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Wei Ren

Publications and source records attributed to Wei Ren.

At least 109 records · Page 6Linked to original sources
arXiv2023

Polarization description of successive ferroelectric switching in hafnia

Intertwined ionic conduction and ferroelectric (FE) switching in HfO2 lead to extensive focuses. To describe its fundamental phenomena, we present a free-energy model describing the potential of ferroelectrics with successive FE switching paths, and extend the domain model of ionic conduction to ferroelectric domains. Associate theoretical analyses and first-principles calculations suggest a nesting-domain pattern with opposite piezoelectric loops during the nucleation-and-growth process in displacive FE-HfO2. A collective oxygen ion conduction mechanism is also proposed with a field-dependent ionic conductivity following the Merz's law. We conclude that the ionic conductibility is concomitant with the ferroelectricity in HfO2, and it may provide a new venue for pursuing low temperature fast oxide-ion conductors and artificial synapses.

cond-mat.mtrl-sci↗
arXiv2023

Safe Stabilization for Stochastic Time-Delay Systems

This paper addresses the safe stabilization problem of stochastic nonlinear time-delay systems. Based on theKrasovskii approach, we first propose a stochastic control Lyapunov-Krasovskii functional to guarantee the stabilization objective and a stochastic control barrier-Krasovskii functional to ensure the safety objective. Both functionals are developed respectively for each control objectives for the first time. Since the optimization problem is not easy to be resolved for stochastic time-delay systems, we derive a sliding mode based approach to combine the proposed two functionals and to mediate stabilization and safety objectives, which allows to achieve the stabilization objective under the safety requirement. The proposed approach is illustrated via a numerical example.

eess.SY↗
arXiv2023

Large Energy Shifts of Crystal-field Excitations in Erbium Orthoferrite Driven by Internal Magnetic Fields

Due to the complex interactions between rare-earth elements and transition metals, as well as between themselves, rare-earth transition-metal oxides are likely to exhibit highly intriguing and novel magnetic structures and dynamic behaviours. Rare-earth elements in these compounds frequently demonstrate unusual behaviours in their crystal-field (CF) excitations, which necessitate thorough studies for in-depth comprehensions. When cooling from 10 K to 1.5 K through the magnetic ordering temperature of $Er^{3+}$ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of $Er^{3+}$ in erbium orthoferrite ($ErFeO_3$) from 0.32 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for $Er^{3+}$ in $ErFeO_3$ by fitting to the observed CF excitation peaks, which enables to explain all the observed experimental results in a very consistent manner. According to the model, the ground crystal field level of $Er^{3+}$, which corresponds to the lowest Kramers doublet supposed to be at zero energy transfer, has been shifted by the internal magnetic fields induced by both $Er^{3+}$ and $Fe^{3+}$ spin orders below and above the $Er^{3+}$ ordering temperature, respectively. Additional measurements in various magnetic fields offer compelling evidence in favour of this hypothesis. The measured external field dependence of the CF excitation energy led to a derivation of the internal field of $Er^{3+}$ as 0.54 T, which is strongly corroborated by theoretical modelling. Additionally, the g-factor for the $Er^{3+}$ ground state in $ErFeO_3$ shows an exceptionally significant anisotropy.

cond-mat.str-el↗
arXiv2023

Intrinsic Piezoelectric Anisotropy of Tetragonal ABO3 Perovskites: A High-Throughput Study

A comprehensive understand of the intrinsic piezoelectric anisotropy stemming from diverse chemical and physical factors is a key step for the rational design of highly anisotropic materials. We performed high-throughput calculations on tetragonal ABO3 perovskites to investigate the piezoelectricity and the interplay between lattice, displacement, polarization and elasticity. Among the 123 types of perovskites, the structural tetragonality is naturally divided into two categories: normal tetragonal (c/a ratio < 1.1) and super-tetragonal (c/a ratio > 1.17), exhibiting distinct ferroelectric, elastic, and piezoelectric properties. Charge analysis revealed the mechanisms underlying polarization saturation and piezoelectricity suppression in the super-tetragonal region, which also produces an inherent contradiction between high d33 and large piezoelectric anisotropy ratio |d33/d31|. The polarization axis and elastic softness direction jointly determine the maximum longitudinal piezoelectric response d33 direction. The validity and deficiencies of the widely utilized |d33/d31| ratio for representing piezoelectric anisotropy were reevaluated.

cond-mat.mtrl-sci↗
arXiv2023

Giant shifts of crystal-field excitations in ErFeO3 driven by internal magnetic fields

Due to the complex interactions between rare-earth elements and transition metals, as well as/or themselves, rare-earth transition-metal oxides are likely to exhibit highly intriguing and novel magnetic structures and dynamic behaviours. Rare-earth elements in these compounds frequently demonstrate unusual behaviours in their crystal-field (CF) excitations, which necessitate thorough research for in-depth comprehensions. When cooling from 10 K to 1.5 K via the magnetic ordering temperature of Er3+ at 4.1 K, we observed a significant energy shift of the low-lying CF excitation of Er3+ in ErFeO3 from 0.35 meV to 0.75 meV utilizing the inelastic neutron-scattering technique. A sound CF model was proposed for Er3+ in ErFeO3 by fitting to the observed CF excitation peaks, which enables to explain all the observed experimental results in a very consistent manner. According to the model, the ground crystal field level of Er3+, which corresponds to the lowest Kramers doublet supposed to be at zero energy, has been shifted by the internal magnetic fields induced by both Er3+ and Fe3+ spin orders below and above the Er3+ ordering temperature, respectively. Additional measurements in various magnetic fields offer compelling evidence in favour of this hypothesis. The measured external field dependence of the CF excitation energy led to the derivation of the internal field of Er3+ as 0.33 meV, which is strongly corroborated by theoretical modelling. Additionally, the effective g-factor for Er3+ in ErFeO3 showed an exceptionally significant anisotropy.

cond-mat.str-el↗
arXiv2023

Multipiezo effect in altermagnetic V2SeTeO monolayer

Inspired by recent theoretical proposal on the interesting piezomagnetism and C-paired valley polarization in V2Se2O monolayer, we predict a stable antiferromagnetic Janus monolayer V2SeTeO with altermagnetic configuration using density functional theory calculations. It exhibits a novel multi-piezo effect combining piezoelectric, piezovalley and piezomagnetism. Most interestingly, the valley polarization and the net magnetization under strain in V2SeTeO exceed these in V2Se2O, along with the additional large piezoelectric coefficient of e31 (0.322*10-10 C m-1). The multi-piezo effect makes antiferromagnetic Janus monolayer V2SeTeO a tantalizing material for potential applications in nanoelectronics, optoelectronics, spintronics and valleytronics.

cond-mat.mtrl-sci↗
arXiv2023

A Unified Distributed Method for Constrained Networked Optimization via Saddle-Point Dynamics

This paper develops a unified distributed method for solving two classes of constrained networked optimization problems, i.e., optimal consensus problem and resource allocation problem with non-identical set constraints. We first transform these two constrained networked optimization problems into a unified saddle-point problem framework with set constraints. Subsequently, two projection-based primal-dual algorithms via Optimistic Gradient Descent Ascent (OGDA) method and Extra-gradient (EG) method are developed for solving constrained saddle-point problems. It is shown that the developed algorithms achieve exact convergence to a saddle point with an ergodic convergence rate $O(1/k)$ for general convex-concave functions. Based on the proposed primal-dual algorithms via saddle-point dynamics, we develop unified distributed algorithm design and convergence analysis for these two networked optimization problems. Finally, two numerical examples are presented to demonstrate the theoretical results.

math.OC↗
arXiv2023

The curvature-induced magnetization in CrI3 bilayer: flexomagnetic effect enhancement in van der Waals antiferromagnets

The bilayer of CrI3 is a prototypical van der Waals 2D antiferromagnetic material with magnetoelectric effect. It is not generally known, however, that for symmetry reasons the flexomagnetic effect, i.e., the strain gradient-induced magnetization, is also possible in this material. In the present paper, based on the first principle calculations, we estimate the flexomagnetic effect to be 200 μBÅ that is two orders of magnitude higher than it was predicted for the referent antiperovskite flexomagnetic material Mn3GaN. The two major factors of flexomagnetic effect enhancement related to the peculiarities of antiferromagnetic structure of van der Waals magnets is revealed: the strain-dependent ferromagnetic coupling in each layer and large interlayer distance separating antiferromagnetically coupled ions. Since 2D systems are naturally prone to mechanical deformation, the emerging field of flexomagnetism is of special interest for application in spintronics of van der Waals materials and straintronics in particular.

cond-mat.mtrl-sci↗
arXiv2023

The structural stability and polarization analysis of rhombohedral phase HfO2

A comparative theoretical study is presented for the rhombohedral R3 and R3m phase HfO2, of two possible forms in its heavily Zr-doped ferroelectric thin films found recently in experiments. Their structural stability and polarization under the in-plane compressive strain are comprehensively investigated. We discovered that there is a phase transition from R3 to R3m phase under the biaxial compressive strain. Both the direction and amplitude of their polarization can be tuned by the strain. By performing a symmetry mode analysis, we are able to understand its improper nature of the ferroelectricity. These results may help to shed light on the understanding of the hafnia ferroelectric thin films.

cond-mat.mtrl-sci↗
arXiv2023

Terahertz field-driven magnon upconversion in an antiferromagnet

Tailored light excitation and nonlinear control of lattice vibrations have emerged as powerful strategies to manipulate the properties of quantum materials out of equilibrium. Generalizing the use of coherent phonon-phonon interactions to nonlinear couplings among other types of collective modes would open unprecedented opportunities in the design of novel dynamic functionalities in solids. For example, the collective excitations of magnetic order -- magnons -- can carry information with little energy dissipation, and their coherent and nonlinear control would provide an attractive route to achieve collective-mode-based information processing and storage in forthcoming spintronics and magnonics. Here, we discover that intense terahertz (THz) fields can initiate processes of magnon upconversion mediated by an intermediate magnetic resonance. By using a suite of advanced spectroscopic tools, including a newly demonstrated two-dimensional (2D) THz polarimetry technique enabled by single-shot detection, we unveil the unidirectional nature of coupling between distinct magnon modes of a canted antiferromagnet. Calculations of spin dynamics further suggest that this coupling is a universal feature of antiferromagnets with canted magnetic moments. These results demonstrate a route to inducing desirable energy transfer pathways and THz-induced coupling between coherent magnons in solids and pave the way for a new era in the development of ultrafast control of magnetism.

cond-mat.mtrl-sci↗
arXiv2023

Intertwined charge and pair density orders in a monolayer high-Tc iron-based superconductor

Symmetry-breaking electronic phase in unconventional high-temperature (high-Tc) superconductors is a fascinating issue in condensed-matter physics, among which the most attractive phases are charge density wave (CDW) phase with four unit-cell periodicity in cuprates and nematic phase breaking the C4 rotational symmetry in iron-based superconductors (FeSCs). Recently, pair density wave (PDW), an exotic superconducting phase with non-zero momentum Cooper pairs, has been observed in high-Tc cuprates and the monolayer FeSC. However, the interplay between the CDW, PDW and nematic phase remains to be explored. Here, using scanning tunneling microscopy/spectroscopy, we detected commensurate CDW and CDW-induced PDW orders with the same period of lambda = 4aFe (aFe is the distance between neighboring Fe atoms) in a monolayer high-Tc Fe(Te,Se) film grown on SrTiO3(001) substrate. Further analyses demonstrate the observed CDW is a smectic order, which breaks both translation and C4 rotational symmetry. Moreover, the smecticity of the CDW order is strongest near the superconducting gap but weakens near defects and in an applied magnetic field, indicating the interplay between the smectic CDW and PDW orders. Our works provide a new platform to study the intertwined orders and their interactions in high-Tc superconductors.

cond-mat.supr-con↗
arXiv2023

Collatz Dynamics is Partitioned by Residue Class Regularly

We propose Reduced Collatz Conjecture that is equivalent to Collatz Conjecture, which states that every positive integer can return to an integer less than it, instead of 1. Reduced Collatz Conjecture is easier to explore because certain structures must be presented in reduced dynamics, rather than in original dynamics (as original dynamics is a mixture of original dynamics). Reduced dynamics is a computation sequence from starting integer to the first integer less than it, in terms of ``I'' that represents (3*x+1)/2 and ``O'' that represents x/2. We formally prove that all positive integers are partitioned into two halves and either presents ``I'' or ``O'' in next ongoing computation. More specifically, (1) if any positive integer x that is i module $2^t$ (i is an odd integer) is given, then the first t computations (each one is either ``I'' or ``O'' corresponding to whether current integer is odd or even) will be identical with that of i. (2) If current integer after t computations (in terms of ``I'' or ``O'') is less than x, then reduced dynamics of x is available. Otherwise, the residue class of x (namely, i module $2^t$) can be partitioned into two halves (namely, i module $2^{t+1}$ and $i+2^t$ module $2^{t+1}$), and either half presents ``I'' or ``O'' in intermediately forthcoming (t+1)-th computation. This discovery will be helpful to the final proof of Collatz conjecture - if the union of residue classes who present reduced dynamics that become larger with the growth of residue module, equals all positive integers asymptotically, then Reduced Collatz Conjecture (or equivalently, Collatz Conjecture) will be true.

math.GM↗
arXiv2023

Spin-Dependent Graph Neural Network Potential for Magnetic Materials

The development of machine learning interatomic potentials has immensely contributed to the accuracy of simulations of molecules and crystals. However, creating interatomic potentials for magnetic systems that account for both magnetic moments and structural degrees of freedom remains a challenge. This work introduces SpinGNN, a spin-dependent interatomic potential approach that employs the graph neural network (GNN) to describe magnetic systems. SpinGNN consists of two types of edge GNNs: Heisenberg edge GNN (HEGNN) and spin-distance edge GNN (SEGNN). HEGNN is tailored to capture Heisenberg-type spin-lattice interactions, while SEGNN accurately models multi-body and high-order spin-lattice coupling. The effectiveness of SpinGNN is demonstrated by its exceptional precision in fitting a high-order spin Hamiltonian and two complex spin-lattice Hamiltonians with great precision. Furthermore, it successfully models the subtle spin-lattice coupling in BiFeO3 and performs large-scale spin-lattice dynamics simulations, predicting its antiferromagnetic ground state, magnetic phase transition, and domain wall energy landscape with high accuracy. Our study broadens the scope of graph neural network potentials to magnetic systems, serving as a foundation for carrying out large-scale spin-lattice dynamic simulations of such systems.

physics.comp-ph↗
arXiv2023

Gorenstein homological dimension and some invariants of groups

For any group $G$, the Gorenstein homological dimension ${\rm Ghd}_RG$ is defined to be the Gorenstein flat dimension of the coefficient ring $R$, which is considered as an $RG$-module with trivial group action. We prove that ${\rm Ghd}_RG < \infty$ if and only if the Gorenstein flat dimension of any $RG$-module is finite, if and only if there exists an $R$-pure $RG$-monic $R\rightarrow A$ with $A$ being $R$-flat and ${\rm Ghd}_RG = {\rm fd}_{RG}A$, where $R$ is a commutative ring with finite Gorenstein weak global dimension. As applications, properties of ${\rm Ghd}$ on subgroup, quotient group, extension of groups as well as Weyl group are investigated. Moreover, we compare the relations between some invariants such as ${\rm sfli}RG$, ${\rm silf}RG$, ${\rm spli}RG$, ${\rm silp}RG$, and Gorenstein projective, Gorenstein flat and PGF dimensions of $RG$-modules; a sufficient condition for Gorenstein projective-flat problem over group rings is given.

math.AC↗
arXiv2023

Reduced Collatz Dynamics is Periodical and the Period Equals 2 to the Power of the Count of x/2

In this paper, we prove that reduced dynamics on Collatz conjecture is periodical, and its period equals 2 to the power of the count of x/2 computation in the reduced dynamics. More specifically, if there exists reduced dynamics of x (that is, start from an integer x and the computation will go to an integer less than x), then there must also exist reduced dynamics of x+P (that is, if starting from an integer x+P, then computation will go to an integer less than x+P), where P equals 2 to the power of L, and L is the total count of x/2 computations (i.e., computational times) in the reduced dynamics of x (note that, equivalently, L is also the length of the reduced dynamics of x). Therefore, the power (or output) of this period property, which is discovered and proved in this paper, is - the study of the existence of reduced dynamics of x will result in the existence of reduced dynamics of x+P (and iteratively x+n*P, n is a positive integer). Hence, only partition of integers needs to be verified for the existence of their reduced dynamics. Finally, if any starting integer x can be verified for the existence of its reduced dynamics, then Collatz Conjecture will be True (due to our proposed Reduced Collatz Conjecture).

math.GM↗
arXiv2023

Minimal-time Deadbeat Consensus and Individual Disagreement Degree Prediction for High-order Linear Multi-agent Systems

In this paper, a Hankel matrix-based fully distributed algorithm is proposed to address a minimal-time deadbeat consensus prediction problem for discrete-time high-order multi-agent systems (MASs). Therein, each agent can predict the consensus value with the minimum number of observable historical outputs of its own. Accordingly, compared to most existing algorithms only yielding asymptotic convergence, the present method can attain deadbeat consensus instead. Moreover, based on the consensus value prediction, instant individual disagreement degree value of MASs can be calculated in advance as well. Sufficient conditions are derived to guarantee both the minimal-time deadbeat consensus and the instant individual disagreement degree prediction. Finally, both the effectiveness and superiority of the proposed deadbeat consensus algorithm are substantiated by numerical simulations.

eess.SY↗
arXiv2023

Structural disorder by octahedral tilting in inorganic halide perovskites: New insight with Bayesian optimization

Structural disorder is common in metal-halide perovskites and important for understanding the functional properties of these materials. First-principles methods can address structure variation on the atomistic scale, but they are often limited by the lack of structure-sampling schemes required to characterize the disorder. In this work, structural disorder in the benchmark inorganic halide perovskites CsPbI$_3^{}$ and CsPbBr$_3^{}$ is computationally studied in terms of the three octahedral-tilting angles. The consequent variation in energetics and properties are described by three-dimensional potential-energy surfaces (PESs) and property landscapes, delivered by Bayesian Optimization Structure Search method with integrated density-functional-theory (DFT) calculations. The rapid convergence of the PES with about 200 DFT data points in three-dimensional searches demonstrates the power of active learning and strategic sampling with Bayesian optimization. Further analysis indicates that disorder grows with increasing temperature, and reveals that the materials band gap at finite temperatures is a statistical mean over disordered structures.

cond-mat.mtrl-sci↗
arXiv2023

Molecular dynamics simulation of the transformation of Fe-Co alloy by machine learning force field based on atomic cluster expansion

The force field describing the calculated interaction between atoms or molecules is the key to the accuracy of many molecular dynamics (MD) simulation results. Compared with traditional or semi-empirical force fields, machine learning force fields have the advantages of faster speed and higher precision. We have employed the method of atomic cluster expansion (ACE) combined with first-principles density functional theory (DFT) calculations for machine learning, and successfully obtained the force field of the binary Fe-Co alloy. Molecular dynamics simulations of Fe-Co alloy carried out using this ACE force field predicted the correct phase transition range of Fe-Co alloy.

physics.comp-ph↗