arXiv subjects

H. H. Wang

Publications and source records attributed to H. H. Wang.

16 recordsLinked to original sources

arXiv2025

Precession for the mode change in a gamma-ray pulsar

PSR J2021+4026 is a gamma-ray pulsar having variations in its spin-down rate and gamma-ray flux. Its variations in timing and emission are correlated, e.g., a larger spin-down rate for a low gamma-ray flux. We show that the mode change in PSR J2021+4026 can be understood in the precession scenario. In the precession model, the inclination angle is modulated due to precession. At the same time, the wobble angle may decay with time. This results in damping of the precession. Combined with magnetospheric torque model and the outer gap model, the damped precession can explain: (1) when the inclination angle is larger, the spin-down rate will be larger, accompanied by a lower gamma-ray flux. (2) The variation amplitude of the gamma-ray flux and spin-down rate is smaller than previous results due to the damping of the precession. The modulation period is becoming shorter due to a smaller wobble angle. In the end, we propose that there are two kinds of modulations in pulsars. Long-term modulations in pulsars may be due to precession. Short-term modulations may be of magnetospheric origin.

astro-ph.HE↗

arXiv2024

Suppression of Edge Localized Modes in ITER Baseline Scenario in EAST using Edge Localized Magnetic Perturbations

We report the suppression of Type-I Edge Localized Modes (ELMs) in the EAST tokamak under ITER baseline conditions using $n = 4$ Resonant Magnetic Perturbations (RMPs), while maintaining energy confinement. Achieving RMP-ELM suppression requires a normalized plasma beta ($β_N$) exceeding 1.8 in a target plasma with $q_{95}\approx 3.1$ and tungsten divertors. Quasi-linear modeling shows high plasma beta enhances RMP-driven neoclassical toroidal viscosity torque, reducing field penetration thresholds. These findings demonstrate the feasibility and efficiency of high $n$ RMPs for ELM suppression in ITER.

physics.plasm-ph↗

arXiv2024

Evidence of the gamma-ray counterpart from nova FM Cir with Fermi-LAT

We report the analysis results of X-ray and gamma-ray data of the nova FM Cir taken by Swift and Fermi-LAT. The gamma-ray emission from FM Cir can be identified with a significance level of 3sigma within 40 days after the nova eruption (2018 January 19) while we bin the light curve per day. The significance can further exceed 4 sigma confidence level if we accumulate longer time (i.e., 20 days) to bin the light curve. The gamma-ray counterpart could be identified with a Test Statistic (TS) above 4 until 180 days after the eruption. The duration of the gamma-ray detection was longer than those reported in the previous studies of the other novae detected in the GeV range. The significant X-ray emission was observed after the gamma-ray flux level fell below the sensitivity of Fermi-LAT. The hardness ratio of the X-ray emission decreased rapidly with time, and the spectra were dominated by blackbody radiation from the hot white dwarf. Except for the longer duration of the gamma-ray emission, the multi-wavelength properties of FM Cir closely resemble those of other novae detected in the GeV range.

astro-ph.HE↗

arXiv2023

Extension of ELM suppression window using n=4 RMPs in EAST

The q95 window for Type-I Edge Localized Modes (ELMs) suppression using n=4 even parity Resonant Magnetic Perturbations (RMPs) has been significantly expanded to a range from 3.9 to 4.8, which is demonstrated to be reliable and repeatable in EAST over the last two years. This window is significantly wider than the previous one, which is around q95=3.7pm0.1, and is achieved using n=4 odd parity RMPs. Here, n represents the toroidal mode number of the applied RMPs and q95 is the safety factor at the 95% normalized poloidal magnetic flux. During ELM suppression, there is only a slight drop in the stored energy (<=10%). The comparison of pedestal density profiles suggests that ELM suppression is achieved when the pedestal gradient is kept lower than a threshold. This wide q95 window for ELM suppression is consistent with the prediction made by MARS-F modeling prior to the experiment, in which it is located at one of the resonant q95 windows for plasma response. The Chirikov parameter taking into account plasma response near the pedestal top, which measures the plasma edge stochasticity, significantly increases when q95 exceeds 4, mainly due to denser neighboring rational surfaces. Modeling of plasma response by the MARS-F code shows a strong coupling between resonant and non-resonant components across the pedestal region, which is characteristic of the kink-peeling like response observed during RMP-ELM suppression in previous studies on EAST. These promising results show the reliability of ELM suppression using the n=4 RMPs and expand the physical understanding on ELM suppression mechanism.

physics.plasm-ph↗

arXiv2021

Field-induced metal-to-insulator transition and colossal anisotropic magnetoresistance in a nearly Dirac material EuMnSb$_2$

How to realize applicably appreciated functionalities based on the coupling between charge and spin degrees of freedom is still a challenge in the field of spintronics. For example, anisotropic magnetoresistance (AMR) effect is utilized to read out the information stored by various magnetic structures, which usually originates from atomic spin-orbit coupling (SOC). However, the application of AMR in antiferromagnet-based spintronics is still hindered by rather small AMR value. Here, we discover a colossal AMR effect during the field-induced metal-to-insulator transition (MIT) in a nearly Dirac material EuMnSb$_2$ with an antiferromagnetic order of Eu$^{2+}$ moments. The colossal AMR reaches to an unprecedented value of 1.84$\times$10$^6$% at 2 K, which is four orders of magnitude larger than previously reported values in antiferromagnets. Based on density functional theory calculations, a Dirac-like band structure, which is strongly dependent on SOC, is confirmed around Y point and dominates the overall transport properties in the present sample with predominant electron-type carriers. Moreover, it is also revealed that the indirect band gap around Fermi level is dependent on the magnetic structure of Eu$^{2+}$ moments, which leads to the field-induced MIT and plays a key role on the colossal AMR effect. Finally, our present work suggests that the similar antiferromagnetic topological materials as EuMnSb$_2$, in which Dirac-like fermions is strongly modulated by SOC and antiferromagnetism, would be a fertile ground to explore applicably appreciated AMR effect.

cond-mat.mtrl-sci↗

arXiv2021

Influence of Micro-turbulence on Neoclassical Tearing Mode Onset

Direct evidence of micro-turbulence effect on the onset of neoclassical tearing mode (NTM) is reported for the first time in this letter. A puzzling positive correlation between critical width of seed island of NTM and normalized plasma pressure beta_p is first observed employing a novel method for clearly separating the processes of seed island and the onset of NTM in the EAST tokamak. Different from the methods developed before, the width of the seed island is well controlled by slowly ramping up the current in resonant magnetic perturbation coils. It is revealed that the positive correlation is mainly attributed to the enhancement of perpendicular transport by micro-turbulence, which overcomes the destabilizing effect of beta_p on the onset of NTM. Reduced magnetohydrodynamics (MHD) modeling well reproduced the two states of nonlinear bifurcations observed in this experiment by including the finite transport effect. This result provides a new route for understanding multi-scale interaction in plasma physics.

physics.plasm-ph↗

arXiv2021

Investigation of gamma-ray variability and glitches of PSR J1420-6048

PSR J1420-6048 is a young gamma-ray pulsar with recurrent glitches. Utilizing long-term monitoring data obtained from the Fermi Gamma-ray Space Telescope, we found that PSR J1420-6048 has shown gamma-ray flux variation and we also detected four glitches between 2008 and 2019. Two of the glitches are previously unknown, and their gamma-ray spectrum also shows variability between each glitch. Since the results might be contaminated by background sources, we discuss whether the observed changes in flux and spectra were caused by artificial misallocations of photons from a nearby pulsar wind nebula (HESS J1420-607) and a pulsar (PSR J1418-6058), or a change of the emission geometry from the target pulsar itself. We examine the correlation of the flux changes and the alternating pulse structure to investigate whether the emission geometry in the outer magnetosphere was changing. By assuming the observational features were not totally resulted from the background environment, we compare our results with similar phenomena observed in other gamma-ray pulsars and propose that a strong crust crack can cause timing anomaly of a neutron star, which can affect the particle accelerations or pair creation regions resulting in the changes of emission behaviors.

astro-ph.HE↗

arXiv2020

An X-ray study for white dwarf binary AR Scorpii

We report a study of the X-ray emission from the white dwarf/M-type star binary system AR Scorpii using archival data taken in 2016-2020. It has been known that the X-ray emission is dominated by the optically thin thermal plasma emission, and its flux level varies significantly over the orbital phase. The X-ray emission also contains a component that modulates with the beat frequency between the white dwarf's spin frequency and orbital frequency. In this new analysis, the 2020 data taken by NICER shows that the X-ray emission is modulating with the spin frequency as well as the beat frequency, indicating that part of the X-ray emission is coming from the white dwarf's magnetosphere. It is found that the signal of the spin frequency appears only at a specific orbital phase, while the beat signal appears over the orbital phase. We interpret the X-ray emission modulating with the spin frequency and the beat frequency as a result of the synchrotron emission from electrons with a smaller and larger pitch angle, respectively. In a long-term evolution, the beat pulse profile averaged over the orbital phase changed from a single-peak structure in 2016/2018 to a double-peak structure in 2020. The observed X-ray flux levels measured in 2016/2017 are higher than those measured in 2018/2020. The plasma temperature and amplitude of the orbital waveform might vary with time too. These results indicate that the X-ray emission from AR Scorpii evolves on a timescale of years. This long-term evolution would be explained by a super-orbital modulation related to, for example, a precession of the white dwarf, or a fluctuation of the system related to activity of the companion star.

astro-ph.HE↗

arXiv2020

Repeated state change of variable gamma-ray pulsar, PSR J2021+4026

PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that shows state change of the gamma-ray emission and spin-down rate. The state change of PSR J2021+4026 was first observed at 2011 October, at which the pulsar changes the state from high gamma-ray flux/low spin-down rate state to low gamma-ray flux/high spin-down rate st\ ate. In December 2014, PSR J2021+4026 recovered the state before the 2011 state change over a timescale of a few months. We report that the long term evolution of the gamma-ray flux and timing behavior suggests that PSR J2021+4026 changed the state near 2018 February 1st and entered a new low gamma-ray flux/high spin-down rate state. At the 2018 state change, the averaged flux dropped from $(1.29\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1}}$ to $(1.12\pm 0.01)\times 10^{-6} {\rm cts~cm^{-2}s^{-1\ }}$, which has the similar behavior to the case of 2011 event. The spin-down rate has increased by $\sim 3\%$ in the new state since the 2018 state change. The shapes of pulse profile and spectrum in GeV bands also changed at the 2018 event, and they are consistent with behavior at the 2011 state change. Our results probably suggest that PSR J2021+4026 is switching between different states with a timescale of several years, like some radio pulsars (e.g. PSR~B1828-11). PSR J2021+4026 will provide a unique opportunity to study the mechanism of the state switching.

astro-ph.HE↗

arXiv2019

Preformed Cooper pairs in layered FeSe-based superconductors

Superconductivity arises from two distinct quantum phenomena: electron pairing and long-range phase coherence. In conventional superconductors, the two quantum phenomena generally take place simultaneously, while the electron pairing occurs at higher temperature than the long-range phase coherence in the underdoped high-Tc cuprate superconductors. Recently, whether electron pairing is also prior to long-range phase coherence in single-layer FeSe film on SrTiO3 substrate is under debate. Here, by measuring Knight shift and nuclear spin-lattice relaxation rate, we unambiguously reveal a pseudogap behavior below Tp ~ 60 K in two layered FeSe-based superconductors with quasi-two-dimension. In the pseudogap regime, a weak diamagnetic signal and a remarkable Nernst effect are also observed, which indicate that the observed pseudogap behavior is related to superconducting fluctuations. These works confirm that strong phase fluctuation is an important character in the two-dimensional iron-based superconductors as widely observed in high-Tc cuprate superconductors.

cond-mat.supr-con↗

arXiv2019

Anchoring Magnetic Fields in Turbulent Molecular Clouds II - from 0.1 to 0.01 parsec

We (Li et al. 2009; Paper-I) compared the magnetic field directions inferred from polarimetry data obtained from 100-pc scale inter-cloud media (ICM) and from sub-pc scale molecular cloud cores. The highly correlated result led us to conclude that cloud turbulence must be sub-Alfvenic. Here we extend the study with 0.01-pc cores observed by interferometers. The inferred field directions at this scale significantly deviate from that of the surrounding ICM. An obvious question to ask is whether this high-resolution result contradicts the sub-Alfvenic picture concluded earlier. We performed MHD simulations of a slightly super-critical (magnetic criticality = 2) clouds with Alfvenic Mach number $M_A = 0.63$, which can reproduce the Paper-I results, and observed the development towards smaller scales. Interestingly, all subregions hosting cores with $n_H$$_2$ > $10^{5}$/cc (the typical density observed by interferometers) possess $M_A = 2-3$. Not too surprisingly, these slightly super-Alfvenic cores result in B-field orientation offsets comparable to the interferometer observations. The result suggests that gravity can concentrate (and maybe also contribute to, which takes more study to confirm) turbulent energy and create slightly super-Alfvenic cores out from sub-Alfvenic clouds. The results of our simulations also agree with the observed velocity-scale (Kauffmann et al. 2013), mass-scale (Lombardi et al. 2010) and field strength-density (Li et al. 2015; Crutcher et al. 2010) relations.

astro-ph.GA↗

arXiv2018

X-ray study of Variable Gamma-ray Pulsar PSR J2021+4026

PSR~J2021+4026 showed a sudden decrease in the gamma-ray emission at the glitch that occurred around 2011, October 16, and a relaxation of the flux to the pre-glitch state at around 2014 December. We report X-ray analysis results of the data observed by XMM-Newton on 2015 December 20 in the post-relaxation state. To examine any change in the X-ray emission, we compare the properties of the pulse profiles and spectra at the low gamma-ray flux state and at the post-relaxation state. The phase-averaged spectra for both states can be well described by a power-law component plus a blackbody component. The former is dominated by unpulsed emission and is probably originated from the pulsar wind nebula as reported by Hui et al (2015). The emission property of the blackbody component is consistent with the emission from the polar cap heated by the back-flow bombardment of the high-energy electrons or positrons that were accelerated in the magnetosphere. We found no significant change in the X-ray emission properties between two states. We suggest that the change of the X-ray luminosity is at an order of ~4%, which is difficult to measure with the current observations. We model the observed X-ray light curve with the heated polar cap emission and we speculate that the observed large pulsed fraction is owing to asymmetric magnetospheric structure.

astro-ph.HE↗

arXiv2018

Magnetic-field enhanced high-thermoelectric performance in topological Dirac semimetal Cd$_3$As$_2$ crystal

Thermoelectric materials can be used to convert heat to electric power through the Seebeck effect. We study magneto-thermoelectric figure of merit (ZT) in three-dimensional Dirac semimetal Cd$_3$As$_2$ crystal. It is found that enhancement of power factor and reduction of thermal conductivity can be realized at the same time through magnetic field although magnetoresistivity is greatly increased. ZT can be highly enhanced from 0.17 to 1.1 by more than six times around 350 K under a perpendicular magnetic field of 7 Tesla. The huge enhancement of ZT by magnetic field arises from the linear Dirac band with large Fermi velocity and the large electric thermal conductivity in Cd$_3$As$_2$. Our work paves a new way to greatly enhance the thermoelectric performance in the quantum topological materials.

cond-mat.mtrl-sci↗

arXiv2016

Charge fluctuations and superconductivity in organic conductors: the case of $β"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$

A $^{13}$C NMR study of the normal and superconducting states of the all-organic charge-transfer salt $β"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ is presented. We find that the normal state is a charge-ordered metal configured as vertical stripes, produced by a combination of 1/4-filling, correlations, and a polar counterion sublattice. The NMR properties associated with the superconducting state are consistent with gap nodes and singlet pairing, and therefore similar to other organic superconductors. Quite distinct, however, is the absence of evidence for low-energy antiferromagnetic spin fluctuations for $T>T_c=4.5$ K. Both aspects are discussed in the context of a proposal that the pairing in this compound is driven by charge fluctuations.

cond-mat.str-el↗

arXiv2002

Nickel Antidot Arrays on Anodic Alumina Substrates

Large area nickel antidot arrays with density up to 10^10 /cm^2 have been fabricated by depositing nickel onto anodic aluminum oxide membranes that contain lattices of nanopores. Electron microscopy images show a high degree of order of the antidot arrays. Various sizes and shapes of the antidots were observed with increasing thickness of the deposited nickel. New features appear in the antidot arrays in both magnetization and transport measurements when the external magnetic field is parallel to the current direction, including an enhancement and a nonmonotonous field dependence of the magnetoresistance, larger values of the coercive field and remanence moment, and smaller saturation field.

cond-mat.mtrl-sci↗

arXiv1999

Low Temperature Penetration depth of κ-(ET)_2Cu[N(CN)_2]Br and κ-(ET)_2Cu(NCS)_2

We present high precision measurements of the penetration depth λof single crystals of κ-(ET)_2Cu[N(CN)2]Br and κ-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, λ\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, λ(0)=100\pm 20 μm.

cond-mat.supr-con↗