-free process, $\overline{K}N \rightarrow \overline{K}N$ followed by the non-resonant absorption by the two spectator-nucleons $\overline{K}NN \rightarrow ΛN $. We found that the simplest fit to the observed peak gives us a Breit-Wigner pole position at $B_{\rm {\it Kpp}} = 47 \pm 3 \, (stat.) \,^{+3}_{-6} \,(sys.)$ MeV having a width $Γ_{\rm {\it Kpp}} = 115 \pm 7 \, (stat.) \,^{+10}_{-9} \,(sys.)$ MeV, and the $S$-wave Gaussian reaction form-factor parameter $Q_{\rm {\it Kpp}} = 381 \pm 14 \, (stat.)\,^{+57}_{-0} \,(sys.)$ MeV/$c$, as a new form of the nuclear bound system with strangeness -- "$K^-pp$".

nucl-ex↗
arXiv2018

Superconductivity in the metastable 1T' and 1T''' phases of MoS$_2$ crystals

Transition-metal dichalcogenides open novel opportunities for the exploration of exciting new physics and devices. As a representative system, 2H-MoS$_2$ has been extensively investigated owing to its unique band structure with a large band gap, degenerate valleys and non-zero Berry curvature. However, experimental studies of metastable 1T polytypes have been a challenge for a long time, and electronic properties are obscure due to the inaccessibility of single phase without the coexistence of 1T', 1T'' and 1T''' lattice structures, which hinder the broad applications of MoS$_2$ in future nanodevices and optoelectronic devices. Using ${K_x(H_2O)_yMoS_2}$ as the precursor, we have successfully obtained high-quality layered crystals of the metastable 1T'''-MoS$_2$ with $\sqrt{3}a\times\sqrt{3}a$ superstructure and metastable 1T'-MoS$_2$ with a$\times$2a superstructure, as evidenced by structural characterizations through scanning tunneling microscopy, Raman spectroscopy and X-ray diffraction. It is found that the metastable 1T'-MoS$_2$ is a superconductor with onset transition temperature (${T_c}$) of 4.2 K, while the metastable 1 T'''-MoS$_2$ shows either superconductivity with Tc of 5.3 K or insulating behavior, which strongly depends on the synthesis procedure. Both of the metastable polytypes of MoS$_2$ crystals can be transformed to the stable 2H phase with mild annealing at about 70 $^{\circ}$C in He atmosphere. These findings provide pivotal information on the atomic configurations and physical properties of 1T polytypes of MoS$_2$.

cond-mat.supr-con↗
arXiv2018

Fragmentation and disk formation during high-mass star formation: The IRAM NOEMA (Northern Extended Millimeter Array) large program CORE

Aims: We aim to understand the fragmentation as well as the disk formation, outflow generation and chemical processes during high-mass star formation on spatial scales of individual cores. Methods: Using the IRAM Northern Extended Millimeter Array (NOEMA) in combination with the 30m telescope, we have observed in the IRAM large program CORE the 1.37mm continuum and spectral line emission at high angular resolution (~0.4'') for a sample of 20 well-known high-mass star-forming regions with distances below 5.5kpc and luminosities larger than 10^4Lsun. Results: We present the overall survey scope, the selected sample, the observational setup and the main goals of CORE. Scientifically, we concentrate on the mm continuum emission on scales on the order of 1000AU. We detect strong mm continuum emission from all regions, mostly due to the emission from cold dust. The fragmentation properties of the sample are diverse. We see extremes where some regions are dominated by a single high-mass core whereas others fragment into as many as 20 cores. A minimum-spanning-tree analysis finds fragmentation at scales on the order of the thermal Jeans length or smaller suggesting that turbulent fragmentation is less important than thermal gravitational fragmentation. The diversity of highly fragmented versus singular regions can be explained by varying initial density structures and/or different initial magnetic field strengths. Conclusions: The smallest observed separations between cores are found around the angular resolution limit which indicates that further fragmentation likely takes place on even smaller spatial scales. The CORE project with its numerous spectral line detections will address a diverse set of important physical and chemical questions in the field of high-mass star formation.

astro-ph.GA↗
arXiv2018

Measurement of mechanical deformations induced by enhanced electromagnetic stress on a parallel metallic-plate system

We measured the electromagnetic stress-induced local strain distribution on a centimeter-sized parallel-plate metallic resonant unit illuminated with microwave. Using a fiber interferometer, we found that the strain changes sign across the resonant unit, in agreement with theoretical predictions that the attractive electric and repulsive magnetic forces act at different locations. The enhancement of the corresponding maximum local electromagnetic stress is stronger than the enhancement of the net force, reaching a factor of >600 compared to the ordinary radiation pressure.

physics.app-ph↗
arXiv2018

Modeling the Multi-band Afterglow of GRB 060908 with the Double Power-Law Hard Electron Energy Spectrum Model

Electrons accelerated in relativistic collisionless shocks are usually assumed to follow a power-law energy distribution with an index of $p$. Observationally, although most gamma-ray bursts (GRBs) have afterglows that are consistent with $p>2$, there are still a few GRBs suggestive of a hard ($p<2$) electron energy spectrum. Our previous works found strong evidence for the exist of a double power-law hard electron energy (DPLH) spectrum with $1 2$ and an injection break assumed as $γ_{\rm b}\propto γ^{q}$ in the relativistic regime. Moreover, these works suggested a possibly universal value of $q\sim0.5$. GRB~060908 is another case that shows a flat spectrum in the optical/near-infrared band and requires a hard electron energy distribution, which, along with the rich multi-band afterglow data, provides us an opportunity to test this conjecture. Based on the model of \citet{Resmi08}, we explain the multi-band afterglow of GRB~060908 in a wind medium and take also the synchrotron self-Compton effect. We show that though the DPLH spectrum is favored by the afterglow data, the value of $q$ is badly constrained due to the relatively large uncertainties of the spectral indices. However, the afterglow can be well reproduced if we adopt $q=0.5$, implying the compatibility of the above conjecture with the data.

astro-ph.HE↗
arXiv2018

Modeling the Multi-band Afterglows of GRB 060614 and GRB 060908: Further Evidence for a Double Power-Law Hard Electron Energy Spectrum

Electrons accelerated in relativistic collisionless shocks are usually assumed to follow a power-law energy distribution with an index of $p$. Observationally, although most gamma-ray bursts (GRBs) have afterglows that are consistent with $p>2$, there are still a few GRBs suggestive of a hard ($p<2$) electron energy spectrum. Our previous work showed that GRB 091127 gave strong evidence for a double power-law hard electron energy (DPLH) spectrum with $1 2$ and an "injection break" assumed as $γ_{\rm b}\propto γ^q$ in the highly relativistic regime, where $γ$ is the bulk Lorentz factor of the jet. In this paper, we show that GRB 060614 and GRB 060908 provide further evidence for such a DPLH spectrum. We interpret the multi-band afterglow of GRB 060614 with the DPLH model in an homogeneous interstellar medium by taking into account a continuous energy injection process, while for GRB 060908, a wind-like circumburst density profile is used. The two bursts, along with GRB 091127, suggest a similar behavior in the evolution of the injection break, with $q\sim0.5$. Whether this represents a universal law of the injection break remains uncertain and more such afterglow observations are needed to test this conjecture.

astro-ph.HE↗
arXiv2018

Love wave gas sensor based on DWNTs sensitive material

This work focuses on the application related to the detection of low moisture and environmental pollutants. A novel gas sensor with inkjet printed Double Walled Carbon Nano Tubes (DWNTs) on a Love wave sensor platform was developed for Volatile Organic Compounds (VOCs) and humidity detection application. The experiments were conducted in real-time at ambient conditions. Results demonstrate the adsorption of vapor compounds on DWNTs sensitive material and leads for example to frequency shifts of 1.97 kHz and 2.93 kHz with 120 ppm of ethanol vapor and 6.22 % RH, respectively.

physics.ins-det↗
arXiv2018

SH-SAW VOCs sensor based on ink-jet printed MWNTs / polymer nanocomposite films

This study presents Shear Horizontal Surface Acoustic Wave (SH-SAW) sensor based on ink-jet printed poly (3,4-ethylenedioxythiophene) polystyrene sulfonate -- multi wall carbon nanotubes (PEDOT:PSS-MWCNTs) and MWNTs-based inks as sensitive gas material. Experiments show the validation of fabrication process of acoustic platform and ink-jet printed sensitive layers. The characterization of two devices under different concentrations of ethanol vapor shows promising results in terms of reproducibility of the measurements. A sensitivity of 12Hz/ppm was recorded with the sensor based on ink-jet printed 600nm thickness sensitive layer.

physics.ins-det↗
arXiv2017

Search for the Signatures of a New-Born Black Hole from the Collapse of a Supra-massive Millisecond Magnetar in Short GRB Light Curves

`Internal plateau' followed by a sharp decay is commonly seen in short gamma-ray burst (GRB) light curves. The plateau component is usually interpreted as the dipole emission from a supra-massive magnetar, and the sharp decay may imply the collapse of the magnetar to a black hole (BH). Fall-back accretion onto the new-born BH could produce long-lasting activities via the Blandford-Znajek (BZ) process. The magnetic flux accumulated near the BH would be confined by the accretion disks for a period of time. As the accretion rate decreases, the magnetic flux is strong enough to obstruct gas infall, leading to a magnetically-arrested disk (MAD). Within this scenario, we show that the BZ process could produce two types of typical X-ray light curves: type I exhibits a long-lasting plateau, followed by a power-law decay with slopes ranging from 5/3 to 40/9; type II shows roughly a single power-law decay with slope of 5/3. The former requires low magnetic filed strength, while the latter corresponds to relatively high values. We search for such signatures of the new-born BH from a sample of short GRBs with an internal plateau, and find two candidates: GRB 101219A and GRB 160821B, corresponding to type II and type I light curve, respectively. It is shown that our model can explain the data very well.

astro-ph.HE↗
arXiv2017

Star formation in a high-pressure environment: An SMA view of the Galactic centre dust ridge

The star formation rate in the Central Molecular Zone (CMZ) is an order of magnitude lower than predicted according to star formation relations that have been calibrated in the disc of our own and nearby galaxies. Understanding how and why star formation appears to be different in this region is crucial if we are to understand the environmental dependence of the star formation process. Here, we present the detection of a sample of high-mass cores in the CMZ's "dust ridge" that have been discovered with the Submillimeter Array as part of the CMZoom survey. These cores range in mass from ~ 50 - 2150 Msun within radii of 0.1 - 0.25 pc. All appear to be young (pre-UCHII), meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. We report that at least two of these cores ('c1' and 'e1') contain young, high-mass protostars. We compare all of the detected cores with high-mass cores in the Galactic disc and find that they are broadly similar in terms of their masses and sizes, despite being subjected to external pressures that are several orders of magnitude greater - ~ 10^8 K/cm^3, as opposed to ~ 10^5 K/cm^3. The fact that > 80% of these cores do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is further evidence for an increased critical density threshold for star formation in the CMZ due to turbulence.

astro-ph.GA↗
arXiv2017

High-pressure behavior of superconducting boron-doped diamond

This work investigates the high-pressure structure of freestanding superconducting ($T_{c}$ = 4.3\,K) boron doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30\,GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients and the grain boundary components undergo an irreversible phase change at 14\,GPa. We show that the blue shift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_{c}$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_{0}$=510$\pm$28\,GPa. The comparative analysis of high-pressure data clarified that the sp$^{2}$ carbons in the grain boundaries transform into hexagonal diamond.

cond-mat.supr-con↗
arXiv2017

A magnetic topological semimetal Sr1-yMn1-zSb2 (y, z< 0.1)

Weyl (WSMs) evolve from Dirac semimetals in the presence of broken time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in TaAs-class materials and photonic crystals are due to the loss of space-inversion symmetry. For TRS-breaking WSMs, despite numerous theoretical and experimental efforts, few examples have been reported. In this Article, we report a new type of magnetic semimetal Sr1-yMn1-zSb2 (y,z<0.1) with nearly massless relativistic fermion behaviour (m*=0.04-0.05m0, where m0 is the free electron mass). This material exhibits a ferromagnetic order for 304K < T < 565K, but a canted antiferromagnetic order with a ferromagnetic component for T < 304K. The combination of relativistic fermion behaviour and ferromagnetism in Sr1-yMn1-zSb2 offers a rare opportunity to investigate the interplay between relativistic fermions and spontaneous TRS breaking.

cond-mat.mtrl-sci↗
arXiv2017

Druse-Induced Morphology Evolution in Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is a key site of pathogenesis for many retina diseases. The formation of drusen in the retina is characteristic of retinal degeneration. We investigate morphological changes in the RPE in the presence of soft drusen using an integrated experimental and modeling approach. We collect RPE flat mount images from donated human eyes and develop 1) statistical tools to quantify the images and 2) a cell-based model to simulate the morphology evolution. We compare three different mechanisms of RPE repair evolution, cell apoptosis, cell fusion, and expansion, and Simulations of our RPE morphogenesis model quantitatively reproduce deformations of human RPE morphology due to drusen, suggesting that a purse-string mechanism is sufficient to explain how RPE heals cell loss caused by drusen-damage. We found that drusen beneath tissue promote cell death in a number that far exceeds the cell numbers covering the drusen. Tissue deformations are studied using area distributions, Voronoi domains and a texture tensor.

q-bio.TO↗
arXiv2017

The ALMA view of W33A: A Spiral Filament Feeding the Candidate Disc in MM1-Main

We targeted the massive star forming region W33A using the Atacama Large Sub/Millimeter Array (ALMA) in band 6 (230 GHz) and 7 (345 GHz) to search for a sub-1000au disc around the central O-type massive young stellar object (MYSO) W33A MM1-Main. Our data achieves a resolution of ~0.2" (~500au) and resolves the central core, MM1, into multiple components and reveals complex and filamentary structures. There is strong molecular line emission covering the entire MM1 region. The kinematic signatures are inconsistent with only Keplerian rotation although we propose that the shift in the emission line centroids within ~1000au of MM1-Main could hint at an underlying compact disc with Keplerian rotation. We cannot however rule out the possibility of an unresolved binary or multiple system. A putative smaller disc could be fed by the large scale spiral `feeding filament' we detect in both gas and dust emission. We also discuss the nature of the now-resolved continuum sources.

astro-ph.SR↗