arXiv subjects

Q. Zhang

Publications and source records attributed to Q. Zhang.

At least 55 records · Page 3Linked to original sources

arXiv2023

Multi-k magnetic structure and large anomalous Hall effect in candidate magnetic Weyl semimetal NdAlGe

The magnetic structure, magnetoresistance, and Hall effect of non-centrosymmetric magnetic semimetal NdAlGe are investigated revealing an unusual magnetic state and anomalous transport properties that are associated with the electronic structure of this non-centrosymmetric compound. The magnetization and magnetoresistance measurements are both highly anisotropic and indicate an Ising-like magnetic system. The magnetic structure is complex in that it involves three magnetic ordering vectors including an incommensurate spin density wave and commensurate ferrimagnetic state in zero field. We have discovered a large anomalous Hall conductivity that reaches = 430 Ω-1cm-1 implying that it originates from an intrinsic Berry curvature effect stemming from Weyl nodes found in the electronic structure. These electronic structure calculations indicate the presence of nested Fermi surface pockets with nesting wave vectors similar to the measured magnetic ordering wavevector and the presence of Weyl nodes in proximity to the Fermi surface. We associate the incommensurate magnetic structure with the large anomalous Hall response to be the result of the combination of Fermi surface nesting and the Berry curvature associated with Weyl nodes.

cond-mat.mtrl-sci↗

arXiv2023

The Lobster Eye Imager for Astronomy Onboard the SATech-01 Satellite

The Lobster Eye Imager for Astronomy (LEIA), a pathfinder of the Wide-field X-ray Telescope of the Einstein Probe (EP) mission, was successfully launched onboard the SATech-01 satellite of the Chinese Academy of Sciences on 27 July 2022. In this paper, we introduce the design and on-ground test results of the LEIA instrument. Using state-of-the-art Micro-Pore Optics (MPO), a wide field-of-view (FoV) of 346 square degrees (18.6 degrees * 18.6 degrees) of the X-ray imager is realized. An optical assembly composed of 36 MPO chips is used to focus incident X-ray photons, and four large-format complementary metal-oxide semiconductor (CMOS) sensors, each of 6 cm * 6 cm, are used as the focal plane detectors. The instrument has an angular resolution of 4 - 8 arcmin (in FWHM) for the central focal spot of the point spread function, and an effective area of 2 - 3 cm2 at 1 keV in essentially all the directions within the field of view. The detection passband is 0.5 - 4 keV in the soft X-rays and the sensitivity is 2 - 3 * 10-11 erg s-1 cm-2 (about 1 mini-Crab) at 1,000 second observation. The total weight of LEIA is 56 kg and the power is 85 W. The satellite, with a design lifetime of 2 years, operates in a Sun-synchronous orbit of 500 km with an orbital period of 95 minutes. LEIA is paving the way for future missions by verifying in flight the technologies of both novel focusing imaging optics and CMOS sensors for X-ray observation, and by optimizing the working setups of the instrumental parameters. In addition, LEIA is able to carry out scientific observations to find new transients and to monitor known sources in the soft X-ray band, albeit limited useful observing time available.

astro-ph.IM↗

arXiv2023

Infall and Outflow Towards High-mass Starless Clump Candidates

The evolutionary sequence for high-mass star formation starts with massive starless clumps that go on to form protostellar, young stellar objects and then compact HII regions. While there are many examples of the three later stages, the very early stages have proved to be elusive. We follow-up a sample of 110 mid-infrared dark clumps selected from the ATLASGAL catalogue with the IRAM telescope in an effort to identify a robust sample of massive starless clumps. We have used the HCO+ (1-0) and HNC (1-0) transitions to identify clumps associated with infall motion and the SiO (2-1) transition to identity outflow candidates. We have found blue asymmetric line profile in 65% of the sample, and have measured the infall velocities and mass infall rates (0.6-$36 \times 10^{-3}$ Msun/yr) for 33 of these clumps. We find a trend for the mass infall rate decreasing with an increase of bolometric luminosity to clump mass i.e. star formation within the clumps evolves. Using the SiO 2-1 line, we have identified good outflow candidates. Combining the infall and outflow tracers reveals that 67% of quiescent clumps are already undergoing gravitational collapse or are associated with star formation; these clumps provide us with our best opportunity to determined the initial conditions and study the earliest stages of massive star formation. Finally, we provide an overview of a systematic high-resolution ALMA study of quiescent clumps selected that allows us to develop a detailed understanding of earliest stages and their subsequent evolution.

astro-ph.GA↗

arXiv2023

Kinematics and stability of high-mass protostellar disk candidates at sub-arcsecond resolution -- Insights from the IRAM NOEMA large program CORE

The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars ($M\gtrsim 8M_\odot$) are still not well understood. To this end, we have undertaken a large observational program (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous ($L>10^4L_\odot$) protostellar objects in the 1.37 mm wavelength regime in both continuum and line emission, reaching $\sim$0.4" resolution (800 au at 2 kpc). Using the dense gas tracer CH$_3$CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. Specific angular momentum ($j$) radial profiles are on average $\sim10^{-3}$ km /s pc and follow $j \propto r^{1.7}$, consistent with a poorly resolved rotating and infalling envelope/disk model. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of $\sim 10-25$ $M_\odot$. Modelling the level population of CH$_3$CN lines, we present temperature maps and find median gas temperatures in the range $70-210$ K. We create Toomre $Q$ maps to study the stability of the disks and find almost all (11 of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations. In particular, disks with masses greater than $\sim10-20\%$ of the mass of their host (proto)stars are Toomre unstable, and more luminous protostellar objects tend to have disks that are more massive and hence more prone to fragmentation. Our finings show that most disks around high-mass protostars are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.

astro-ph.GA↗

arXiv2023

Mother of Dragons: A Massive, quiescent core in the dragon cloud (IRDC G028.37+00.07)

Context: Core accretion models of massive star formation require the existence of massive, starless cores within molecular clouds. Yet, only a small number of candidates for such truly massive, monolithic cores are currently known. Aims: Here we analyse a massive core in the well-studied infrared-dark cloud (IRDC) called the 'dragon cloud' (also known as G028.37+00.07 or 'Cloud C'). This core (C2c1) sits at the end of a chain of a roughly equally spaced actively star-forming cores near the centre of the IRDC. Methods: We present new high-angular resolution 1 mm ALMA dust continuum and molecular line observations of the massive core. Results: The high-angular resolution observations show that this region fragments into two cores C2c1a and C2c1b, which retain significant background-subtracted masses of 23 Msun and 2 Msun (31 Msun and 6 Msun without background subtraction), respectively. The cores do not appear to fragment further on the scales of our highest angular resolution images (0.200 arcsec, 0.005 pc ~ 1000 AU). We find that these cores are very dense (nH2 > 10^6 cm-3) and have only trans-sonic non-thermal motions (Ms ~ 1). Together the mass, density and internal motions imply a virial parameter of < 1, which suggests the cores are gravitationally unstable, unless supported by strong magnetic fields with strengths of ~ 1 - 10 mG. From CO line observations, we find that there is tentative evidence for a weak molecular outflow towards the lower-mass core, and yet the more massive core remains devoid of any star formation indicators. Conclusions: We present evidence for the existence of a massive, pre-stellar core, which has implications for theories of massive star formation. This source warrants follow-up higher-angular-resolution observations to further assess its monolithic and pre-stellar nature.

astro-ph.GA↗

arXiv2023

Motion and teleportation of polar bubbles in ultra-thin ferroelectrics

Polar bubble domains are complex topological defects akin to magnetic skyrmions that can spontaneously form in ferroelectric thin films and superlattices. They can be deterministically written and deleted and exhibit a set of properties, such as sub-10 nm radius and room-temperature stability, that are highly attractive for dense data storage and reconfigurable nano-electronics technologies. However, possibilities of controlled motion of electric bubble skyrmions, a critical technology requirement currently remains missing. Here we present atomistic simulations that demonstrate how external electric-field perturbations can induce two types of motion of bubble skyrmions in low-dimensional tetragonal PbZr$_{0.4}$Ti$_{0.6}$O$_3$ systems under residual depolarizing field. Specifically, we show that, depending on the spatial profile and magnitude of the external field, bubble skyrmions can exhibit either a continuous motion driven by the external electric field gradient or a discontinuous, teleportation-like, skyrmion domain transfer. These findings provide the first analysis of dynamics and controlled motion of polar skyrmions that are essential for functionalization of these particle-like domain structures.

cond-mat.mtrl-sci↗

arXiv2023

Dynamical mean-field driven spinor condensate physics beyond the single-mode approximation

$^{23}$Na spin-1 Bose-Einstein condensates are used to experimentally demonstrate that mean-field physics beyond the single-mode approximation can be relevant during the non-equilibrium dynamics. The experimentally observed spin oscillation dynamics and associated dynamical spatial structure formation confirm theoretical predictions that are derived by solving a set of coupled mean-field Gross-Pitaevskii equations [J. Jie et al., Phys. Rev. A 102, 023324 (2020)]. The experiments rely on microwave dressing of the $f=1$ hyperfine states, where $f$ denotes the total angular momentum of the $^{23}$Na atom. The fact that beyond single-mode approximation physics at the mean-field level, i.e., spatial mean-field dynamics that distinguishes the spatial density profiles associated with different Zeeman levels, can -- in certain parameter regimes -- have a pronounced effect on the dynamics when the spin healing length is comparable to or larger than the size of the Bose-Einstein condensate has implications for using Bose-Einstein condensates as models for quantum phase transitions and spin squeezing studies as well as for non-linear SU(1,1) interferometers.

cond-mat.quant-gas↗

arXiv2023

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -XIII. Ongoing triggered star formation within clump-fed scenario found in the massive ($\sim1500$ $\rm M_\odot$) clump

Whether ionization feedback triggers the formation of massive stars is highly debated. Using ALMA 3 mm observations with a spatial resolution of $\sim 0.05$ pc and a mass sensitivity of 1.1 $\rm M_\odot$ beam$^{-1}$ at 20 K, we investigate the star formation and gas flow structures within the ionizing feedback-driven structure, a clump-scale massive ($\gtrsim 1500$ $\rm M_\odot$) bright-rimmed cloud (BRC) associated with IRAS 18290-0924. This BRC is bound only if external compression from ionized gas is considered. A small-scale ($\lesssim1$ pc) age sequence along the direction of ionizing radiation is revealed for the embedded cores and protostars, which suggests triggered star formation via radiation-driven implosion (RDI). Furthermore, filamentary gas structures converge towards the cores located in the BRC's center, indicating that these filaments are fueling mass towards cores. The local core-scale mass infall rate derived from H$^{13}$CO$^+$ $J=1-0$ blue profile is of the same order of magnitude as the filamentary mass inflow rate, approximately 1 $\rm M_\odot$ kyr$^{-1}$. A photodissociation region (PDR) covering the irradiated clump surface is detected in several molecules, such as CCH, HCO$^+$, and CS whereas the spatial distribution stratification of these molecules is indistinct. CCH spectra of the PDR possibly indicate a photoevaporation flow leaving the clump surface with a projected velocity of $\sim2$ km s$^{-1}$. Our new observations show that RDI accompanied by a clump-fed process is operating in this massive BRC. Whether this combined process works in other massive BRCs is worth exploring with dedicated surveys.

astro-ph.GA↗

arXiv2022

First wide field-of-view X-ray observations by a lobster eye focusing telescope in orbit

As a novel X-ray focusing technology, lobster eye micro-pore optics (MPO) feature both a wide observing field of view and true imaging capability, promising sky monitoring with significantly improved sensitivity and spatial resolution in soft X-rays. Since first proposed by Angel (1979), the optics have been extensively studied, developed and trialed over the past decades. In this Letter, we report on the first-light results from a flight experiment of the Lobster Eye Imager for Astronomy ($LEIA$), a pathfinder of the wide-field X-ray telescope of the Einstein Probe mission. The piggyback imager, launched in July 2022, has a mostly un-vignetted field of view of $18.6^\circ \times 18.6^\circ $. Its spatial resolution is in the range of 4$-$7 arcmin in FWHM and the focal spot effective area is 2$-$3 cm$^2$, both showing only mild fluctuations across the field of view. We present images of the Galactic center region, Sco X-1 and the diffuse Cygnus Loop nebular taken in snapshot observations over 0.5$-$4 keV. These are truly wide-field X-ray images of celestial bodies observed, for the first time, by a focusing imaging telescope. Initial analyses of the in-flight data show excellent agreement between the observed images and the on-ground calibration and simulations. The instrument and its characterization are briefly described, as well as the flight experiment. The results provide a solid basis for the development of the present and proposed wide-field X-ray missions using lobster eye MPO.

astro-ph.HE↗

arXiv2022

Ferroelectric Solitons Crafted in Epitaxial Bismuth Ferrite Superlattices

In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been reported in multiferroics. Here, we report a range of soliton topologies in bismuth ferrite strontium titanate superlattices. High-resolution piezoresponse force microscopy and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy reveal a zoo of topologies, and polarization displacement mapping of planar specimens reveals center-convergent and divergent topological defects as small as 3 nm. Phase field simulations verify that some of these topologies can be classed as bimerons, with a topological charge of plus and minus one, and first-principles-based effective Hamiltonian computations show that the co-existence of such structures can lead to non-integer topological charges, a first observation in a BiFeO3-based system. Our results open new opportunities in multiferroic topotronics.

cond-mat.mtrl-sci↗

arXiv2022

Pole position of $Λ(1405)$ measured in $d(K^-,n)πΣ$ reactions

We measured a set of $π^\pmΣ^\mp$, $π^0Σ^0$, and $π^-Σ^0$ invariant mass spectra below and above the $\bar{K}N$ mass threshold in $K^-$-induced reactions on deuteron. We deduced the $S$-wave $\bar{K}N\rightarrowπΣ$ and $\bar{K}N\rightarrow\bar{K}N$ scattering amplitudes in the isospin 0 channel in the framework of a $\bar{K}N$ and $πΣ$ coupled channel. We find that a resonance pole corresponding to $Λ(1405)$ is located at 1417.7$^{+6.0}_{-7.4}$(fitting errors)$^{+1.1}_{-1.0}$(systematic errors) + $[-26.1^{+6.0}_{-7.9}$(fitting errors)$^{+1.7}_{-2.0}$(systematic errors)]$i$ MeV/$c^2$, closer to the $\bar{K}N$ mass threshold than the value determined by the Particle Data Group.

nucl-ex↗

arXiv2022

Interplay of itinerant electrons and Ising moments in a hybrid honeycomb quantum magnet TmNi$_3$Al$_9$

The interplay between itinerant electrons and local magnetic moments in quantum materials brings about rich and fascinating phenomena and stimulates various developments in the theoretical framework. In this work, thermodynamic, electric transport, and neutron diffraction measurements were performed on a newly synthesized honeycomb lattice magnet TmNi$_3$Al$_9$. Based on the experimental data, a magnetic field temperature phase diagram was constructed, exhibiting three essentially different magnetic regions. Below ${T_{\rm N}=2.97 \pm 0.02}\ \rm K$ Tm$^{3+}$ moments order antiferromagnetically in zero field. We found that the Tm$^{3+}$ ions form a pseudo-doublet ground state with the Ising-like moments lying normal to the two-dimensional honeycomb layers. Application of a magnetic field along the easy axis gradually suppresses the antiferromagnetic order in favor of an induced ferromagnetic state above the critical field ${B_c=0.92 \pm 0.05}\ \rm T$. In the vicinity of $B_c$, a strong enhancement of the quantum spin fluctuations was observed. The quantum Ising nature of the local moments and the coupling to itinerant electrons are discussed.

cond-mat.str-el↗

arXiv2022

Flat-Band Itinerant Antiferromagnetism in the Kagome Metal CoSn1-xInx

Destructive interference of electron hopping on the frustrated kagome lattice generates Dirac nodes, saddle points, and flat bands in the electronic structure. The latter provides the narrow bands and a peak in the density of states that can generate correlated electron behavior when the Fermi level lies within them. In the kagome metal CoSn, this alignment is not realized, and the compound is a Pauli paramagnet. Here we show that replacing part of the tin with indium (CoSn1-xInx) moves the Fermi energy into the flat band region, with support from band structure calculations, heat capacity measurements, and angle resolved photoemission spectroscopy. The associated instability results in the emergence of itinerant antiferromagnetism with a Neel temperature up to 30K. Long range magnetic order is confirmed by neutron diffraction measurements, which indicate an ordered magnetic moment of 0.1-0.2 Bohr magnetons per Co (for x = 0.4). Thus, CoSn1-xInx provides a rare example of an itinerant antiferromagnet with a small ordered moment. This work provides clear evidence that flat bands arising from frustrated lattices in bulk crystals represent a viable route to new physics, evidenced here by the emergence of magnetic order upon introducing a non-magnetic dopant into a non-magnetic kagome metal.

cond-mat.str-el↗

arXiv2021

Cross-over behavior of the localized to itinerant transition of 5f electrons in the antiferromagnetic Kondo lattice USb2

In Uranium-based heavy fermion system, the 5f electrons display an intermediate character between partial localization and partial itinerancy, which makes the Kondo problem more complicated. Here we use scanning tunneling microscopy/spectroscopy to investigate the (001) surface of the Kondo lattice antiferromagnet USb2. Temperature-dependent dI/dV spectra from 4.7 K up to 140 K reveal several peak structures around the Fermi level. Two pronounced peaks are originated from the hybridization between the conduction and 5f electrons. We did not observe the mysteriously abrupt change of the electronic state at 45 K, which is previously reported by another STM group and attributed to a novel first-order like transition. Instead, we only observe continuous evolution of the dI/dV spectra with temperature. Furthermore, in some scanning regions, we find significant Sb atoms missing from the top layer, which gives us the opportunity to investigate the electronic structure of the U-terminated surface. For the U-terminated surface, an additional sharp peak emerges, which is closely related to the magnetic order.

cond-mat.str-el↗

arXiv2021

Recent results and future prospects of kaonic nuclei at J-PARC

$\bar K$-nuclear bound systems, kaonic nuclei, have been widely discussed as products of the strongly attractive $\bar K N$ interaction in $I = 0$ channels. Recently, we demonstrated that kaonic nuclei can be produced via in-flight $(K^-,N)$ reactions using the low-momentum DC kaon beam at the J-PARC E15 experiment. We observed the simplest kaonic nuclei, $K^-pp$, having a much deeper binding energy than normal nuclei. For further studies, we have proposed a series of experimental programs for the systematic investigation of light kaonic nuclei, from $\bar K N$ ($Λ(1405)$) to $\bar K NNNN$. In the new experiment approved as J-PARC E80, we will measure the $\bar K NNN$ ($A=3$) system as a first step toward a comprehensive study.

nucl-ex↗

arXiv2021

Discovery of a Highly Collimated Flow from the High-Mass Protostar ISOSS J23053+5953 SMM2

We present Very Large Array C, X, and Q-band continuum observations, as well as 1.3 mm continuum and CO(2-1) observations with the Submillimeter Array toward the high-mass protostellar candidate ISOSS J23053+5953 SMM2. Compact cm continuum emission was detected near the center of the SMM2 core with a spectral index of 0.24 between 6 and 3.6 cm, and a radio luminosity of 1.3 mJy kpc$^2$. The 1.3 mm thermal dust emission indicates a mass of the SMM2 core of 45.8 Msun. The CO(2-1) observations reveal a large, massive molecular outflow centered on the SMM2 core. This fast outflow ($>$ 50 km/s from the cloud systemic velocity) is highly collimated, with a broader, lower-velocity component. The large values for outflow mass (45.2 Msun), and momentum rate (6 x 10$^{-3}$ Msun km/s/yr) derived from the CO emission are consistent with those of flows driven by high-mass YSOs. The dynamical timescale of the flow is between 1.5 - 7.2 x 10$^4$ yr. We also found from the C18O to thermal dust emission ratio that CO is depleted by a factor of about 20, possibly due to freeze out of CO molecules on dust grains. Our data are consistent with previous findings that ISOSS J23053+5953 SMM2 is an emerging high-mass protostar in an early phase of evolution, with an ionized jet, and a fast, highly collimated, and massive outflow.

astro-ph.GA↗

arXiv2021

Orientation effect on the ligh curve shape of periodic methanol maser sources

We report the results of our pilot millimeter observations of periodic maser sources. Using SMA, we carried out 1.3\,mm observations of G22.357$+$0.066 and G25.411$+$0.105, while ALMA 1.3 mm archival data was used in the case of G9.62+0.19E. Two continuum cores (MM1 and MM2) were detected in G22.357$+$0.066, while 3 cores (MM1 -- MM3) detected in G25.411$+$0.105. Assuming dust-to-gas ratio of 100, we derived the masses of the detected cores. Using the $^{12}$CO (2-1) and $^{13}$CO (2-1) line emission, we observed gas kinematics tracing the presence of bipolar outflows in all three star-forming regions. In the cases of G22.357$+$0.066 and G9.62+0.19E, both with similar periodic maser light curve profiles, the outflowing gas is seen in the north-west south-east direction. This suggest edge-on view of the 2 sources. G25.411$+$0.105, with a contrasting light curve profile, show a spatially co-located blue and red outflow lobes, suggesting a face-on view. Our results suggest that orientation effects may play a role in determining the characteristics of the light curves of periodic methanol masers.

astro-ph.GA↗

arXiv2021

Magnetic Fields in Massive Star-Forming Regions (MagMaR) I. Linear Polarized Imaging of the UCHII Region G5.89-0.39

We report 1.2 mm polarized continuum emission observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star formation region G5.89-0.39. The observations show a prominent 0.2 pc north-south filamentary structure. The UCHII in G5.89-0.39 breaks the filament in two pieces. Its millimeter emission shows a dusty belt with a mass of 55-115 M$_{\odot}$ and 4,500 au in radius, surrounding an inner part comprising mostly ionized gas with a dust emission only accounting about 30% of the total millimeter emission. We also found a lattice of convex arches which may be produced by dragged dust and gas from the explosive dispersal event involving the O5 Feldt's star. The north-south filament has a mass between 300-600 M$_{\odot}$ and harbours a cluster of about 20 millimeter envelopes with a median size and mass of 1700 au and 1.5 M$_{\odot}$, respectively, some of which are already forming protostars. We interpret the polarized emission in the filament as mainly coming from magnetically aligned dust grains. The polarization fraction is ~4.4% in the filaments and 2.1% at the shell. The magnetic fields are along the North Filament and perpendicular to the South Filament. In the Central Shell, the magnetic fields are roughly radial in a ring surrounding the dusty belt between 4,500 and 7,500 au, similar to the pattern recently found in the surroundings of Orion BN/KL. This may be an independent observational signpost of explosive dispersal outflows and should be further investigated in other regions.

astro-ph.GA↗