arXiv subjects

J. F. Wang

Publications and source records attributed to J. F. Wang.

At least 19 recordsLinked to original sources

arXiv2026

EP-FXT Observations of Abell 1795: X-ray properties and structure out to $R_{200}$

We present deep X-ray observations of the nearby X-ray-luminous galaxy cluster Abell 1795 obtained with the Einstein Probe Follow-up X-ray Telescope EP-FXT, with a total exposure time of $\sim 480$ ks. Exploiting the large field of view and low particle background of EP-FXT, we directly measured the radial temperature profile of A1795 out to $R_{200}$ with full azimuthal coverage, which was shown to increase in line with the radius within 6' and then gradually decline toward larger radii. The surface-brightness residual map and 2D thermodynamic maps reveal a clockwise spiral structure extending from the cluster core towards the south-east, which traces low-temperature and low-entropy gas, which is also consistent with sloshing-induced cold fronts. In the north-west, the surface-brightness-enhanced region exhibits an arc-like high-temperature feature and both the temperature-derived and density-derived Mach numbers support the assumption that it is a weak shock. These substructures can be explained by a binary merger scenario: the perturbing subcluster induces sloshing during its first passage past the primary core and its subsequent return passage through the ICM might drive the shock towards the north-west. Our results indicate that relaxed galaxy clusters such as A1795 might still retain signatures of dynamical activity.

astro-ph.HE↗

arXiv2025

Triplon Bose-Einstein condensation and proximate magnetism in dimerized antiferromagnets

Dimerized quantum magnets provide a useful arena for novel quantum states and phases transitions with the singlet-triplet type of triplon excitations. Here we study the triplon physics and the Bose-Einstein condensation in two isostructural dimerized antiferromagnets $A$Cu(SeO$_3$)$_2$ ($A$ = Hg, Cd). With the systematic measurements, we demonstrate a dimer singlet ground state in HgCu(SeO$_3$)$_2$ with a triplon gap $\sim$ 7.9 K and a triplon Bose-Einstein condensation with an antiferromagnetic order in CdCu(SeO$_3$)$_2$ below 4.4 K. We further adopt the bond-operator technique and show that the elemental replacement preserves the Hamiltonian and allows the study in a unified theoretical framework with tunable interdimer and intradimer interactions on the opposite sides of the quantum critical point. With the peculiar Cu$_2$O$_8$ dimer configuration and effective ferromagnetic interdimer interaction, $A$Cu(SeO$_3$)$_2$ is distinguished from other $S$ = 1/2 dimerized antiferromagnets. Our results represent a global understanding of the magnetic ground states as well as the magnetic transitions in the dimerized magnets of this unusual crystal structure.

cond-mat.str-el↗

arXiv2024

Suppression and excitation condition of collision on instabilities of electrostatic plasmas

Two-stream (TS) and Bump-On-Tail (BOT) electron distributions can induce instabilities in collisionless plasmas, which is closely related to phenomena in space and fusion plasmas. Collisions can lead to unexpected plasma behavior, especially in dense and/or low temperature plasmas. In this work, the Vlasov-Poisson system with Krook collisions are used to study the effect of collisions. Normally, the collision can dissipate the system energy which causes the suppression of the instabilities. Against the traditional suppression effect of collision on the instability, it is found in our simulation that the collision can also excite the instability even in the forbidden beam velocity range predicted by the cold-beam theory. With collision, the beam velocity range can be divided into suppression area [vth/2, vc + vth], transition area [vc - vth, vc + vth], excitation area [vc + vth, 2vc] and forbidden area [2vc, +infinity] for TS instability. where vc is the critical velocity from cold-beam theory and vth is thermal velocity or the beam width in our simulation. The collision dissipation effect and the excitation of beam instability can compete with each other, which evoked the excitation of collision on TS instability. The collision can change the suppression and excitation condition from beam theory. However, for BOT instability, there is only suppression effect of collision on the instability. These results can expand the view of collision effect on instability of electrostatic plasmas.

physics.plasm-ph↗

arXiv2024

High-field magnetoelectric coupling and successive magnetic transitions in Mn-doped polar antiferromagnet Ni3TeO6

Among the 3d transition metal ions doped polar Ni3TeO6, Mn-doped Ni3TeO6 has stimulated great interest due to its high magnetic ordering temperature and complex magnetic phases, but the mechanism of magnetoelectric (ME) coupling is far from understood. Herein we report our systematic investigation of the chemical control of magnetism, metamagnetic transition, and ME properties of Ni3-xMnxTeO6 single crystals in high magnetic field (H) up to 52 T. We present a previously unreported weak ferromagnetic behavior appeared in the ab plane below 9.5 K in addition to the incommensurate helical and commensurate collinear antiferromagnetic states. In the low-field region, a spin-flop type metamagnetic transition without any hysteresis occurs at Hc1 for H // c, while another metamagnetic transition accompanied with a change in electric polarization is observed at Hc2 in the high-field region both for H // c and H // ab above 30 K, which can be attributed to the sudden rotation of magnetic moments at Ni2 sites. The ME measurements reveal that a first-order ME effect is observed in the low-T and low-H regions, while a second-order ME coupling term appears above 30 K in the magnetic field range of Hc1 < H < Hc2 for H // c and H < Hc2 for H // ab, both becoming significant with increasing temperature. Eventually, they are dominated by the second-order ME effect near the antiferromagnetic transition temperature. The present work demonstrates that Ni3-xMnxTeO6 is an exotic magnetoelectric material compared with Ni3TeO6 and its derivatives, thereby providing insights to better understand the magnetism and ME coupling in Ni3TeO6 and its derivatives.

cond-mat.mtrl-sci↗

arXiv2023

The effect of solar wind on the charged particles' diffusion coefficients

The transport of energetic charged particles through magnetized plasmas is ubiquitous in interplanetary space and astrophysics, and the important physical quantities are the along-field and cross-field spatial diffusion coefficients of energetic charged particles. In this paper, the influence of solar wind on particle transport is investigated. Using the focusing equation, we obtain along- and cross-field diffusion coefficient accounting for the solar wind effect. For different conditions, the relative importance of solar wind effect to diffusion are investigated. It is shown that when energetic charged particles are close to the sun, for along-field diffusion the solar wind effect needs to be taken into account. These results are important for studying energetic charged particle transport processes in the vicinity of the sun.

physics.space-ph↗

arXiv2023

Electrochemically-controlled metasurfaces with high-contrast switching at visible frequencies

Recently in nanophotonics, a rigorous evolution from passive to active metasurfaces has been witnessed. This advancement not only brings forward interesting physical phenomena but also elicits opportunities for practical applications. However, active metasurfaces operating at visible frequencies often exhibit low performance due to design and fabrication restrictions at the nanoscale. In this work, we demonstrate electrochemically controlled metasurfaces with high intensity contrast, fast switching rate, and excellent reversibility at visible frequencies. We use a conducting polymer, polyaniline (PANI), that can be locally conjugated on preselected gold nanorods to actively control the phase profiles of the metasurfaces. The optical responses of the metasurfaces can be in situ monitored and optimized by controlling the PANI growth of subwavelength dimension during the electrochemical process. We showcase electrochemically controlled anomalous transmission and holography with good switching performance. Such electrochemically powered optical metasurfaces lay a solid basis to develop metasurface devices for real-world optical applications.

physics.optics↗

arXiv2023

Relationship of transport coefficients with statistical quantities of charged particles

In the previous studies, from the Fokker-Planck equation the general spatial transport equation, which contains an infinite number of spatial derivative terms $T_n=κ_{nz}\partial^n{F}/ \partial{z^n}$ with $n=1, 2, 3, \cdots$, was derived. Due to the complexity of the general equation, some simplified equations with finite spatial derivative terms have been used in astrophysical researches, e.g., the diffusion equation, the hyperdiffusion one, subdiffusion transport one, etc. In this paper, the simplified equations with the highest order spatial derivative terms up to the first-, second-, third-, fourth-, and fifth-order are listed, and their transport coefficient formulas are derived, respectively. We find that most of the transport coefficients are determined by the corresponding statistical quantities. In addition, we find that the well-known statistical quantities, skewness $\mathcal{S}$ and kurtosis $\mathcal{K}$, are determined by some transport coefficients. The results can help one to use different transport coefficients determined by the statistical quantities, including many that are relatively new found in this paper, to study charged particle parallel transport processes.

astro-ph.SR↗

arXiv2021

Study of momentum diffusion with the effect of adiabatic focusing

Momentum diffusion of the energetic charged particles is an important mechanism of the transport process in astrophysics, physics of the fusion devices, and laboratory plasmas. In addition to the uniform field momentum diffusion, we obtain the modifying term due to the focusing effect of the large-scale magnetic field. After evaluating the modifying term, we find that it is determined by the sign of the focusing characteristic length and the Fokker-Planck coefficients $D_{μμ}$, $D_{μp}$, $D_{pμ}$, and $D_{pp}$. It is shown that we get a new second order acceleration mechanism in this work.

physics.plasm-ph↗

arXiv2021

Antiferromagnetic multi-level memristor using linear magnetoelectricity

The explosive growth of artificial intelligence and data-intensive computing has brought crucial challenge to modern information science and technology, i.e. conceptually new devices with superior properties are urgently desired. Memristor is recognized as a very promising circuit element to tackle the barriers, because of its fascinating advantages in imitating neural network of human brain, and thus realizing in-memory computing. However, there exist two core and fundamental issues: energy efficiency and accuracy, owing to the electric current operation of traditional memristors. In the present work, we demonstrate a new type of memristor, i.e. charge q and magnetic flux ϕ space memristor, enabled by linear magnetoelectricity of Co4Nb2O9. The memory states show distinctly linear magnetoelectric coefficients with a large ratio of about 10, ensuing exceptional accuracy of related devices. The present q-ϕ type memristor can be manipulated by magnetic and electric fields without involving electric current, paving the way to develop ultralow-energy-consuming devices. In the meanwhile, it is worth to mention that Co4Nb2O9 hosts an intrinsic compensated antiferromagnetic structure, which suggests interesting possibility of further integrating the unique merits of antiferromagnetic spintronics such as ultrahigh density and ultrafast switching. Linear magnetoelectricity is proposed to essential to the q-ϕ type memristor, which would be accessible in a broad class of multiferroics and other magnetoelectric materials such as topological insulators. Our findings could therefore advance memristors towards new levels of functionality.

cond-mat.mtrl-sci↗

arXiv2020

The invariance of the diffusion coefficient with the iterative operations of charged particles' transport equation

The Spatial Parallel Diffusion Coefficient (SPDC) is one of the important quantities describing energetic charged particle transport. There are three different definitions for the SPDC, i.e., the Displacement Variance definition $κ_{zz}^{DV}=\lim_{t\rightarrow t_{\infty}}dσ^2/(2dt)$, the Fick's Law definition $κ_{zz}^{FL}=J/X$ with $X=\partial{F}/\partial{z}$, and the TGK formula definition $κ_{zz}^{TGK}=\int_0^{\infty}dt \langle v_z(t)v_z(0) \rangle$. For constant mean magnetic field, the three different definitions of the SPDC give the same result. However, for focusing field it is demonstrated that the results of the different definitions are not the same. In this paper, from the Fokker-Planck equation we find that different methods, e.g., the general Fourier expansion and perturbation theory, can give the different Equations of the Isotropic Distribution Function (EIDFs). But it is shown that one EIDF can be transformed into another by some Derivative Iterative Operations (DIOs). If one definition of the SPDC is invariant for the DIOs, it is clear that the definition is also an invariance for different EIDFs, therewith it is an invariant quantity for the different Derivation Methods of EIDF (DMEs). For the focusing field we suggest that the TGK definition $κ_{zz}^{TGK}$ is only the approximate formula, and the Fick's Law definition $κ_{zz}^{FL}$ is not invariant to some DIOs. However, at least for the special condition, in this paper we show that the definition $κ_{zz}^{DV}$ is the invariant quantity to the kinds of the DIOs. Therefore, for spatially varying field the displacement variance definition $κ_{zz}^{DV}$, rather than the Fick's law definition $κ_{zz}^{FL}$ and TGK formula definition $κ_{zz}^{TGK}$, is the most appropriate definition of the SPDCs.

math-ph↗

arXiv2019

Diffusion coefficient with displacement variance of energetic particles with adiabatic focusing

The equation $κ_{zz}=dσ^2/(2dt)$ (hereafter DCDV) is a well-known formula of energetic particles describing the relation of parallel diffusion coefficient $κ_{zz}$ with the parallel displacement variance $σ^2$. In this study, we find that DCDV is only applicable to two kinds of transport equations of isotropic distribution function, one is without cross terms, the other is without convection term. Here, by employing the more general transport equation, i.e., the variable coefficient differential equation derived from the Fokker-Planck equation, a new equation of $κ_{zz}$ as a function of $σ^2$ is obtained. We find that DCDV is the special case of the new equation. In addition, another equation of $κ_{zz}$ as a function of $σ^2$ corresponding to the telegraph equation is also investigated preliminarily.

astro-ph.HE↗

arXiv2019

Magnetism study on a frustration-free spatially anisotropic $S$ = 1 square lattice antiferromagnet Ni[SC(NH$_2$)$_2$]$_6$Br$_2$

Magnetism of the $S$ = 1 Heisenberg antiferromagnets on the spatially anisotropic square lattice has been scarcely explored. Here we report a study of the magnetism, specific heat, and thermal conductivity on Ni[SC(NH$_2$)$_2$]$_6$Br$_2$ (DHN) single crystals. Ni$^{2+}$ ions feature an $S$ = 1 rectangular lattice in the $bc$ plane, which can be viewed as an unfrustrated spatially anisotropic square lattice. A long-range antiferromagnetic order is developed at $T \rm_N =$ 2.23 K. Below $T\rm_N$, an upturn is observed in the $b$-axis magnetic susceptibility and the resultant minimum might be an indication for the $XY$ anisotropy in the ordered state. A gapped spin-wave dispersion is confirmed from the temperature dependence of the magnetic specific heat. Anisotropic temperature-field phase diagrams are mapped out and possible magnetic structures are proposed.

cond-mat.mtrl-sci↗

arXiv2018

Parallel and perpendicular diffusion coefficients of energetic charged particles with adiabatic focusing

It is very important to understand stochastic diffusion of energetic charged particles in non-uniform background magnetic field in plasmas of astrophysics and fusion devices. Using different methods considering along-field adiabatic focusing effect, various authors derived parallel diffusion coefficient $κ_\parallel$ and its correction $T$ to $κ_{\parallel 0}$, where $κ_{\parallel 0}$ is the parallel diffusion coefficient without adiabatic focusing effect. In this paper, using the improved perturbation method developed by He \& Schlickeiser and iteration process, we obtain a new correction $T'$ to $κ_{\parallel 0}$. Furthermore, by employing the isotropic pitch-angle scattering model $D_{μμ}=D(1-μ^2)$, we find that $T'$ has the different sign as that of $T$. In this paper the spatial perpendicular diffusion coefficient $κ_\bot$ with the adiabatic focusing effect is also obtained.

physics.space-ph↗

arXiv2017

Magnetization, specific heat, and thermal conductivity of hexagonal ErMnO$_3$ single crystals

We report a study of magnetism and magnetic transitions of hexagonal ErMnO$_3$ single crystals by magnetization, specific heat and heat transport measurements. Magnetization data show that the $c$-axis magnetic field induces three magnetic transitions at 0.8, 12 and 28 T. The specific heat shows a peak at 2.2 K, which is due to a magnetic transition of Er$^{3+}$ moments. For low-$T$ thermal conductivity ($κ$), a clear dip-like feature appears in $κ(H)$ isotherm at 1--1.25 T for $H \parallel ab$; while in the case of $H \parallel c$, a step-like increase is observed at 0.5--0.8 T. The transition fields in $κ(H)$ are in good agreement with those obtained from magnetization, and the anomaly of $κ$ can be understood by a spin-phonon scattering scenario. The natures of magnetic structures and corresponding field-induced transitions at low temperatures are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Perpendicular diffusion coefficient of cosmic rays in the presence of weak adiabatic focusing

The influence of adiabatic focusing on particle diffusion is an important topic in astrophysics and plasma physics. In the past several authors have explored the influence of along-field adiabatic focusing on parallel diffusion of charged energetic particles. In this paper by using the Unified NonLinear Transport (UNLT) theory developed by Shalchi (SH2010) and the method of He and Schlickeiser (HS2014) we derive a new nonlinear perpendicular diffusion coefficient for non-uniform background magnetic field. This formula demonstrates that particle perpendicular diffusion coefficient is modified by along-field adiabatic focusing. For isotropic pitch-angle scattering and weak adiabatic focusing limit the derived perpendicular diffusion coefficient is independent of the sign of adiabatic focusing characteristic length. For two-component model we simplify the perpendicular diffusion coefficient up to second order of the power series of adiabatic focusing characteristic quantity. We find that the first order modifying factor is equal to zero and the sign of the second one is determined by the energy of particles.

astro-ph.SR↗

arXiv2017

Magnetic field line random walk in two-dimensional dynamical turbulence

The field line random walk (FLRW) of magnetic turbulence is one of the important topics in plasma physics and astrophysics. In this article by using the field line tracing method mean square displacements (MSD) of FLRW is calculated in all possible length scales for pure two-dimensional turbulence with damping dynamical model. We demonstrate that in order to describe FLRW with damping dynamical model a new dimensionless quantity $R$ is needed to be introduced. In different length scales dimensionless MSD shows different relationship with the dimensionless quantity $R$. Although temporal effect impacts MSD of FLRW and even changes regimes of FLRW, it dose not affect the relationship between the dimensionless MSD and dimensionless quantity $R$ in all possible length scales.

astro-ph.HE↗

arXiv2016

Covariant gaussian approximation in Ginzburg - Landau model

Condensed matter systems undergoing second order transition away from the critical fluctuation region are usually described sufficiently well by the mean field approximation. The critical fluctuation region, determined by the Ginzburg criterion, $\left \vert T/T_{c}-1\right \vert \ll Gi$, is narrow even in high $T_{c}$ superconductors and has universal features well captured by the renormalization group method. However recent experiments on magnetization, conductivity and Nernst effect suggest that fluctuations effects are large in a wider region both above and below $T_{c}$. In particular some "pseudogap" phenomena and strong renormalization of the mean field critical temperature $T_{mf}$ can be interpreted as strong fluctuations effects that are nonperturbative (cannot be accounted for by "gaussian fluctuations"). The physics in a broader region therefore requires more accurate approach. Self consistent methods are generally "non - conserving" in the sense that the Ward identities are not obeyed. This is especially detrimental in the symmetry broken phase where, for example, Goldstone bosons become massive. Covariant gaussian approximation remedies these problems. The Green's functions obey all the Ward identities and describe the fluctuations much better. The results for the order parameter correlator and magnetic penetration depth of the Ginzburg - Landau model of superconductivity are compared with both Monte Carlo simulations and experiments in high $T_{c}$ cuprates.

cond-mat.supr-con↗

arXiv2016

Hydride Vapor Phase Epitaxy of GaN on Silicon Covered by Nanostructures

Several ten $μ$m GaN have been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates have been covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features allow for an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear, and the luminescence of this part of the GaN layer is dominated by the donor-bound exciton. For templates consisting of both a thin AlN buffer and GaN nanostructures, a silicon incorporation into the GaN grown by HVPE is not observed. Moreover, the growth mode of the (high-T) HVPE step depends on the specific structure of the AlN/GaN template, where in a first case, the epitaxy is dominated by the formation of slowly growing facets, while in a second case, the epitaxy proceeds directly along the c-axis.

cond-mat.mtrl-sci↗