arXiv subjects

W. Yu

Publications and source records attributed to W. Yu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Measurement of Absolute Single and Double Electron Capture Cross Sections for O6+ Ions Collision with CO2, CH4, H2 and N2

The absolute electron capture cross sections for single and double charge exchanges between the highly charged ions O6+ and CO2, CH4, H2, N2, the dominant collision processes in the solar wind, have been measured in the energy from 7 keV*q (2.63 keV/u) to 52 keV*q (19.5 keV/u). These measurements were carried out in the new experimental instrument set up at Fudan University, and the error of cross sections for single and double charge exchanges at the 1σ confidence level are about 11% and 16%, respectively. Limited agreement is achieved with single electron capture results calculated by the classical over-barrier model. These cross sections data are useful for simulation ion-neutral processes in astrophysical environments and to improve the present theoretical model of fundamental atomic processes.

physics.atom-ph↗

arXiv2020

No pulsed radio emission during a bursting phase of a Galactic magnetar

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients of unknown origin observed at extragalactic distances. It has been long speculated that magnetars are the engine powering repeating bursts from FRB sources, but no convincing evidence has been collected so far\cite{sun19}. Recently, the Galactic magnetar SGR J1935+2154 entered an active phase by emitting intense soft Gamma-ray bursts. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft Gamma-ray / hard X-ray flare. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard X-rays) data. During the third session, 29 soft Gamma-ray repeater (SGR) bursts were detected in Gamma-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB -- SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.

astro-ph.HE↗

arXiv2020

Zero Bias Conductance Peak in Dirac Semimetal-Superconductor Devices

Majorana zero modes (MZMs), fundamental building blocks for realizing topological quantum computers, can appear at the interface between a superconductor and a topological material. One of the experimental signatures that has been widely pursued to confirm the existence of MZMs is the observation of a large, quantized zero-bias conductance peak (ZBCP) in the differential conductance measurements. In this Letter, we report observation of such a large ZBCP in junction structures of normal metal (titanium/gold Ti/Au) + Dirac semimetal (cadmium arsenide Cd3As2) + conventional superconductor (aluminum Al), with a value close to four times that of the normal state conductance. Our detailed analyses suggest that this large ZBCP is most likely not caused by MZMs. We attribute the ZBCP, instead, to the existence of a supercurrent between two far-separated superconducting Al electrodes, which shows up as a zero-bias peak because of the circuitry and thermal fluctuations of the supercurrent phase, a mechanism conceived by Ivanchenko and Zil'berman more than 50 years ago [JETP 28, 1272 (1969)]. Our results thus call for extreme caution when assigning the origin of a large ZBCP to MZMs in a multiterminal semiconductor or topological insulator/semimetal setup. We thus provide criteria for identifying when the ZBCP is definitely not caused by an MZM. Furthermore, we present several remarkable experimental results of a supercurrent effect occurring over unusually long distances and clean perfect Andreev reflection features.

cond-mat.mes-hall↗

arXiv2018

The Magnetic Field Geometry of Small Solar Wind Flux Ropes Inferred from their Twist Distribution

This work extends recent efforts on the force-free modeling of large flux rope-type structures (magnetic clouds, MCs) to much smaller spatial scales. We first select small flux ropes (SFRs) by eye whose duration is unambiguous and which were observed by the Solar Terrestrial Relations Observatory (STEREO) or Wind spacecraft during solar maximum years. We inquire into which analytical technique is physically most appropriate. We consider three models: (i) linear force-free field ($\bigtriangledown\times$ B = $α(r) $ B) with a specific, prescribed constant $α$ (Lundquist solution), and (ii) with $α$ as a free constant parameter (Lundquist-alpha solution), (iii) uniform twist field (Gold-Hoyle solution). We retain only those cases where the impact parameter is less than one-half the FR radius, $R$, so the results should be robust (29 cases). The SFR radii lie in the range [$\sim$ 0.003, 0.059] AU. Comparing results, we find that the Lundquist-alpha and uniform twist solutions yielded comparable and small normalized $χ^2$ values in most cases. We then use Grad-Shafranov (GS) reconstruction to analyze these events further. We then considered the twist per unit length, $τ$, both its profile through the FR and its absolute value. We find $τ$ to lie in the range [5.6, 34] turns/AU. The GH model-derived $τ$ values are comparable to those obtained from GS reconstruction. We find that twist unit length ($L$) is inversely proportional to $R$, as $τ\sim 0.17/R$. We combine MC and SFR results on $τ(R)$ and give a relation which is approximately valid for both sets. The axial and azimuthal fluxes, $F_z$ and $F_ϕ$, vary as $\approx 2.1 B_0 R^2 \times10^{21}$ Mx and $F_ϕ/L \approx 0.36 B_0 R \times10^{21}$Mx/AU. The relative helicity per unit length, $H/L \approx 0.75 B_0^2 R^3$$\times 10^{42}$ Mx$^2$/AU.

physics.space-ph↗

arXiv2018

Mermin-Wagner physics, (H,T) phase diagram, and candidate quantum spin-liquid phase in the spin-1/2 triangular-lattice antiferromagnet Ba8CoNb6O24

Ba$_8$CoNb$_6$O$_{24}$ presents a system whose Co$^{2+}$ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the $S = 1/2$ TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an nexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the $(H,T)$ phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low- and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba$_8$CoNb$_6$O$_{24}$ therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the $S = 1/2$ Heisenberg TLAFM.

cond-mat.str-el↗

arXiv2017

Experimental Approach to the Thermodynamics of the Pure Two-Dimensional Spin-1/2 Triangular Lattice Antiferromagnet in Ba8CoNb6O24

Frustrated quantum magnets pose well-defined questions concerning quantum fluctuation effects and the nature of the many-body wavefunction, which challenge theory, numerics, experiment and materials synthesis. The S = 1/2 triangular-lattice antiferromagnet (TLAF) presents a case where classical order is strongly suppressed by quantum fluctuations, leading to extensive renormalization of physical properties at all energy scales. However, purely two-dimensional (2D) models are difficult to realise in the 3D world and their physics is controlled by the Mermin-Wagner theorem, which describes the dominant effects of additional thermal fluctuations. Here we report the magnetic properties Ba8CoNb6O24, whose Co2+ions have an effective spin 1/2 and construct a regular TLAF with very large interlayer spacing. We find no magnetic ordering down to 0.028 K, strong low-energy spin fluctuations in qualitative agreement with theoretical analysis and a diverging correlation length, all indicating a Mermin-Wagner trend towards zero-temperature ordering in this ideal 2D system.

cond-mat.str-el↗

arXiv2017

Vizic: A Jupyter-based Interactive Visualization Tool for Astronomical Catalogs

The ever-growing datasets in observational astronomy have challenged scientists in many aspects, including an efficient and interactive data exploration and visualization. Many tools have been developed to confront this challenge. However, they usually focus on displaying the actual images or focus on visualizing patterns within catalogs in a predefined way. In this paper we introduce Vizic, a Python visualization library that builds the connection between images and catalogs through an interactive map of the sky region. Vizic visualizes catalog data over a custom background canvas using the shape, size and orientation of each object in the catalog. The displayed objects in the map are highly interactive and customizable comparing to those in the images. These objects can be filtered by or colored by their properties, such as redshift and magnitude. They also can be sub-selected using a lasso-like tool for further analysis using standard Python functions from inside a Jupyter notebook. Furthermore, Vizic allows custom overlays to be appended dynamically on top of the sky map. We have initially implemented several overlays, namely, Voronoi, Delaunay, Minimum Spanning Tree and HEALPix grid layers, which are helpful for visualizing large-scale structure. All these overlays can be generated, added or removed interactively with one line of code. The catalog data is stored in a non-relational database, and the interfaces were developed in JavaScript and Python to work within Jupyter Notebook, which allows to create custom widgets, user generated scripts to analyze and plot the data selected/displayed in the interactive map. This unique design makes Vizic a very powerful and flexible interactive analysis tool. Vizic can be adopted in variety of exercises, for example, data inspection, clustering analysis, galaxy alignment studies, outlier identification or simply large-scale visualizations.

astro-ph.IM↗

arXiv2017

Particle-in-cell simulations of laser-plasma interactions at solid densities and relativistic intensities: the role of atomic processes

Direct studies of intense laser-solid interactions is still of great challenges, because of the many coupled physical mechanisms, such as direct laser heating, ionization dynamics, collision among charged particles, and electrostatic or electromagnetic instabilities, to name just a few. Here, we present a full particle-in-cell simulation (PIC) framework, which enables us to calculate laser-solid interactions in a "first principle" way, covering almost "all" the coupled physical mechanisms. Apart from the mechanisms above, the numerical self-heating of PIC simulations, which usually appears in solid-density plasmas, is also well controlled by the proposed "layered-density" method. This method can be easily implemented into the state-of-the-art PIC codes. Especially, the electron heating/acceleration at relativistically intense laser-solid interactions in the presence of large scale pre-formed plasmas is re-investigated by this PIC code. Results indicate that collisional damping (even though it is very week) can significantly influence the electron heating/acceleration in front of the target. Furthermore the Bremsstrahlung radiation will be enhanced by $2\sim3$ times when the solid is dramatically heated and ionized. For the considered case, where laser is of intensity $10^{20}\ \text{W}/\text{cm}^2$ and pre-plasma in front of the solid target is of scale-length $10\ μ\text{m}$, collision damping coupled with ionization dynamics and Bremsstrahlung radiations is shown to lower the "cut-off" electron energy by $25\%$. In addition, the resistive electromagnetic fields due to Ohmic-heating also play a non-ignorable role and must be included in real laser-solid interactions.

physics.plasm-ph↗

arXiv2017

Evidence for an excitonic insulator phase in a zero-gap InAs/GaSb bilayer

Many-body interactions can produce novel ground states in a condensed-matter system. For example, interacting electrons and holes can spontaneously form excitons, a neutral bound state, provided that the exciton binding energy exceeds the energy separation between the single particle states. Here we report on electrical transport measurements on spatially separated two-dimensional electron and hole gases with nominally degenerate energy subbands, realized in an InAs(10 nm)/GaSb(5 nm) coupled quantum well. We observe a narrow and intense maximum (~500 kΩ) in the four-terminal resistivity in the charge neutrality region, separating the electron-like and hole-like regimes, with a strong activated temperature-dependence above T = 7 K and perfect stability against quantizing magnetic fields. By quantitatively comparing our data with early theoretical predictions, we show that such unexpectedly large resistance in our nominally zero-gap semi-metal system is probably due to the formation of an excitonic insulator state.

cond-mat.mtrl-sci↗

arXiv2017

Containing intense laser light in circular cavity with magnetic trap door

It is shown by particle-in-cell simulation that intense circularly polarized (CP) laser light can be contained in the cavity of a solid-density circular Al-plasma shell for hundreds of light-wave periods before it is dissipated by laser-plasma interaction. A right-hand CP laser pulse can propagate almost without reflection into the cavity through a highly magnetized overdense H-plasma slab filling the entrance hole. The entrapped laser light is then multiply reflected at the inner surfaces of the slab and shell plasmas, gradually losing energy to the latter. Compared to that of the incident laser, the frequency is only slightly broadened and the wave vector slightly modified by appearance of weak nearly isotropic and homogeneous higher harmonics.

physics.plasm-ph↗

arXiv2016

Monte-Carlo approach to calculate the ionization of warm dense matter within particle-in-cell simulations

A physical model based on a Monte-Carlo approach is proposed to calculate the ionization dynam- ics of warm dense matters (WDM) within particle-in-cell simulations, and where the impact (col- lision) ionization (CI), electron-ion recombination (RE) and ionization potential depression (IPD) by surrounding plasmas are taken into consideration self-consistently. When compared with other models, which are applied in the literature for plasmas near thermal equilibrium, the temporal re- laxation of ionization dynamics can also be simulated by the proposed model. Besides, this model is general and can be applied for both single elements and alloys with quite different composi- tions. The proposed model is implemented into a particle-in-cell (PIC) code, with (final) ionization equilibriums sustained by competitions between CI and its inverse process (i.e., RE). Comparisons between the full model and model without IPD or RE are performed. Our results indicate that for bulk aluminium in the WDM regime, i) the averaged ionization degree increases by including IPD; while ii) the averaged ionization degree is significantly over estimated when the RE is neglected. A direct comparison from the PIC code is made with the existing models for the dependence of averaged ionization degree on thermal equilibrium temperatures, and shows good agreements with that generated from Saha-Boltzmann model or/and FLYCHK code.

physics.plasm-ph↗

arXiv2016

Millihertz Quasi-periodic Oscillations in 4U 1636-536: Putting Possible Constraints on the Neutron Star Size

Based on previous studies of quasi-periodic oscillations in neutron star LMXBs, mHz quasi-periodic oscillations (QPO) are believed to be related to `marginally stable' burning on the neutron star (NS) surface. Our study of phase resolved energy spectra of these oscillations in 4U 1636-53 shows that the oscillations are not caused by variations in the blackbody temperature of the neutron star, but reveals a correlation between the change of the count rate during the mHz QPO pulse and the spatial extend of a region emitting blackbody emission. The maximum size of the emission area $R^2_{\mathrm{BB}}=216.7^{+93.2}_{-86.4}$km$^2$, provides the direct evidence that the oscillations originate from a variable surface area constrained on the NS and are therefore not related to instabilities in the accretion disk. The obtained lower limit on the size of the neutron star (11.0 km) rules out equations of state that prefer small NS radii. Observations of mHz QPOs in NS LMXBs with NICER and eXTP will reduce the statistical uncertainty in the lower limit on the NS radius, which together with better estimates of the hardening factor and distance, will allow improving discrimination between different equations of state and compact star models. Furthermore, future missions will allow us to measure the peak blackbody emission area for a single mHz QPO pulse, which will push the lower limit to larger radii.

astro-ph.HE↗

arXiv2016

Monte-Carlo approach to calculate the proton stopping in warm dense matter within particle-in-cell simulations

A Monte-Carlo approach to proton stopping in warm dense matter is implemented into an existing particle-in-cell code. The model is based on multiple binary-collisions among electron-electron, electron-ion and ion-ion, taking into account contributions from both free and bound electrons, and allows to calculate particle stopping in much more natural manner. At low temperature limit, when ``all'' electron are bounded at the nucleus, the stopping power converges to the predictions of Bethe-Bloch theory, which shows good consistency with data provided by the NIST. With the rising of temperatures, more and more bound electron are ionized, thus giving rise to an increased stopping power to cold matter, which is consistent with the report of a recently experimental measurement [Phys. Rev. Lett. 114, 215002 (2015)]. When temperature is further increased, with ionizations reaching the maximum, lowered stopping power is observed, which is due to the suppression of collision frequency between projected proton beam and hot plasmas in the target.

physics.plasm-ph↗

arXiv2016

Sources of The Slow Solar wind During the Solar Cycle 23/24 Minimum

We investigate the characteristics and the sources of the slow (< 450 km/s) solar wind during the four years (2006-2009) of low solar activity between Solar Cycles 23 and 24. We use a comprehensive set of in-situ observations in the near-Earth solar wind (Wind and ACE) and remove the periods when large-scale interplanetary coronal mass ejections were present. The investigated period features significant variations in the global coronal structure, including the frequent presence of low-latitude active regions in 2006-2007, long-lived low- and mid-latitude coronal holes in 2006 - mid-2008 and mostly the quiet Sun in 2009. We examine both Carrington Rotation averages of selected solar plasma, charge state and compositional parameters and distributions of these parameters related to Quiet Sun, Active Region Sun and the Coronal Hole Sun. While some of the investigated parameters (e.g., speed, the C^{+6}/C^{+4} and He/H ratio) show clear variations over our study period and with solar wind source type, some (Fe/O) exhibit very little changes. Our results highlight the difficulty in distinguishing between the slow solar wind sources based on the inspection of the solar wind conditions.

astro-ph.SR↗

arXiv2016

Using XMM-Newton to study the energy dependent variability of H 1743-322 during its 2014 outburst

Black hole transients during bright outbursts show distinct changes of their spectral and variability properties as they evolve during an outburst, that are interpreted as evidence for changes in the accretion flow and X-ray emitting regions. We obtained an anticipated XMM-Newton ToO observation of H 1743-322 during its outburst in September 2014. Based on data of eight outbursts observed in the last 10 years we expected to catch the start of the hard-to-soft state transition. The fact that neither the general shape of the observed power density spectrum nor the characteristic frequency show an energy dependence implies that the source still stays in the low-hard state at the time of our observation near outburst peak. The spectral properties agree with the source being in the low-hard state and a Swift/XRT monitoring of the outburst reveals that H 1743-322 stays in the low-hard state during the entire outburst (a. k. a. 'failed outburst'). We derive the averaged QPO waveform and obtain phase-resolved spectra. Comparing the phase-resolved spectra to the phase averaged energy spectrum reveals spectral pivoting. We compare variability on long and short time scales using covariance spectra and find that the covariance ratio does not show an increase towards lower energies as has been found in other black hole X-ray binaries. There are two possible explanations: either the absence of additional disc variability on longer time scales is related to the rather high inclination of H 1743-322 compared to other black hole X-ray binaries or it is the reason why we observe H 1743-322 during a failed outburst. More data on failed outbursts and on high-inclination sources will be needed to investigate these two possibilities further.

astro-ph.HE↗

arXiv2016

Quantum Oscillations at Integer and Fractional Landau Level Indices in ZrTe5

A three-dimensional (3D) Dirac semimetal (DS) is an analogue of graphene, but with linear energy dispersion in all (three) momentum directions.3D DSs have been a fertile playground in discovering novel quantum particles, for example Weyl fermions, in solid state systems.Many 3D DSs (e.g., ZrTe5) were theoretically predicted. We report here the results from the studies of aberration-corrected scanning transmission electron microscopy and low temperature magneto-transport measurements in exfoliated ZrTe5 thin flakes.Several unique results were observed. First, an anomalous-Hall-effect-like behavior was observed around zero magnetic field (B).Second, a non-trivial Berry's phase of π was obtained from the Landau level fan diagram of the Shubnikov-de Haas oscillations in the longitudinal resistivity. Third, the longitudinal resistivity shows linear B field dependence in the quantum limit. Most surprisingly, quantum oscillations were observed at fractional Landau level indices N = 2/3 and 2/5, demonstrating strong electron-electron interactions effects in ZrTe5.

cond-mat.mtrl-sci↗

arXiv2016

The controllable super-high energetic electrons by external magnetic fields at relativistic laser-solid interactions in the presence of large scale pre-plasmas

The two stage electron acceleration model [arXiv: 1512.02411 and arXiv: 1512.07546] is extended to the study of laser magnetized-plasmas interactions at relativistic intensities and in the presence of large-scale preformed plasmas. It is shown that the cut-off electron kinetic energy is controllable by the external magnetic field strength and directions. Further studies indicate that for a right-hand circularly polarized laser (RH-CP) of intensity $10^{20}\ \text{W}/\text{cm}^2$ and pre-plasma scale length $10\ μ\text{m}$, the cut-off electron kinetic energy can be as high as $500\ \text{MeV}$, when a homogeneous external magnetic field of exceeding $10000\ \text{T}$ (or $B=ω_{c}/ω_0>1$) is loaded along the laser propagation direction, which is a significant increase compared with that $120\ \text{MeV}$ without external magnetic field. A laser front sharpening mechanism is identified at relativistic laser magnetized-plasmas interactions with $B=ω_{c}/ω_0>1$, which is responsible for these super-high energetic electrons.

physics.plasm-ph↗

arXiv2016

The generation of unexpected super-high energetic electrons at relativistic circularly polarized laser-solid interactions in the presence of large scale pre-plasmas

As an extension of the previous work [arXiv: 1512.02411], we have investigated the role of circularly polarized (CP) laser pulses while keeping other conditions the same. It is found that in the presence of large scale pre-formed plasmas, super-high energetic electrons can be generated at relativistic CP laser-solid interactions. For laser of intensity $10^{20}\ \text{W}/\text{cm}^2$ and pre-plasma scale-length $10\ μ\text{m}$, the cut-off energy of electron by CP laser is $120\ \text{MeV}$ compared with $100\ \text{MeV}$ in the case of linearly polarized (LP) laser. The unexpected super-high energetic electron acceleration can also be explained by the two-stage acceleration model, {by taking into account the envelop modulation effects of the reflected CP laser pulse.} The underlying physics of this envelop modulation is figured out, and a modified first-stage electron acceleration scaling law in the presence of the modulated-CP laser is also obtained.

physics.plasm-ph↗