arXiv subjects

Z. Y. Liu

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

At least 19 recordsLinked to original sources

arXiv2026

A study of periodic nulling in PSR B0751+32 with FAST

We report new results from a nulling study of PSR~B0751+32 (PSR J0754+3231), observed at 1250~MHz with the Five hundred meter Aperture Spherical radio Telescope (FAST). Our analysis confirms the presence of periodic nulling in this pulsar. Using the recently developed mixture model method, we obtained a nulling fraction (NF) of $35.1\% \pm 0.6\%$. Three independent approaches were employed to estimate the nulling periodicity, and the results reveal significant temporal evolution of the modulation both within individual observations and across different \textbf{observing} essions. The pulsar exhibits an asymmetric two-component mean pulse profile, with the leading component brighter and narrower than the trailing one. Pulse energy analysis shows that both components remain stable immediately after the onset of the burst state, but subsequently undergo a progressive decline, with the trailing component most severely affected prior to burst termination. Notably, no evidence of the previously reported subpulse drifting was detected in our data. Our results challenge previous models that ascribed periodic nulling to purely geometric effects.

astro-ph.HE↗

arXiv2026

Commissioning and Full Realization of the PLASEN System at BRIF

A PLASEN (Precision LAser Spectroscopy for Exotic Nuclei) system, consisting of a compact radio-frequency quadrupole cooler-buncher (RFQ-cb) and a collinear resonance ionization spectroscopy setup, has now been fully commissioned with radioactive ion beams at the Beijing Radioactive Ion-beam Facility (BRIF). Using both stable and radioactive Rb ion beams from BRIF, we demonstrated that the large beam energy spread observed at BRIF has been successfully handled by employing the RFQ-cb, enabling the delivery of high-quality bunched radioactive ion beams for collinear resonance ionization spectroscopy experiments. Under these conditions, we performed laser spectroscopy of exotic nuclei, achieving high resolution (about 100 MHz spectral linewidth) and high sensitivity (up to 1:200 efficiency). This fully operational PLASEN system will serve as a state-of-the-art experimental platform at BRIF for research in multiple fields such as nuclear, atomic and molecular physics.

physics.ins-det↗

arXiv2025

Development and Characterization of a High-Resolution and High-Sensitivity Collinear Resonance Ionization Spectroscopy Setup

With the recent implementation of a radio-frequency quadrupole (RFQ) cooler-buncher and a multi-step laser resonance ionization technique, our previously developed collinear laser spectroscopy setup has been successfully upgraded into a fully functional collinear resonance ionization spectroscopy system. The new system was fully characterized using a bunched ion beam at 30~keV, during which hyperfine structure spectra of $^{85,87}$Rb isotopes were measured. An overall efficiency exceeding 1:200 (one resonant ion detected for every 200 ions after the RFQ cooler-buncher) was achieved while maintaining a spectral resolution of 100 MHz. Under these conditions, the extracted hyperfine structure parameters and isotope shift for $^{85,87}$Rb show excellent agreement with the literature values. These results demonstrate the system's capability to perform high-resolution and high-sensitivity laser spectroscopy of neutron-rich Rb isotopes, which are expected to be produced at the Beijing Radioactive Ion-beam Facility at a rate of approximately 100 particles per second.

physics.ins-det↗

arXiv2025

Superconductivity up to 17 K in the high-pressure rhombohedral-I phase of ReO3: a potential oxide analogy of hydride superconductors

As an A-site-vacant perovskite-type oxide, ReO3 undergoes sequential pressure-driven structural transitions associated with the rotation of ReO6 octahedra. The rhombohedral-I phase in the pressure range of 12-39 GPa is featured by a lattice of nearly close-packed oxygen layers intercalated with Re cations, in reminiscent of the recently discovered superhydride superconductors. A combined study of first-principles calculations and transport measurements under high pressures enabled us to discover superconductivity in the rhombohedral-I phase, and it shows a dome-shaped Tc(P) with a maximum Tc of 17 K at about 30 GPa. In addition to the enhanced density of states at Fermi level compared to that of the ambient phase, the vibrations of hexagonal-close-packed oxygen lattice significantly strengthen the electron-phonon coupling, which is responsible for observed superconductivity with a relatively high Tc. The present work thus establishes a rare case among oxide superconductors that the light-element oxygen lattice plays a crucial role in inducing superconductivity.

cond-mat.supr-con↗

arXiv2024

Prediction of Fishbone Linear Instability in Tokamaks with Machine Learning Methods

A machine learning based surrogate model for fishbone linear instability in tokamaks is constructed. Hybrid simulations with the kinetic-magnetohydrodynamic (MHD) code M3D-K is used to generate the database of fishbone linear instability, through scanning the four key parameters which are thought to determine the fishbone physics. The four key parameters include (1) central total beta of both thermal plasma and fast ions, (2) the fast ion pressure fraction, (3) central value of safety factor $q$ and (4) the radius of $q=1$ surface. Four machine learning methods including linear regression, support vector machines (SVM) with linear kernel, SVM with nonlinear kernel and multi-layer perceptron are used to predict the fishbone instability, growth rate and real frequency, mode structure respectively. Among the four methods, SVM with nonlinear kernel performs very well to predict the linear instability with accuracy $\approx$95%, growth rate and real frequency with $R^2\approx$98%, mode structure with $R^2\approx$98%.

physics.plasm-ph↗

arXiv2024

Development of a gyrokinetic-MHD energetic particle simulation code Part II: Linear simulations of Alfvén eigenmodes driven by energetic particles

We have developed a hybrid code GMEC: Gyro-kinetic Magnetohydrodynamics (MHD) Energetic-particle Code that can numerically simulate energetic particle-driven Alfvén eigenmodes and energetic particle transport in tokamak plasmas. In order to resolve the Alfvén eigenmodes with high toroidal numbers effectively, the field-aligned coordinates and meshes are adopted. The extended MHD equations are solved with five-points finite difference method and fourth order Runge-Kutta method. The gyrokinetic equations are solved by particle-in-cell (PIC) method for the perturbed energetic particle pressures that are coupled into the MHD equations. Up to now, a simplified version of the hybrid code has been completed with several successful verifications including linear simulations of toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes.

physics.plasm-ph↗

arXiv2024

Development of a gyrokinetic-MHD energetic particle simulation code Part I: MHD version

A new magnetohydrodynamics (MHD) code based on initial value approach, GMEC_I, has been developed for simulating various MHD physics in tokamak plasmas, as the MHD foundation of the gyrokinetic-MHD energetic particle simulation code (GMEC) family. GMEC_I solves multi-level reduced-MHD models that form a hierarchy of physics complexity, which provide conveniences for the cross-code verification and the identification of key physics effect in tokamak geometry. The field-aligned coordinates are used to represent mode structure efficiently. High-order finite difference methods are used for spatial discretization. The shifted metric methods are used for numerical stability. The discrete expansion forms of physics equations in the code are generated symbolically using the compile-time symbolic solver (CSS), which is specifically developed to reduce the complexity of the high-order finite difference form of the MHD equations. Advanced computational techniques have been implemented for optimizing memory access and code parallelization that show a good efficiency using both Thread Building Block (TBB) and Message Passing Interface (MPI). Benchmarks between GMEC_I and the eigenvalue code MAS are presented for ballooning modes without and with diamagnetic drift effects, and tearing modes, which show excellent agreements.

physics.plasm-ph↗

arXiv2023

Emergence of high-temperature superconducting phase in the pressurized La3Ni2O7 crystals

The recent report of pressure-induced structure transition and signature of superconductivity with Tc = 80 K above 14 GPa in the La3Ni2O7 crystals has garnered considerable attention. To further elaborate this discovery, we carried out comprehensive resistance measurements on the La3Ni2O7 crystals grown with the optical-image floating zone furnace under oxygen pressure (15 bar) by using the diamond anvil cell (DAC) and cubic anvil cell (CAC), which employs the solid and liquid pressure transmitting medium, respectively. For the sample #1 measured in DAC, it exhibits a semiconducting-like behavior with large resistance at low pressures and becomes metallic gradually upon compression. At the pressures P >= 13.7 GPa, we observed the appearance of resistance drop as large as ~50% around 70 K, which evolves into a kink-like anomaly at pressures above 40 GPa and shifts to lower temperatures gradually with increasing magnetic field. These observations are consistent with the recent report mentioned above. On the other hand, the sample #2 measured in CAC retains the metallic behavior in the investigated pressure range up to 15 GPa. The hump-like anomaly in resistance around ~130 K at ambient pressure disappears at P >= 2 GPa. In the pressure range from 11 to 15 GPa, we observed the gradual development of a shoulder-like anomaly in resistance at low temperatures, which evolves into a pronounced drop of resistance by 98% below 62 K at 15 GPa, reaching a temperature-independent resistance of 20 uOhm below 20 K. Similarly, this resistance anomaly can be shifted to lower temperatures progressively by applying external magnetic fields, resembling a typical superconducting transition.

cond-mat.supr-con↗

arXiv2022

A Nuclear-Based Diagnostic Scheme for Nonthermal Ion Spectrum in High Energy Density Plasmas Experiments

The nuclear reactions in a plasma system with energy distribution deviated from Maxwellian are proved to have some unique characteristics including those in their product energy spectrum. Based on this, a new nuclear diagnostic scheme for measuring the nonthermal ion energy spectrum in high-energy-density plasmas is proposed, where the effective temperature and spectral peak of the nonthermal ion energy spectrum are uniquely determined by the spectral width and spectral peak of the product energy spectrum. Then, taking the laser-driven magnetic reconnection experiment as an example, according to the particle-in-cell (PIC) simulation results coupled with the proton-boron (pB) fusion reaction, the pB reaction rate is increased by 4 orders of magnitude; meanwhile, the product spectral peak and spectral width are shifted towards higher energy, deviating from the predictions of the thermal equilibrium assumption, both of which are on account of the accelerated and heated suprathermal protons in magnetic reconnection. Furthermore, analyzing a series of simulation results, the new product spectrum analysis approach suitable for the parameter range of laser-driven magnetic reconnection is found.

physics.plasm-ph↗

arXiv2022

A fast radio burst source at a complex magnetised site in a barred galaxy

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio bursts. Recent observations of a Galactic FRB suggest that at least some FRBs originate from magnetars, but the origin of cosmological FRBs is still not settled. Here we report the detection of 1863 bursts in 82 hr over 54 days from the repeating source FRB~20201124A. These observations show irregular short-time variation of the Faraday rotation measure (RM), which probes the density-weighted line-of-sight magnetic field strength, of individual bursts during the first 36 days, followed by a constant RM. We detected circular polarisation in more than half of the burst sample, including one burst reaching a high fractional circular polarisation of 75%. Oscillations in fractional linear and circular polarisations as well as polarisation angle as a function of wavelength were detected. All of these features provide evidence for a complicated, dynamically evolving, magnetised immediate environment within about an astronomical unit (au; Earth-Sun distance) of the source. Our optical observations of its Milky-Way-sized, metal-rich host galaxy reveal a barred spiral, with the FRB source residing in a low stellar density, interarm region at an intermediate galactocentric distance. This environment is inconsistent with a young magnetar engine formed during an extreme explosion of a massive star that resulted in a long gamma-ray burst or superluminous supernova.

astro-ph.HE↗

arXiv2022

Electronic and magnetic properties of intermetallic Kagome magnets $R$V$_6$Sn$_6$ ($R$ = Tb - Tm)

We present a systematic study of the structure, electronic, and magnetic properties of a new branch of intermetalllic compounds, $R$V$_6$Sn$_6$ ($R$ = Tb - Tm) by using X-ray diffraction, magnetic susceptibility, magnetization, electrical transport, and heat-capacity measurements. These compounds feature a combination of a non-magnetic vanadium kagome sublattice and a magnetic rare-earth triangular sublattice that supports various spin anisotropies based on different $R$ ions. We find magnetic orders for the $R$ = Tb, Dy, and Ho compounds at 4.4, 3, 2.5 K, respectively, while no ordering is detected down to 0.4 K for the $R$ = Er and Tm compounds with easy-plane anisotropies. Electronically, we found no superconductivity or charge ordering transition down to 0.4 K for any member of this family, while all compounds exhibit multi-band transport properties that originate from the band topology of the vanadium kagome sublattice.

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↗

arXiv2022

Emergence of superconductivity on the border of antiferromagnetic order in RbMn6Bi5 under high pressure: A new family of Mn-based superconductors

The advances in the field of unconventional superconductivity are largely driven by the discovery of novel superconducting systems. Here we report on the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional RbMn6Bi5 via measurements of resistivity and magnetic susceptibility under high pressures. With increasing pressure, its antiferromagnetic transition with TN = 83 K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and exhibits a dome-like Tc(P) with a maximal Tc_onset = 9.5 K at about 15 GPa. Its temperature-pressure phase diagram resembles those of many magnetism-mediated superconducting systems. In addition, the superconducting state around Pc is characterized by a large upper critical field μ0Hc2(0) exceeding the Pauli limit, elaborating a possible unconventional paring mechanism. The present study, together with our recent work on KMn6Bi5 (Tcmax = 9.3 K), makes AMn6Bi5 (A= Alkali metal) a new family of Mn-based superconductors with relatively high Tc.

cond-mat.supr-con↗

arXiv2022

Pressure-induced superconductivity up to 9 K in the quasi-one-dimensional KMn6Bi5

The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in KMn6Bi5, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic anvil cell apparatus, we find that its antiferromagnetic transition can be suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and displays a superconducting dome with the maximal Tconset = 9.3 K achieved at about 14.2 GPa. The close proximity of superconductivity to a magnetic instability in the temperature-pressure phase diagram of KMn6Bi5 and an unusually large μ0Hc2(0) = 18.9 T exceeding the Pauli limit suggests an unconventional magnetism-mediated paring mechanism. In contrast to the binary MnP, the flexibility of the crystal structure and chemical compositions in the ternary AMn6Bi5 (A = alkali metal) can open a new avenue for finding more Mn-based superconductors.

cond-mat.supr-con↗

arXiv2021

Quasi-one-dimensional superconductivity in the pressurized charge-density-wave conductor HfTe3

HfTe3 single crystal undergoes a charge-density-wave (CDW) transition at TCDW = 93 K without the appearance of superconductivity (SC) down to 50 mK at ambient pressure. Here, we determined its CDW vector q = 0.91(1) a* + 0.27(1) c* via low-temperature transimission electron microscope and then performed comprehensive high-pressure transport measurements along three major crystallographic axes. Our results indicate that the superconducting pairing starts to occur within the quasi-one-dimensional (Q1D) -Te2-Te3- chain at 4-5 K but the phase coherence between the superconducting chains cannot be realized along either the b- or c-axis down to at least 1.4 K, giving rise to an extremely anisotropic SC rarely seen in real materials. We have discussed the prominent Q1D SC in pressurized HfTe3 in terms of the anisotropic Fermi surfaces arising from the unidirectional Te-5px electronic states and the local pairs formed along the -Te2-Te3- chains based on the first-principles electronic structure calculations.

cond-mat.supr-con↗

arXiv2021

Thermal dynamics of charge density wave pinning in ZrTe3

Impurity pinning has long been discussed to have a profound effect on the dynamics of an incommensurate charge density wave (CDW), which would otherwise slide through the lattice without resistance. Here we visualize the impurity pinning evolution of the CDW in ZrTe3 using the variable temperature scanning tunneling microscopy (STM). At low temperatures, we observe a quasi-1D incommensurate CDW modulation moderately correlated to the impurity positions, indicating a weak impurity pinning. As we raise the sample temperature, the CDW modulation gets progressively weakened and distorted, while the correlation with the impurities becomes stronger. Above the CDW transition temperature, short-range modulations persist with the phase almost all pinned by impurities. The evolution from weak to strong impurity pinning through the CDW transition can be understood as a result of losing phase rigidity.

cond-mat.str-el↗

arXiv2021

Fast radio burst detection in the presence of coloured noise

In this paper, we investigate the impact of correlated noise on fast radio burst (FRB) searching. We found that 1) the correlated noise significantly increases the false alarm probability; 2) the signal-to-noise ratios (S/N) of the false positives become higher; 3) the correlated noise also affects the pulse width distribution of false positives, and there will be more false positives with wider pulse width. We use 55-hour observation for M82 galaxy carried out at Nanshan 26m radio telescope to demonstrate the application of the correlated noise modelling. The number of candidates and parameter distribution of the false positives can be reproduced with the modelling of correlated noise. We will also discuss a low S/N candidate detected in the observation, for which we demonstrate the method to evaluate the false alarm probability in the presence of correlated noise.Possible origins of the candidate are discussed, where two possible pictures, an M82-harbored giant pulse and a cosmological FRB, are both compatible with the observation.

astro-ph.HE↗

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↗