arXiv subjects

Zhijun Wang

Publications and source records attributed to Zhijun Wang.

At least 91 records · Page 5Linked to original sources

arXiv2023

Trend-Based SAC Beam Control Method with Zero-Shot in Superconducting Linear Accelerator

The superconducting linear accelerator is a highly flexiable facility for modern scientific discoveries, necessitating weekly reconfiguration and tuning. Accordingly, minimizing setup time proves essential in affording users with ample experimental time. We propose a trend-based soft actor-critic(TBSAC) beam control method with strong robustness, allowing the agents to be trained in a simulated environment and applied to the real accelerator directly with zero-shot. To validate the effectiveness of our method, two different typical beam control tasks were performed on China Accelerator Facility for Superheavy Elements (CAFe II) and a light particle injector(LPI) respectively. The orbit correction tasks were performed in three cryomodules in CAFe II seperately, the time required for tuning has been reduced to one-tenth of that needed by human experts, and the RMS values of the corrected orbit were all less than 1mm. The other transmission efficiency optimization task was conducted in the LPI, our agent successfully optimized the transmission efficiency of radio-frequency quadrupole(RFQ) to over $85\%$ within 2 minutes. The outcomes of these two experiments offer substantiation that our proposed TBSAC approach can efficiently and effectively accomplish beam commissioning tasks while upholding the same standard as skilled human experts. As such, our method exhibits potential for future applications in other accelerator commissioning fields.

physics.acc-ph↗

arXiv2023

Phase Field Model for Non-equilibrium Interface Conditions

This article presents a new phase-field formulation for non-equilibrium interface conditions in rapid phase transformations. With a particular way of defining concentration fields, the classical sharp and diffuse (thick) interface theories are unified into the present phase-field model. Each point inside the diffuse interface is comparable to the classical sharp interface model, governed by phase boundary migration and short-range atomic exchanges and connected by long-range diffusion. Their thermodynamic forces and fluxes are following the Onsager reciprocal relation and consistent with the classical irreversible thermodynamics. Furthermore, we establish the balance of driving forces and energy dissipations for both the diffuse interface and the single point inside it. The model is then validated by rapid solidification of Al-Cu alloys. With an effective mobility of considering non-equilibrium long-range diffusion, the model represents the main characteristics of non-equilibrium interface kinetics, i.e., solute trapping and solute drag. Especially, we reproduce the complete trapping at the high-velocity regime and provide a thermodynamic consistent description of the partial drag phenomena at the low-velocity region. Moreover, the dissipation analysis indicates that even the far-from-equilibrium diffuse interface is composed of numerous representative volume elements near the equilibrium, providing a new understanding of non-equilibrium interfaces.

cond-mat.mtrl-sci↗

arXiv2023

Thermodynamics of Non-equilibrium Diffuse-Interfaces in Mesoscale Phase Transformations

We present a new phase-field formulation for the non-equilibrium interface kinetics. The diffuse interface is considered an integral of numerous representative volume elements (RVEs), in which there is a two-phase mixture with two conserved and two non-conserved concentration fields. This particular way of separating concentration fields leads to two distinct dissipative processes besides the phase boundary migration, i.e., trans-sharp-interface diffusion between different phases within a single RVE and trans-diffuse-interface diffusion between different RVEs within a single phase. A two-part mechanism is proposed to balance the driving forces and energy dissipation for diffusionless and diffusional processes in RVEs. Moreover, the classical interface friction and solute drag can be recovered by integrating the diffuse interface. Compared with the sharp interface theory, the present diffuse interface can reproduce the atomic simulated partial-drag self-consistently in thermodynamics.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic field filtering of the hinge supercurrent in unconventional metal NiTe$_2$-based Josephson junctions

Topological materials with boundary (surface/edge/hinge) states have attracted tremendous research interest. Besides, unconventional (obstructed atomic) materials have recently drawn lots of attention owing to their obstructed boundary states. Experimentally, Josephson junctions (JJs) constructed on materials with boundary states produce the peculiar boundary supercurrent, which was utilized as a powerful diagnostic approach. Here, we report the observations of conspicuous hinge supercurrent in NiTe$_2$-based JJs. Particularly, applying an in-plane magnetic field along the Josephson current could rapidly suppress the bulk supercurrent and retain the nearly pure hinge supercurrent, namely the magnetic field filtering of supercurrent. Further systematic comparative analysis and theoretical calculations demonstrate the existence of unconventional nature and obstructed hinge states in NiTe$_2$. Our results revealed the unique hinge states in unconventional metal NiTe$_2$, and demonstrated in-plane magnetic field as an efficient method to filter out the futile bulk contributions and thereby to highlight the hinge states hidden in topological/unconventional materials.

cond-mat.mes-hall↗

arXiv2022

Prediction of nonlinear interface dynamics in the unidirectional freezing of particle suspensions with rigid compacted layer

Water freezing in particle suspensions widely exists in nature. As a typical physical system of free boundary problem, the spatiotemporal evolution of the solid/liquid interface not only origins from phase transformation but also from permeation flow in front of ice. Physical models have been proposed in previous efforts to describe the interface dynamic behaviors in unidirectional freezing of particle suspensions. However, there are several physical parameters difficult to be determined in previous investigations dedicated to describing the spatiotemporal evolution in unidirectional freezing of particle suspensions. Here, based on the fundamental momentum theorem, we propose a consistent theoretical framework to address the unidirectional freezing process in the particle suspensions coupled with the effect of water permeation. An interface undercooling-dependent pushing force exerted on the compacted layer with a specific formula is derived based on the surface tension. Then a dynamic compacted layer is considered and analyzed. Numerical solutions of the nonlinear models reveal the dependence of system dynamics on some typical physical parameters, particle radius, initial particle concentration in the suspensions, freezing velocity and so on. The system dynamics are characterized by interface velocity, interface undercooling and interface recoil as functions of time. The models allow us to reconsider the formation mechanism of ice spears in freezing of particle suspensions in a simpler but novel way, with potential implications for both understanding and controlling not only ice formation in porous media but also crystallization processes in other complex systems.

cond-mat.mtrl-sci↗

arXiv2022

Breaking the Representation Bottleneck of Chinese Characters: Neural Machine Translation with Stroke Sequence Modeling

Existing research generally treats Chinese character as a minimum unit for representation. However, such Chinese character representation will suffer two bottlenecks: 1) Learning bottleneck, the learning cannot benefit from its rich internal features (e.g., radicals and strokes); and 2) Parameter bottleneck, each individual character has to be represented by a unique vector. In this paper, we introduce a novel representation method for Chinese characters to break the bottlenecks, namely StrokeNet, which represents a Chinese character by a Latinized stroke sequence (e.g., "ao1 (concave)" to "ajaie" and "tu1 (convex)" to "aeaqe"). Specifically, StrokeNet maps each stroke to a specific Latin character, thus allowing similar Chinese characters to have similar Latin representations. With the introduction of StrokeNet to neural machine translation (NMT), many powerful but not applicable techniques to non-Latin languages (e.g., shared subword vocabulary learning and ciphertext-based data augmentation) can now be perfectly implemented. Experiments on the widely-used NIST Chinese-English, WMT17 Chinese-English and IWSLT17 Japanese-English NMT tasks show that StrokeNet can provide a significant performance boost over the strong baselines with fewer model parameters, achieving 26.5 BLEU on the WMT17 Chinese-English task which is better than any previously reported results without using monolingual data. Code and scripts are freely available at https://github.com/zjwang21/StrokeNet.

cs.CL↗

arXiv2022

Quadrupole topological insulators in Ta2M3Te5 (M= Ni, Pd) monolayers

Higher-order topological insulators have been introduced in the precursory Benalcazar-Bernevig-Hughes quadrupole model, but no electronic compound has been proposed to be a quadrupole topological insulator (QTI) yet. In this work, we predict that Ta$_2M_3$Te$_5$ ($M=$ Pd, Ni) monolayers can be 2D QTIs with second-order topology due to the double-band inversion. A time-reversal-invariant system with two mirror reflections (M$_x$ and M$_y$) can be classified by Stiefel-Whitney numbers ($w_1, w_2$) due to the combined symmetry $TC_{2z}$. Using the Wilson loop method, we compute $w_1=0$ and $w_2=1$ for Ta$_2$Ni$_3$Te$_5$, indicating a QTI with $q^{xy}=e/2$. Thus, gapped edge states and localized corner states are obtained. By analyzing atomic band representations, we demonstrate that its unconventional nature with an essential band representation at an empty site, i.e., $A_g@4e$, is due to the remarkable double-band inversion on Y-$Γ$. Then, we construct an eight-band quadrupole model with $M_x$ and $M_y$ successfully for electronic materials. These transition-metal compounds of $A_2M_{1,3}X_5$ ($A$ = Ta, Nb; $M$ = Pd, Ni; $X$ = Se, Te) family provide a good platform for realizing the QTI and exploring the interplay between topology and interactions.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic-field modulation of topological electronic states and emergent magneto-transport in a magnetic Weyl semimetal

The modulation of topological electronic states by an external magnetic field is highly desired for condensed matter physics. Schemes to achieve this have been proposed theoretically, but few can be realized experimentally. Here, combining transverse transport, theoretical calculations, and scanning tunneling microscopy/spectroscopy (STM/S) investigations, we provide an observation that the topological electronic states, accompanied by an emergent magneto-transport phenomenon, were modulated by applying magnetic fields through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6. A giant unconventional anomalous Hall effect (UAHE) is found during the magnetisation re-orientation from easy axis to hard ones in magnetic fields, with a UAHE peak around the low field of 5 kOe. Under the reasonable spin-canting effect, the folding of the topological anti-crossing bands occurs, generating a strong Berry curvature that accounts for the observed UAHE. Field-dependent STM/S reveals a highly synchronous evolution of electronic density of states, with a dI/dV peak around the same field of 5 kOe, which provides evidence to the folded bands and excited UAHE by external magnetic fields. This finding elucidates the connection between the real-space non-collinear magnetism and the k-space topological electronic states and establishes a novel manner to engineer the magneto-transport behaviours of correlated electrons for future topological spintronics.

cond-mat.mtrl-sci↗

arXiv2022

Large spin Hall conductivity and excellent hydrogen evolution reaction activity in unconventional PtTe1.75 monolayer

Two-dimensional (2D) materials have gained lots of attention due to the potential applications. In this work, we propose that based on first-principles calculations, the (2$\times$2) patterned PtTe$_2$ monolayer with kagome lattice formed by the well-ordered Te vacancy (PtTe$_{1.75}$) hosts large spin Hall conductivity (SHC) and excellent hydrogen evolution reaction (HER) activity. The unconventional nature relies on the $A1@1b$ band representation (BR) of the highest valence band without SOC. The large SHC comes from the Rashba spin-orbit coupling (SOC) in the noncentrosymmetric structure induced by the Te vacancy. Even though it has a metallic SOC band structure, the $\mathbb Z_2$ invariant is well defined due to the existence of the direct band gap and is computed to be nontrivial. The calculated SHC is as large as 1.25$\times 10^3 \frac{\hbar}{e} (Ω~cm)^{-1}$ at the Fermi level ($E_F$). By tuning the chemical potential from $E_F-0.3$ to $E_F+0.3$ eV, it varies rapidly and monotonically from $-1.2\times 10^3$ to 3.1$\times 10^3 \frac{\hbar}{e} (Ω~cm)^{-1}$. In addition, we also find the Te vacancy in the patterned monolayer can induce excellent HER activity. Our results not only offer a new idea to search 2D materials with large SHC, i.e., by introducing inversion-symmetry breaking vacancies in large SOC systems, but also provide a feasible system with tunable SHC (by applying gate voltage) and excellent HER activity.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic band representations, Fu-Kane-like symmetry indicators and magnetic topological materials

To realize novel topological phases and to pursue potential applications in low-energy consumption spintronics, the study of magnetic topological materials is of great interest. Starting from the theory of nonmagnetic topological quantum chemistry [Bradlyn et al., Nature 547, 298 (2017)], we have obtained irreducible (co)representations and compatibility relations (CRs) in momentum space, and we constructed a complete list of magnetic band (co)representations (MBRs) in real space for other MSGs with anti-unitary symmetries (i.e. type-III and type-IV MSGs). The results are consistent with the magnetic topological quantum chemistry [Elcoro et al., Nat. Comm. 12, 5965 (2021)]. Using the CRs and MBRs, we reproduce the symmetry-based classifications for MSGs, and we obtain a set of Fu-Kane-like formulas of symmetry indicators (SIs) in both spinless (bosonic) and spinful (fermionic) systems, which are implemented in an automatic code - TopMat - to diagnose topological magnetic materials. The magnetic topological materials, whose occupied states can not be decomposed into a sum of MBRs, are consistent with nonzero SIs. Lastly, using our online code, we have performed spin-polarized calculations for magnetic compounds in the materials database and find many magnetic topological candidates.

cond-mat.mtrl-sci↗

arXiv2022

Interaction Driven Topological Phase Transition in Monolayer CrCl$_2$(pyrazine)$_2$

The quadratic band crossing points (QBCPs) at Fermi level in two-dimension have been proposed to be unstable under electron-electron interaction. The possible interaction driven states include quantum anomalous Hall (QAH) state and various nematic ordered states. In this work, motivated by the discovery of ferromagnetic van der Waals layered metal-organic framework CrCl$_2$(pyrazine)$_2$, we theoretically propose that the single layer of CrCl$_2$(pyrazine)$_2$ might realize one or some of these interaction driven states based on the QBCP protected by $C_4$ symmetry. By introducing the short-range density-density type repulsion interactions into this system, we have found the phase diagram depending on different interaction range and strength. The exotic phases include the staggered chiral flux state manifesting QAH effect, the site-nematic insulator and the site-nematic Dirac semimetal state. The QAH state is robust against perturbations breaking the QBCP but it is weakened by increasing temperature. The metal-organic framework is tunable by changing the transition-metal elements, which might improve the gap size and stability of this interaction induced QAH state.

cond-mat.mtrl-sci↗

arXiv2022

Twisted nodal wires and three-dimensional quantum spin Hall effect in distorted square-net compounds

Recently, square-net materials have attracted lots of attention for the Dirac semimetal phase with negligible spin-orbit coupling (SOC) gap, e.g. ZrSiS/LaSbTe and CaMnSb$_2$. In this paper, we demonstrate that the Jahn-Teller effect enlarges the nontrivial SOC gap in the distorted structure, e.g. LaAsS and SrZnSb$_2$. Its distorted $X$ square-net layer ($X=$ P, As, Sb, Bi) resembles a quantum spin Hall (QSH) insulator. Since these QSH layers are simply stacked in the $\hat{x}$ direction and weakly coupled, three-dimensional QSH effect can be expected in these distorted materials, such as insulating compounds CeAs$_{1+x}$Se$_{1-y}$ and EuCdSb$_2$. Our detailed calculations show that it hosts two twisted nodal wires without SOC [each consists of two noncontractible time-reversal symmetry- and inversion symmetry-protected nodal lines touching at a fourfold degenerate point], while with SOC it becomes a topological crystalline insulator with symmetry indicators $(000; 2)$ and mirror Chern numbers $(0, 0)$. The nontrivial band topology is characterized by a generalized spin Chern number $C_{s+}=2$ when there is a gap between two sets of $\hat{s}_{x}$ eigenvalues. The nontrivial topology of these materials can be well reproduced by our tight-binding model and the calculated spin Hall conductivity is quantized to $σ^{x}_{yz} = (\frac{\hbar}{e})\frac{G_xe^2}{πh}$ with $G_x$ a reciprocal lattice vector.

cond-mat.mtrl-sci↗

arXiv2022

Unprotected quadratic band crossing points and quantum anomalous Hall effect in FeB2 monolayer

Quadratic band crossing points (QBCPs) and quantum anomalous Hall effect (QAHE) have attracted the attention of both theoretical and experimental researchers in recent years. Based on first-principle calculations, we find that the FeB$_2$ monolayer is a nonmagnetic semimetal with QBCPs at $K$. Through symmetry analysis and $\mathbf{k}\cdot\mathbf{p}$ invariant theory, we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with same chirality (Berry phase of $2π$). Once introducing Coulomb interactions, we find that there is a spontaneous-time-reversal-breaking instability of the spinful QBCPs, which gives rise to a $C=2$ QAH insulator with orbital moment ordering.

cond-mat.mes-hall↗

arXiv2022

Revisiting the transient coarsening kinetics: a new framework in the Lifshitz-Slyozov-Wagner space

Phase coarsening is a fundamental process of microstructure evolution in multiphase materials. A thorough understanding of its kinetics is of great significance for material processing and performance. Generally, coarsening can be divided into the transient stage and the steady stage. Compared with steady coarsening kinetics, the current understanding of transient coarsening is rather limited and contradictory. In the present work, a new framework in the dimensionless Lifshitz-Slyozov-Wagner space is developed to study transient coarsening kinetics co-controlled by interface migration/reaction and matrix diffusion, where the dynamic equation for individual particles is derived from the thermodynamic extremal principle.

cond-mat.mtrl-sci↗

arXiv2022

Multiple mobile excitons manifested as sidebands in quasi-one-dimensional metallic TaSe3

Charge neutrality and their expected itinerant nature makes excitons potential transmitters of information. However, exciton mobility remains inaccessible to traditional optical experiments that only create and detect excitons with negligible momentum. Here, using angle-resolved photoemission spectroscopy, we detect dispersing excitons in the quasi-one-dimensional metallic trichalcogenide, TaSe3. The low density of conduction electrons and the low dimensionality in TaSe3 combined with a polaronic renormalization of the conduction band and the poorly screened interaction between these polarons and photo-induced valence holes leads to various excitonic bound states that we interpret as intrachain and interchain excitons, and possibly trions. The thresholds for the formation of a photo-hole together with an exciton appear as side valence bands with dispersions nearly parallel to the main valence band, but shifted to lower excitation energies. The energy separation between side and main valence bands can be controlled by surface doping, enabling the tuning of certain exciton properties.

cond-mat.str-el↗

arXiv2022

Unconventional Materials: the mismatch between electronic charge centers and atomic positions

The complete band representations (BRs) have been constructed in the work of topological quantum chemistry. Each BR is expressed by either a localized orbital at a Wyckoff site in real space, or by a set of irreducible representations in momentum space. In this work, we define unconventional materials with a common feature of the mismatch between average electronic centers and atomic positions. They can be effectively diagnosed as whose occupied bands can be expressed as a sum of elementary BRs (eBRs), but not a sum of atomic-orbital-induced BRs (aBRs). The existence of an essential BR at an empty site is described by nonzero real-space invariants (RSIs). The "valence" states can be derived by the aBR decomposition, and unconventional materials are supposed to have an uncompensated total "valence" state. The high-throughput screening for unconventional materials has been performed through the first-principles calculations. We have discovered 423 unconventional compounds, including thermoelectronic materials, higher-order topological insulators, electrides, hydrogen storage materials, hydrogen evolution reaction electrocatalysts, electrodes, and superconductors. The diversity of these interesting properties and applications would be widely studied in the future.

cond-mat.mtrl-sci↗

arXiv2022

In-situ comparison of interface instability of basal and edge planes during unidirectional growth of sea ice

The unique anisotropy of ice has endowed sea ice growth a peculiar and attractive subject from both fundamental and applied viewpoints. The distinct growth behaviors between edge and basal plane of ice are one of the central topics in ice growth. And the unidirectional freezing pattern stems from perturbations of both basal and edge planes. To date there is no direct comparison of unidirectional freezing behavior between basal and edge plane ice. Here, we in-situ investigate the planar instability as well as the unidirectional freezing pattern of basal and edge planes of ice by a design of parallel freezing samples with specified ice orientations in a NaCl solution as a modeled sea water. The planar instability is discussed via neutral stability curves with surface tension anisotropy for both basal and edge plane ice. For the first time, we realize the simultaneous observation of solid/liquid interfaces of basal and edge plane ice under the same set of freezing conditions. The results show that planar instability occurs faster for edge plane ice than basal plane ice. The time-lapse observations confirm a transient competitive interaction of perturbations between the basal and edge planes ice, which is explained by the anisotropic growth of perturbations in basal and edge planes of ice. These experimental results provide a link between morphology evolution of unidirectional grown sea ice and different ice orientations and are suggested to enrich our understanding of sea ice growth as well as crystallization pattern of other anisotropic materials.

cond-mat.mtrl-sci↗

arXiv2021

Yielding and jerky plasticity of tilt grain boundaries in high-temperature graphene

Graphene is well known for its extraordinary mechanical properties combining brittleness and ductility. While most mechanical studies of graphene focused on the strength and brittle fracture behavior, its ductility, plastic deformation, and the possible brittle-to-ductile transition, which are important for high-temperature mechanical performance and applications, still remain much less understood. Here the mechanical response and deformation dynamics of graphene grain boundaries are investigated through a phase field crystal modeling, showing the pivotal effects of temperature and local dislocation structure. Our results indicate that even at relatively high temperature (around 3350 K), the system is still governed by a brittle fracture and cracking dynamics as found in previous low-temperature experimental and atomistic studies. We also identify another type of failure dynamics with low-angle grain boundary disintegration. When temperature increases a transition to plastic deformation is predicted. The appearance of plastic flow at ultrahigh temperature, particularly the phenomenon of jerky plasticity, is attributed to the stick and climb-glide motion of dislocations around the grain boundary. The corresponding mechanism is intrinsic to two-dimensional systems, and governed by the competition between the driving force of accumulated local stress and the defect pinning effect, without the traditional pathways of dislocation generation needed in three-dimensional materials.

cond-mat.mtrl-sci↗