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Y. Z. Zhang

Publications and source records attributed to Y. Z. Zhang.

At least 19 recordsLinked to original sources
arXiv2024

Unraveling the untwisting process and upward mass transfer of a twisted prominence driven by vortex motion

Solar filaments/prominences are common features in the Sun's atmosphere that contain cool chromospheric material suspended within the hot corona. However, the intricate topology of these structures and the mechanisms driving their instability and upward material transfer are not well understood. This study is to analyze a specific twisted prominence on February 10, 2021, and to explore its dynamics, including stability, motion, and material transfer. The study utilizes high-resolution H$α$ observations from the 1-m New Vacuum Solar Telescope and space-borne observations from the Solar Dynamics Observatory. We analyzed the data to investigate the characteristics and behavior of the twisted prominence. We also detected and measured the outflow speed surrounding the prominence. The study reveals that the observed prominence exhibited a stretched and twisted structure at its apex, distinguishing it from familiar cloudy prominences. Following more than 30 hours of equilibrium, the prominence destabilized, leading to a series of dynamic phenomena, such as vortex motion, oscillations, resonations, untwisting, and the upward transfer of mass. Consequently, material from the top of the prominence was carried upward and deposited into the overlying magnetic arcades. Noteworthy, outflows surrounding the prominence were characterized by speeds exceeding 40 km $s^{-1}$. We propose, for the first time, a mechanism rooted in the Kármán Vortex Street instability to explain the destabilization of the prominence. The estimated typical Strouhal Number of 0.23$\pm$0.06, which is related to vortex shedding, falls within the expected range for the Kármán Vortex Street effect, as predicted by simulations. These discoveries provide new insights into the dynamics and fundamental topology of solar prominences and reveal a previously unknown mechanism for mass loading into the upper atmosphere.

astro-ph.SR↗
arXiv2023

Magnetic phase diagrams and large magnetocaloric effects of the two-dimensional antiferromagnetic triangular lattice of Gd$^{3+}$ ions in KBaGd(BO$_3$)$_2$

We report a detailed study of the magnetic properties of KBaGd(BO$_3$)$_2$, in which magnetic Gd$^{3+}$ ($S=7/2$) ions form into two-dimensional triangular layers. Magnetization, specific heat and magnetocaloric effect (MCE) measurements have been performed on KBaGd(BO$_3$)$_2$ single crystals. The results show that a long-range antiferromagnetic state is established below $T_{\rm N}=0.24$ K. In zero fields, only about half of the full entropy is released at $T_{\rm N}$, indicating that not all the magnetic moments are frozen below the ordering temperature, as expected from the geometrical frustration of the triangular spin lattice. Further studies under external fields were performed down to 50 mK, and the magnetic phase diagrams are established with magnetic fields applied both within and perpendicular to the triangular plane. KBaGd(BO$_3$)$_2$ serves as an example of a two-dimensional triangular lattice with large spin values ($S=7/2$) and can be directly compared with the iso-structure KBaR(BO$_3$)$_2$ (R = Dy-Yb) family of doublet ground states, which exhibit effective spins of $S=1/2$.

cond-mat.str-el↗
arXiv2022

On the drift wave eigenmode crossing zero frequency in Tokamak

The conventional ion temperature gradient or η_i mode is known to propagate in the ion diamagnetic direction. Investigation of a generic drift fluid model with warm ions and adiabatic electrons, reveals that as η_i decreases, the propagation characteristics of the unstable mode may change drastically, the mode frequency first decreases in magnitude, and reaches zero for a critical η_i. But as η_i goes down further, the mode begins to propagate in the electron diamagnetic direction. The lower toroidal mode number perturbations are more prone to reversal in propagation direction. Even for η_i=0, the mode remains unstable, drawing free energy form the density gradients. Since finite ion temperature appears to be essential for propagation in the electron direction, it is appropriate to introduce new terminology and call this wave the warm ion electron drift (WIED) mode. The model drift wave system is explored within the framework of the two dimensional (2D) weakly asymmetric ballooning theory (WABT) for local eigenmode satisfying natural boundary conditions. The physics behind the excitation of the eigenmode crossing zero frequency is identified to be the reactive instability induced by the curvature coupling between the positive energy wave and the negative energy wave, a damped mode in electron direction coupled to a growing mode in the ion direction in non-dissipative slab limit. Apart from its intrinsic scientific value, this mechanism may shed some light onto the nature of tokamak edge turbulence observed in frequencies moderately lower than the electron diamagnetic frequency; understanding this phenomenon could be helpful in conceptual design around the edge region of future tokamak.

physics.plasm-ph↗
arXiv2021

The intermittent excitation of geodesic acoustic mode by nonlinear Instanton of electron drift wave envelope in L-mode discharge near tokamak edge

There are two distinct phases in the evolution of drift wave envelope in the presence of zonal flow. A long-lived standing wave phase, which we call the Caviton, and a short-lived traveling wave phase (in radial direction) we call the Instanton. For drift wave turbulence driven by ion temperature gradient mode (ITG), these two stages of dynamics were displayed in [Zhang Y Z, Liu Z Y, Xie T, Mahajan S M and Liu J 2017 Physics of Plasmas 24 122304]. In this paper we show that the dynamical attributes of ITG turbulence are readily replicated when the turbulence rotates in the electron direction; our model calculation deals specifically with the toroidal electron drift waves (EDW) in the well-known δ_e model. While the basic calculations are presented in parallel to the ITG counterpart, more emphasis is laid here on the motion of Instanton; several abrupt phenomena observed in tokamaks, such as intermittent excitation of geodesic acoustic mode (GAM) shown in this paper, could be attributed to the sudden and fast radial motion of Instanton. The calculation brings out the defining characteristics of the Instanton: it begins as a linear traveling wave right after the transition. Then, it evolves to a nonlinear stage with increasing frequency all the way to 20 kHz. The modulation to Reynolds stress in zonal flow equation will cause resonant excitation to GAM. The intermittency is shown due to the random phase mixing between multiple central rational surfaces in the reaction region.

physics.plasm-ph↗
arXiv2019

I-mode investigation on the Experimental Advanced Superconducting Tokamak

By analyzing large quantities of discharges in the unfavorable ion $ \vec B\times \nabla B $ drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the $ E_r $ profiles obtained by the eight-channel Doppler backscattering system (DBS8)\cite{J.Q.Hu} show a deeper edge $ E_r $ well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the $ E_r $ fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigations are needed to explain the new properties such as the harmonics and obvious magnetical perturbations.

physics.plasm-ph↗
arXiv2018

Metallic glasses for spintronics: anomalous temperature dependence and giant enhancement of inverse spin Hall effect

Spin-charge conversion via inverse spin Hall effect (ISHE) is essential for enabling various applications of spintronics. The spin Hall response usually follows a universal scaling relation with longitudinal electric resistivity and has mild temperature dependence because elementary excitations play only a minor role in resistivity and hence ISHE. Here we report that the ISHE of metallic glasses shows nearly two orders of magnitude enhancements with temperature increase from a threshold of 80-100 K to glass transition points. As electric resistivity changes only marginally in the temperature range, the anomalous temperature dependence is in defiance of the prevailing scaling law. Such a giant temperature enhancement can be well described by a two-level thermal excitation model of glasses and disappears after crystallization, suggesting a new mechanism which involves unique thermal excitations of glasses. This finding may pave new ways to achieve high spin-charge conversion efficiency at room and higher temperatures for spintronic devices and to detect structure and dynamics of glasses using spin currents.

cond-mat.mtrl-sci↗
arXiv2015

The unified ballooning theory with weak up-down asymmetric mode structure and the numerical studies

A unified ballooning theory, constructed on the basis of two special theories [Y. Z. Zhang, S. M. Mahajan, X. D. Zhang, Phys. Fluids B4, 2729 (1992); Y. Z. Zhang, T. Xie, Nucl. Fusion & Plasma Phys. 33, 193 (2013)], shows that a weak up-down asymmetric mode structure is normally formed in an up-down symmetric equilibrium; the weak up-down asymmetry in mode structure is the manifestation of non-trivial higher order effects beyond the standard ballooning equation. It is shown that the asymmetric mode may have even higher growth rate than symmetric modes. Salient features of the theory are illustrated by investigating a fluid model for the ion temperature gradient (ITG) mode. The two dimensional (2D) analytical form of ITG mode, solved in ballooning representation, is then converted into the radial-poloidal space to provide the natural boundary condition for solving the 2D mathematical local eigenmode problem. We find the analytical expression of mode structure in good agreement with finite difference solution. This sets a reliable framework for quasi-linear computation.

physics.plasm-ph↗
arXiv2012

Revision of Solar Spicule Classification

Solar spicules are the fundamental magnetic structures in the chromosphere and considered to play a key role in channelling the chromosphere and corona. Recently, it was suggested by De Pontieu et al. that there were two types of spicules with very different dynamic properties, which were detected by space- time plot technique in the Ca ii H line (3968 A) wavelength from Hinode/SOT observations. 'Type I' spicule, with a 3-7 minute lifetime, undergoes a cycle of upward and downward motion; in contrast, 'Type II' spicule fades away within dozens of seconds, without descending phase. We are motivated by the fact that for a spicule with complicated 3D motion, the space-time plot, which is made through a slit on a fixed position, could not match the spicule behavior all the time and might lose its real life story. By revisiting the same data sets, we identify and trace 105 and 102 spicules in quiet sun (QS) and coronal hole (CH), respectively, and obtain their statistical dynamic properties. First, we have not found a single convincing example of 'Type II' spicules. Secondly, more than 60% of the identified spicules in each region show a complete cycle, i.e., majority spicules are 'Type I'. Thirdly, the lifetime of spicules in QS and CH are 148 s and 112 s, respectively, but there is no fundamental lifetime difference between the spicules in QS and CH reported earlier. Therefore, the suggestion of coronal heating by 'Type II' spicules should be taken with cautions. Subject headings: Sun: chromosphere Sun:transition region Sun:corona

astro-ph.SR↗
arXiv2009

Thermally activated energy and critical magnetic fields of SmFeAsO$_{0.9}$F$_{0.1}$

Thermally activated flux flow and vortex glass transition of recently discovered SmFeAsO$_{0.9}$F$_{0.1}$ superconductor are studied in magnetic fields up to 9.0 T. The thermally activated energy is analyzed in two analytic methods, of which one is conventional and generally used, while the other is closer to the theoretical description. The thermally activated energy values determined from both methods are discussed and compared. In addition, several critical magnetic fields determined from resistivity measurements are presented and discussed.

cond-mat.supr-con↗
arXiv2008

Energetics in Condensate Star and Wormholes

It is known that the total gravitational energy in localized sources having static spherical symmetry and satisfying energy conditions is negative (attractive gravity). A natural query is how the gravitational energy behaves under circumstances where energy conditions are violated. To answer this, the known expression for the gravitational energy is suitably modified to account for situations like the ones occurring in wormhole spacetime. It is then exemplified that in many cases the modified expression yields desirable answers. The implications are discussed.

gr-qc↗
arXiv2008

Weak coupling Bardeen-Cooper-Schrieffer suerconductivity in the Electron-Doped Cuprate Superconductors

We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap $Δ(T)$ in electron-doped copper oxides with various $T_c$'s and Ce-doping concentrations. We show that the temperature dependence of $Δ(T)$ follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where $Δ(0)/k_{B}T_{c}\approx1.72\pm 0.15$ and $Δ(0)$ is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak coupling BCS superconductors.

cond-mat.supr-con↗
arXiv2007

Pseudogap and Mott Transition Studied by Cellular Dynamical Mean Field Theory

We study metal-insulator transitions between Mott insulators and metals. The transition mechanism completely different from the original dynamical mean field theory (DMFT) emerges from a cluster extension of it. A consistent picture suggests that the quasiparticle weight $Z$ remains nonzero through metals and suddenly jumps to zero at the transition, while the gap opens continuously in the insulators. This is in contrast with the original DMFT, where $Z$ continuously vanishes but the gap opens discontinuously. The present results arising from electron differentiation in momentum space agree with recent puzzling bulk-sensitive experiments on CaVO$_3$ and SrVO$_3$.

cond-mat.str-el↗
arXiv2006

Accelerating Universe as Window for Extra Dimensions

Homogeneous cosmological solutions are obtained in five dimensional space time assuming equations of state $ p = kρ$ and $ p_{5}= γρ$ where p is the isotropic 3 - pressure and $p_{5}$, that for the fifth dimension. Using different values for the constants k and $γ$ many known solutions are rediscovered. Further the current acceleration of the universe has led us to investigate higher dimensional gravity theory, which is able to explain acceleration from a theoretical view point without the need of introducing dark energy by hand. We argue that the terms containing higher dimensional metric coefficients produce an extra negative pressure that apparently drives an acceleration of the 3D space, tempting us to suggest that the accelerating universe seems to act as a window to the existence of extra spatial dimensions. Interestingly the 5D matter field remains regular while the \emph{effective} negative pressure is responsible for the inflation. Relaxing the assumptions of two equations of state we also present a class of solutions which provide early deceleration followed by a late acceleration in a unified manner. Interesting to point out that in this case our cosmology apparently mimics the well known quintessence scenario fuelled by a generalised Chaplygin-type of fluid where a smooth transition from a dust dominated model to a de Sitter like one takes place.

gr-qc↗
arXiv2006

Two-Current-Sheet Reconnection Model of Interdependent Flare and Coronal Mass Ejection

Time-dependent resistive magnetohydrodynamic simulations are carried out to study a flux rope eruption caused by magnetic reconnection with implication in coexistent flare-CME (coronal mass ejection) events. An early result obtained in a recent analysis of double catastrophe of a flux rope system is used as the initial condition, in which an isolated flux rope coexists with two current sheets: a vertical one below and a transverse one above the flux rope. The flux rope erupts when reconnection takes place in the current sheets, and the flux rope dynamics depends on the reconnection sequence in the two current sheets. Three cases are discussed: reconnection occurs (1) simultaneously in the two current sheets, (2) first in the transverse one and then in the vertical, and (3) in an order opposite to case 2. Such a two-current-sheet reconnection exhibits characteristics of both magnetic breakout for CME initiation and standard flare model. We argue that both breakout-like and tether-cutting reconnections may be important for CME eruptions and associated surface activities.

astro-ph↗
arXiv2005

Accelerating Universe from Extra Spatial Dimension

We present a simple higher dimensional FRW type of model where the acceleration is apparently caused by the presence of the extra dimensions. Assuming an ansatz in the form of the deceleration parameter we get a class of solutions some of which shows the desirable feature of dimensional reduction as well as reasonably good physical properties of matter. Interestingly we do not have to invoke an extraneous scalar field or a cosmological constant to account for this acceleration. One argues that the terms containing the higher dimensional metric coefficients produces an extra negative pressure that apparently drives the inflation of the 4D space with an accelerating phase. It is further found that in line with the physical requirements our model admits of a decelerating phase in the early era along with an accelerating phase at present.Further the models asymptotically mimic a steady state type of universe although it starts from a big type of singularity. Correspondence to Wesson's induced matter theory is also briefly discussed and in line with it it is argued that the terms containing the higher dimensional metric coefficients apparently creates a negative pressure which drives the inflation of the 3-space with an accelerating phase.

gr-qc↗
arXiv2005

Manipulating vortex motion by thermal and Lorentz force in high temperature superconductors

By using thermal and Lorentz force, the vortex motion is successfully manipulated in the mixed state of underdoped La$_{2 - x}$Sr$_{x}$CuO$_{4}$ single crystals and optimally doped YBa$_{2}$Cu$_{3}$O$_{7 - δ}$ thin films. A conclusion is drawn that the strong Nernst signal above $T_{c}$ is induced by vortex motion. In the normal state, in order to reduce the dissipative contribution from the quasiparticle scattering and enhance the signal due to the possible vortex motion, a new measurement configuration is proposed. It is found that the in-plane Nernst signal ($H$ $| | $ $c$) can be measurable up to a high temperature in the pseudogap region, while the Abrikosov flux flow dissipation can only be measured up to $T_{c}$. This may point to different vortices below and above $T_{c}$ if we attribute the strong Nernst signal in the pseudogap region to the vortex motion. Below $T_{c}$ the dissipation is induced by the motion of the Abrikosov vortices. Above $T_{c}$ the dissipation may be caused by the motion of the spontaneously generated unbinded vortex-antivortex pairs.

cond-mat.supr-con↗
arXiv2005

Deviations from plastic barriers in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ thin films

Resistive transitions of an epitaxial Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ thin film were measured in various magnetic fields ($H\parallel c$), ranging from 0 to 22.0 T. Rounded curvatures of low resistivity tails are observed in Arrhenius plot and considered to relate to deviations from plastic barriers. In order to characterize these deviations, an empirical barrier form is developed, which is found to be in good agreement with experimental data and coincide with the plastic barrier form in a limited magnetic field range. Using the plastic barrier predictions and the empirical barrier form, we successfully explain the observed deviations.

cond-mat.supr-con↗