arXiv subjects

Yurong Yang

Publications and source records attributed to Yurong Yang.

At least 19 recordsLinked to original sources

arXiv2026

Quadratic piezoelectricity from stacking-engineered interference in multilayer sliding ferroelectrics

Designing nonlinear piezoelectricity requires suppressing the linear piezoelectric coefficient without extinguishing higher-order electromechanical response, yet a general and reconfigurable route remains lacking. Here we introduce stacking-engineered piezoelectric interference as such a mechanism in multilayer sliding ferroelectrics. Combining first-principles calculations with a generalized Ginzburg--Landau framework, we show that each interlayer gap acts as a local piezoelectric channel whose sign and magnitude are determined by stacking. Constructive interference between same-signed channels produces a linear-dominated response, whereas destructive interference between oppositely signed channels suppresses the linear coefficient while preserving a finite quadratic response. Representative MoS$_2$ and NiTe$_2$ multilayers approach the parabolic limit, with BAAC-stacked MoS$_2$ reducing the linear-to-quadratic crossover strain by a factor of 25 relative to CBA-stacked MoS$_2$. Experimentally accessible tetralayer MoS$_2$ sliding pathways further connect linear-dominated, quadratic-dominated and sign-inverted states. Here, we identify stacking-engineered interference as a design principle for programmable nonlinear electromechanics in layered materials.

cond-mat.mtrl-sci↗

arXiv2026

Tencent Advertising Algorithm Challenge 2025: All-Modality Generative Recommendation

Generative recommender systems are rapidly emerging as a new paradigm for recommendation, where collaborative identifiers and/or multi-modal content are mapped into discrete token spaces and user behavior is modelled with autoregressive sequence models. Despite progress on multi-modal recommendation datasets, there is still a lack of public benchmarks that jointly offer large-scale, realistic and fully all-modality data designed specifically for generative recommendation (GR) in industrial advertising. To foster research in this direction, we organised the Tencent Advertising Algorithm Challenge 2025, a global competition built on top of two all-modality datasets for GR: TencentGR-1M and TencentGR-10M. Both datasets are constructed from real de-identified Tencent Ads logs and contain rich collaborative IDs and multi-modal representations extracted with state-of-the-art embedding models. The preliminary track (TencentGR-1M) provides 1 million user sequences with up to 100 interacted items each, where each interaction is labeled with exposure and click signals, while the final track (TencentGR-10M) scales this to 10 million users and explicitly distinguishes between click and conversion events at both the sequence and target level. This paper presents the task definition, data construction process, feature schema, baseline GR model, evaluation protocol, and key findings from top-ranked and award-winning solutions. Our datasets focus on multi-modal sequence generation in an advertising setting and introduce weighted evaluation for high-value conversion events. We release our datasets at https://huggingface.co/datasets/TAAC2025 and baseline implementations at https://github.com/TencentAdvertisingAlgorithmCompetition/baseline_2025 to enable future research on all-modality generative recommendation at an industrial scale. The official website is https://algo.qq.com/2025.

cs.IR↗

arXiv2025

Altermagnetic type-II Multiferroics with N\'{e}el-order-locked Electric Polarization

Altermagnetism, an emergent magnetic phase featuring compensated collinear magnetic moments and momentum-dependent spin splittings, has recently garnered widespread interest. A critical issue concerns whether the unconventional spin structures can generate spontaneous electric polarization in altermagnets, thereby achieving type-II multiferroicity. Here, with the combination of symmetry analysis and microscopic theory, we explicitly demonstrate the generation of electric polarization by altermagnetic N\'eel order, and establish a microscopic mechanism of N\'eel-order-locked electric polarization in altermagnetic multiferroics.~We further reveal these pronounced magnetoelectric coupling behaviors and classify them into eight distinct categories for two-dimensional altermagnets governed by layer group symmetries. Then we take monolayer MgFe$_2$N$_2$ as a prototypical example of altermagnetic type-II multiferroics by first-principles calculations. We also propose to identify the N\'eel order and accompanying electric polarization in altermagnetic multiferroics by magneto-optical microscopy. Bridging type-II multiferroics and altermagnets, our work could pave the way for altermagnetic multifunctional spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Giant energy density nitride dielectrics enabled by a paraelectric-metaparaelectric phase transition

Electrostatic dielectric capacitors are foundational to advance the electronics and electric power devices due to their ultrafast charging/discharging capability and high-power density. However, the low energy density limits the potential for next generation devices in terms of miniaturization and integration. We propose a strategy that relies on inducing a field-driven phase transition that we denote paraelectric-metaparaelectric, which yields an ultrahigh energy density in III-nitrides. III-nitride compounds (Al, Sc, B)N with certain cation concentrations possess a nonpolar hexagonal ground phase which could transform into a polar wurtzite phase under a very large electric field, which is denoted as metaparaelectric with nearly null hysteresis P-E loop. This paraelectric-metaparaelectric transition leads to a polarization saturation at large electric field. The corresponding P-E loop displays a giant energy density of 308 J/cm$^3$ with high efficiency nearly 100%. The proposed paraelectric-metaparaelectric phase transition strategy in nitrides opens an avenue to design of next generation high performance dielectrics.

cond-mat.mtrl-sci↗

arXiv2024

Large Enhancement of Properties in Strained Lead-free Multiferroic Solid Solutions with Strong Deviation from Vegard's Law

Efforts to combine the advantages of multiple systems to enhance functionlities through solid solution design present a great challenge due to the constraint imposed by the classical Vegard law. Here, we successfully navigate this trade off by leveraging the synergistic effect of chemical doping and strain engineering in solid solution system of BiFeO3 BaTiO3. Unlike bulks, a significant deviation from the Vegard law accompanying with enhanced multiferroism is observed in the strained solid solution epitaxial films, where we achieve a pronounced tetragonality, enhanced saturated magnetization, substantial polarization, high ferroelectric Curie temperature, all while maintaining impressively low leakage current. These characteristics surpass the properties of their parent BiFeO3 and BaTiO3 films. Moreover, the superior ferroelectricity has never been reported in corresponding bulks. These findings underscore the potential of strained BiFeO3 BaTiO3 films as lead-free, room-temperature multiferroics.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous Hall effect from nonlinear magnetoelectric coupling

The anomalous Hall effect (AHE) is a topology-related transport phenomenon being of potential interest in spintronics, because this effect enables the efficient probe of magnetic orders (i.e., data readout in memory devices). It is well known that AHE spontaneously occurs in ferromagnets or antiferromagnets with magnetization. While recent studies reveal electric-field induced AHE (via linear magnetoelectric coupling), an AHE originating from {\it nonlinear} magnetoelectric coupling remains largely unexplored. Here, by symmetry analysis, we establish the phenomenological theory regarding the spontaneous and electric-field driven AHE in magnets. We show that a large variety of magnetic point groups host an AHE that is driven by uni-axial, bi-axial, or tri-axial electric field and that comes from nonlinear magnetoelectric coupling. Such electric-field driven anomalous Hall conductivities are reversible by reversing the magnetic orders. Furthermore, our first-principles calculations suggest Cr$_2$O$_3$ and CoF$_2$ as candidates hosting the aforementioned AHE. Our work emphasizes the important role of nonlinear magnetoelectric coupling in creating exotic transport phenomena, and offers alternative avenues for the probe of magnetic orders.

cond-mat.mtrl-sci↗

arXiv2024

Superionic Fluoride Gate Dielectrics with Low Diffusion Barrier for Advanced Electronics

Exploration of new dielectrics with large capacitive coupling is an essential topic in modern electronics when conventional dielectrics suffer from the leakage issue near breakdown limit. To address this looming challenge, we demonstrate that rare-earth-metal fluorides with extremely-low ion migration barriers can generally exhibit an excellent capacitive coupling over 20 $\mu$F cm$^{-2}$ (with an equivalent oxide thickness of ~0.15 nm and a large effective dielectric constant near 30) and great compatibility with scalable device manufacturing processes. Such static dielectric capability of superionic fluorides is exemplified by MoS$_2$ transistors exhibiting high on/off current ratios over 10$^8$, ultralow subthreshold swing of 65 mV dec$^{-1}$, and ultralow leakage current density of ~10$^{-6}$ A cm$^{-2}$. Therefore, the fluoride-gated logic inverters can achieve significantly higher static voltage gain values, surpassing ~167, compared to conventional dielectric. Furthermore, the application of fluoride gating enables the demonstration of NAND, NOR, AND, and OR logic circuits with low static energy consumption. Notably, the superconductor-to-insulator transition at the clean-limit Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ can also be realized through fluoride gating. Our findings highlight fluoride dielectrics as a pioneering platform for advanced electronics applications and for tailoring emergent electronic states in condensed matters.

cond-mat.mes-hall↗

arXiv2024

Ultrafast switching of sliding ferroelectricity and dynamical magnetic field in van der Waals bilayer induced by light

Sliding ferroelectricity is a unique type of polarity recently observed in a properly stacked van der Waals bilayer. However, electric-field control of sliding ferroelectricity is hard and could induce large coercive electric fields and serious leakage currents which corrode the ferroelectricity and electronic properties, which are essential for modern two-dimensional electronics and optoelectronics. Here, we proposed laser-pulse deterministic control of sliding ferroelectricity in bilayer h-BN by first principles and molecular dynamics simulation with machine-learned force fields. The laser pulses excite shear modes which exhibit certain directional movements of lateral sliding between bilayers. The vibration of excited modes under laser pulses is predicted to overcome the energy barrier and achieve the switching of sliding ferroelectricity. Furthermore, it is found that three possible sliding transitions - between AB (BA) and BA (AB) stacking - can lead to the occurrence of dynamical magnetic fields along three different directions. Remarkably, the magnetic fields are generated by the simple linear motion of nonmagnetic species, without any need for more exotic (circular, spiral) pathways. Such predictions of deterministic control of sliding ferroelectricity and multi-states of dynamical magnetic field thus expand the potential applications of sliding ferroelectricity in memory and electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Ferroelectrically tunable topological phase transition in In$_2$Se$_3$ thin films

Materials with ferroelectrically switchable topological properties are of interest for both fundamental physics and practical applications. Using first-principles calculations, we find that stacking ferroelectric $\alpha$-In$_2$Se$_3$ monolayers into a bilayer leads to polarization-dependent band structures, which yields polarization-dependent topological properties. Specifically, we find that the states with interlayer ferroelectric couplings are quantum spin Hall insulators, while those with antiferroelectric polarizations are normal insulators. We further find that In$_2$Se$_3$ trilayer and quadlayer exhibit nontrivial band topology as long as in the structure the ferroelectric In$_2$Se$_3$ bilayer is antiferroelectrically coupled to In$_2$Se$_3$ monolayers or other ferroelectric In$_2$Se$_3$ bilayer. Otherwise the system is topologically trivial. The reason is that near the Fermi level the band structure of the ferroelectric In$_2$Se$_3$ bilayer has to be maintained for the nontrivial band topology. This feature can be used to design nontrivial band topology for the thicker films by a proper combination of the interlayer polarization couplings. The topological properties can be ferroelectrically tunable using the dipole locking effect. Our study reveals switchable band topology in a family of natural ferroelectrics, which provide a platform for designing new functional devices.

cond-mat.mtrl-sci↗

arXiv2023

Engineering ferroelectricity in monoclinic hafnia

Ferroelectricity in the complementary metal-oxide semiconductor (CMOS)-compatible hafnia (HfO$_2$) is crucial for the fabrication of high-integration nonvolatile memory devices. However, the capture of ferroelectricity in HfO$_2$ requires the stabilization of thermodynamically-metastable orthorhombic or rhombohedral phases, which entails the introduction of defects (e.g., dopants and vacancies) and pays the price of crystal imperfections, causing unpleasant wake-up and fatigue effects. Here, we report a theoretical strategy on the realization of robust ferroelectricity in HfO$_2$-based ferroelectrics by designing a series of epitaxial (HfO$_2$)$_1$/(CeO$_2$)$_1$ superlattices. The advantages of the designated ferroelectric superlattices are defects free, and most importantly, on the base of the thermodynamically stable monoclinic phase of HfO$_2$. Consequently, this allows the creation of superior ferroelectric properties with an electric polarization $>$25 $\mu$C/cm$^2$ and an ultralow polarization-switching energy barrier at $\sim$2.5 meV/atom. Our work may open an entirely new route towards the fabrication of high-performance HfO$_2$ based ferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2023

Active learning of effective Hamiltonian for super-large-scale atomic structures

The first-principles-based effective Hamiltonian scheme provides one of the most accurate modeling technique for large-scale structures, especially for ferroelectrics. However, the parameterization of the effective Hamiltonian is complicated and can be difficult for some complex systems such as high-entropy perovskites. Here, we propose a general form of effective Hamiltonian and develop an active machine learning approach to parameterize the effective Hamiltonian based on Bayesian linear regression. The parameterization is employed in molecular dynamics simulations with the prediction of energy, forces, stress and their uncertainties at each step, which decides whether first-principles calculations are executed to retrain the parameters. Structures of BaTiO$_3$, Pb(Zr$_{0.75}$Ti$_{0.25}$)O$_3$ and (Pb,Sr)TiO$_3$ system are taken as examples to show the accuracy of this approach, as compared with conventional parametrization method and experiments. This machine learning approach provides a universal and automatic way to compute the effective Hamiltonian parameters for any considered complex systems with super-large-scale (more than $10^7$ atoms) atomic structures.

cond-mat.mtrl-sci↗

arXiv2023

Negative electrocaloric effect in nonpolar phases of perovskite over wide range of temperature

The electrocaloric effect (ECE) offers a promising alternative to the traditional gas compressing refrigeration due to its high efficiency and environmental friendliness. The unusual negative electrocaloric effect refers to the adiabatic temperature drops due to application of electric field, in contrast with the normal (positive) ECE, and provides ways to improve the electrocaloric efficiency in refrigeration cycles. However, negative ECE is unusual and requires a clear understanding of microscopic mechanisms. Here, we found unexpected and extensive negative ECE in nonpolar orthorhombic, tetragonal, and cubic phases of halide and oxide perovskite at wide range of temperature by means of first-principle-based large scale Monte Carlo methods. Such unexpected negative ECE originates from the octahedral tilting related entropy change rather than the polarization entropy change under the application of electric field. Furthermore, a giant negative ECE with temperature change of 8.6 K is found at room temperature. This giant and extensive negative ECE in perovskite opens up new horizon in the research of caloric effects and broadens the electrocaloric refrigeration ways with high efficiency.

cond-mat.mtrl-sci↗

arXiv2023

Flexoelectricity-stabilized ferroelectric phase with enhanced reliability in ultrathin La:HfO2 films

Doped HfO2 thin films exhibit robust ferroelectric properties even for nanometric thicknesses, are compatible with current Si technology and thus have great potential for the revival of integrated ferroelectrics. Phase control and reliability are core issues for their applications. Here we show that, in (111)-oriented 5%La:HfO2 (HLO) epitaxial thin films deposited on (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates, the flexoelectric effect, arising from the strain gradient along the films normal, induces a rhombohedral distortion in the otherwise Pca21 orthorhombic structure. Density functional calculations reveal that the distorted structure is indeed more stable than the pure Pca21 structure, when applying an electric field mimicking the flexoelectric field. This rhombohedral distortion greatly improves the fatigue endurance of HLO thin films by further stabilizing the metastable ferroelectric phase against the transition to the thermodynamically stable non-polar monoclinic phase during repetitive cycling. Our results demonstrate that the flexoelectric effect, though negligibly weak in bulk, is crucial to optimize the structure and properties of doped HfO2 thin films with nanometric thicknesses for integrated ferroelectric applications.

cond-mat.mtrl-sci↗

arXiv2022

Observation of uniaxial strain tuned spin cycloid in a freestanding BiFeO3 film

Non-collinear spin order that breaks space inversion symmetry and allows efficient electric-field control of magnetism makes BiFeO$_3$ a promising candidate for applications in low-power spintronic devices. Epitaxial strain effects have been intensively studied and exhibit significant modulation of the magnetic order in BiFeO$_3$, but tuning its spin structure with continuously varied uniaxial strain is still lacking up to date. Here, we apply \emph{in situ} uniaxial strain to a freestanding BiFeO$_3$ film and use scanning NV microscope to image the nanoscale magnetic order in real-space. The strain is continuously increased from 0\% to 1.5\% and four images under different strains are acquired during this period. The images show that the spin cycloid tilts by $\sim 12.6^\circ$ when strain approaches 1.5\%. A first principle calculation has been processed to show that the tilting is energetically favorable under such strain. Our \emph{in situ} strain applying method in combination with scanning NV microscope real-space imaging ability paves a new way in studying the coupling between magnetic order and strain in BiFeO$_3$ films.

cond-mat.mtrl-sci↗

arXiv2022

Zeeman effect in centrosymmetric antiferromagnets controlled by an electric field

Centrosymmetric antiferromagnetic semiconductors, although abundant in nature, seem less promising than ferromagnets and ferroelectrics for practical applications in semiconductor spintronics. As a matter of fact, the lack of spontaneous polarization and magnetization hinders the efficient utilization of electronic spin in these materials. Here, we propose a paradigm to harness electronic spin in centrosymmetric antiferromagnets via Zeeman spin splittings of electronic energy levels -- termed as spin Zeeman effect -- which is controlled by electric field.By symmetry analysis, we identify twenty-one centrosymmetric antiferromagnetic point groups that accommodate such a spin Zeeman effect. We further predict by first-principles that two antiferromagnetic semiconductors, Fe$_2$TeO$_6$ and SrFe$_2$S$_2$O, are excellent candidates showcasing Zeeman splittings as large as $\sim$55 and $\sim$30 meV, respectively, induced by an electric field of 6 MV/cm. Moreover, the electronic spin magnetization associated to the splitting energy levels can be switched by reversing the electric field. Our work thus sheds light on the electric-field control of electronic spin in antiferromagnets, which broadens the scope of application of centrosymmetric antiferromagnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2017

Cooperative Couplings between Octahedral Rotations and Ferroelectricity in Perovskites

The structure of ABO3 perovskites is dominated by two types of unstable modes, namely, the oxygen octahedral rotation (AFD) and ferroelectric (FE) mode. It is generally believed that such AFD and FE modes tend to compete and suppress each other. Here we use first-principles methods to show that a dual nature of the AFD-FE coupling, which turns from competitive to cooperative as the AFD mode strengthens, occurs in numerous perovskite oxides. We provide a unified model of such a dual interaction by introducing novel high-order coupling terms, and explain the atomistic origin of the resulting new form of ferroelectricity in terms of universal steric mechanisms. We also predict that such a novel form of ferroelectricity leads to atypical behaviors, such as an enhancement of the electric polarization under hydrostatic pressure.

cond-mat.mtrl-sci↗

arXiv2017

Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology - a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption - reminiscent of piezochromism - which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

cond-mat.mtrl-sci↗

arXiv2017

Novel magnetoelectric effects via penta-linear interactions

Magnetoelectric multiferroic materials, particularly with the perovskite structure, are receiving a lot of attention because of their inherent coupling between electrical polarization and magnetic ordering. However, very few types of direct coupling between polarization and magnetization are known, and it is unclear whether they can be useful to the design of novel spintronic devices exploiting the control of magnetization by electric fields. For instance, the typical bi-quadratic coupling only allows to change the magnitude of the magnetization by an electric field, but it does not permit an electric-field-induced switching of the magnetization. Similarly, the so-called Lifshitz invariants allow an electric-field control of complicated magnetic orderings, but not of the magnetization. Here, we report the discovery of novel direct couplings between polarization and magnetization in epitaxial perovskite films, via the use of first-principles methods and the development of an original Landau-type phenomenological theory. Our results feature penta-linear interactions involving the ferromagnetic and anti-ferromagnetic vectors as well as the polar distortions and oxygen octahedral tilting, and permit a number of striking effects. Examples include a continuous electric-field control of the magnetization magnitude and sign, and the discrete switching of the magnetization magnitude. Thus, the high-order, penta-linear couplings demonstrated in this work may open new paths towards novel magneto-electric effects, as well as, spintronic and magnonic devices.

physics.app-ph↗