Density functional theory calculations are conducted to understand and reveal the origin of the complex shear strain morphology and of the polar Moir & eacute; topological pattern recently observed in twisted BaTiO3 bilayers. Our first-principles calculations, along with an original analysis of them allowing the decomposition of the forces into the acoustic and optical contributions, point to the occurrence of forces mostly acting on the acoustic-related motions to produce standing waves of shear strain. Such acoustic waves naturally generate a striking self-organization of the shear strains, and hence create a peculiar gradient of these shear strains. A Moir & eacute; dipole pattern, consisting of the interpenetrated arrays of vortices and antivortices made of the electric dipoles, then mostly arises due to the coupling of this gradient of the shear strain with the electric dipoles. Results and discussion about harmonic versus nonharmonic effects on the strength of these patterns is also provided.

Magnons, the quanta of spin waves, have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. Here we demonstrate highly efficient magnon transport in a LaFeO3/BiFeO3/LaFeO3 all-antiferromagnetic system, which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin-orbit-driven spin-charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, the non-volatility of the output voltage enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.

We revisit first-principles predictions of structural, ferroelectric, and electronic properties in aluminum-based III-V nitride alloys, focusing on Al1-xScxN and Al1-xBxN. Using density functional theory within a unified 48-atom supercell framework, we systematically assess the role of chemical disorder and exchange-correlation approximations by comparing the virtual crystal approximation (VCA) and special quasirandom structures (SQS), as well as PBE, PBESol, SCAN, and SCAN+rVV10 functionals. We demonstrate that, even amongst the similar PBE and PBESol functionals, big quantitative and qualitative differences emerge. In particular, the VCA or SQS PBESol (a popular functional) strongly underestimate the stability domain of the ferroelectric wurtzite phase in Al1-xScxN compared to SQS PBE or SQS SCAN. We demonstrate that the 5-fold coordinated hexagonal phase predicted in 2002 by Farrer and Bellaiche [Phys. Rev. B 66, 201203] is a low-energy metastable state between the four-fold coordinated ferroelectric wurtzite phase and the six-fold coordinated rocksalt phase near the transition point upon increasing the Sc content. In contrast, Al1-xBxN shows a much faster destabilization of the wurtzite ferroelectric phase, with bond breaking which strongly distorts the wurtzite structure (with enhanced polarization) and eventually favor a zincblende phase and a threefold coordinated hexagonal layer phase. Our analysis highlights the critical importance of both local disorder and exchange-correlation treatment in predicting the functional properties of III-V nitride ferroelectrics. Overall, SQS combined with SCAN provides the most consistent theoretical framework for understanding and optimizing emerging nitride-based ferroelectric materials.

Polar skyrmion bubbles are nanoscale ferroelectric domain configurations with swirling polarization textures, and often emerge in ferroelectric oxide systems. Owing to their inhomogeneous polarization patterns, which endow them with distinct topologies and electrical responses from homogeneous monodomains, polar skyrmion bubbles are envisaged to be promising candidates for non-volatile memory devices. In such device, the recorded information density is directly proportional to the density of bubbles, underscoring the need for precise control over bubble nucleation. Here, using first-principles-based calculations, we demonstrate that when assisted with a DC electric field, twisted light, which has a spatially inhomogeneous field pattern, can robustly tune the density of polar skyrmion bubbles in ferroelectric ultrathin films between 102 ~ 104bit/μm2. Moreover, by modulating DC and optical field strengths together with the beam radius, the nucleation rate, which characterizes the creation and annihilation speed of polar skyrmion bubbles, can also be well controlled. These findings highlight the unique response of ferroelectric nanofilms to optical and electric fields, which is crucial for employing polar skyrmion bubbles in the next-generation of ultrahigh-density memory technologies.

Recent theoretical and experimental advances in quantum ferroelectrics suggest that ferroelectricity can also emerge in non-polar space group, highlighting the limitations of conventional polar space group criteria in identifying ferroelectric materials. Here, we introduce a unified definition based on switchable polarization differences between energetically equivalent states, which naturally encompasses conventional and quantum ferroelectrics. Guided by this principle, we implement a high-throughput screening strategy that systematically identifies both conventional and quantum ferroelectrics among experimentally synthesized materials. In particular, we identify a new type of quantum ferroelectric in which the quantized polarization arises from arbitrary ionic displacements, in contrast to previous quantum ferroelectrics (including both fractional and integer quantum ferroelectrics) where quantized polarization results from fractional or integer ionic displacements. Notably, we find that materials such as Ba3I6 and Cs2PdC2 exhibit low switching barriers and robust insulating behavior, highlighting their experimental viability. Our results reconcile conventional and quantum ferroelectrics, expand the accessible materials landscape, and provide a practical roadmap for discovering next-generation ferroelectrics with advanced switchable functionalities.

Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a multiferroic phase of BiFeO3 that not only sustains both order parameters at room temperature with no dead layer but also exhibits signatures of emergent altermagnetism in the four-unit-cell, ultrathin limit. First-principles calculations, spin symmetry analysis, atomic-resolution imaging, and angle-resolved magnetic imaging reveal that short-circuit electrostatic boundary conditions, together with epitaxial strain, drive a continuous second-order, thickness-driven phase transition that enables the formation of multiferroic topological textures. Moreover, the imposed boundary conditions stabilize a d-wave altermagnetic time-reversal symmetry breaking, with corresponding signatures observed in magnetic circular dichroism. Collectively, these results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.

The study of light-matter interaction in ferroelectric and related materials is a rapidly evolving field, bridging state-of-the-art optical techniques with widely used functional materials. This research area leverages advanced experimental methodologies, particularly ultrafast light pulses, to probe the optical properties and complex dynamic behaviors of these materials under light-induced conditions. This review summarizes recent advances in the interaction of light with ferroelectric and related materials, emphasizing breakthroughs in simulations and their implications for material design. We explore a range of light-induced phenomena across various spectral regions, including photostriction, photoinduced structural phase transitions, and ferroelectric modulation in superlattices under band gap illumination. We delve into terahertz-induced ferroelectric hidden states relevant to neuromorphic computing and examine structured illumination effects, such as ferroelectric solitons and dynamical multiferroicity induced by twisted light, primarily in the terahertz region. Mid-infrared light interactions are discussed, focusing on their resonance with infrared-active phonons and localized vibrational modes. Additionally, we cover studies on natural optical activity and gyrotropy, electro-optic and elasto-optic effects, and magnon-phonon quasiparticles. Furthermore, we provide an overview of the theoretical frameworks and simulation tools that underpin these investigations. This review offers illustrative examples of how light-matter interaction can be used to resonantly control the properties of ferroelectric materials.

Alloyed aluminium nitride compounds constitute a promising class of ferroelectric materials due to their high remanent electric polarizations, large band gaps and structural compatibility with a growth on Si substrates. Such materials nonetheless possess large coercive fields and polarization-switching mechanisms are still debated. We performed first-principles calculations to investigate the stability of isolated point defects in the vicinity of a vertical inversion domain wall (DW). We found that all studied defects are energetically more stable at or near the DW. Depending on their nature, they can have the opposite effect on the displacement of the DW, which occurs during polarization switching. Finally, we discuss how likely the different defects may be responsible for leaking currents and degraded ferroelectric properties.

Reversible ultrafast switching of surface thermodynamics is highly desirable for hydrogen storage and catalysis yet remains elusive at the nanoscale. Here, we demonstrate that photoinduced ferroic-order switching in two-dimensional ionic ferroelectric monolayers enables rapid, reversible control of hydrogen binding. In TiGeSe3, carrier-density-driven redistribution of transition-metal 3d orbital occupations triggers a sequential evolution from the ferroelectric ground state to paraelectric phases with staggered or zig-zag antiferromagnetic order. This switch continuously tunes the hydrogen adsorption free energy from 0.33 to 1.11 eV, shifting the interface from near-thermoneutrality to spontaneous desorption. Nonadiabatic dynamics simulations indicate that electron-phonon coupling promotes nonthermal H release, while picosecond carrier recombination rapidly restores the initial ferroic order, closing an ultrafast reversible cycle. Generality is further validated in AgBiP2Se6 and CuInP2S6, establishing ferroic order as an optically addressable knob for dynamic thermodynamic reconfiguration beyond static design.

Understanding the magnetic properties of rare-earth iron garnet ultrathin films subjected to a strain is of essence from both fundamental science and technological perspectives. In this work, we report on the results of a combined first-principles calculations and classical Monte Carlo simulations study of magnetic properties of gadolinium iron garnet thin-films subjected to a uniaxial strain. We employ first-principles calculations to compute the magnetic exchange coupling constants for thin films subjected to uniaxial strains including both compressive and tensile cases. The magnetic exchange coupling constants were then used to construct an effective magnetic Hamiltonian, which includes symmetry-breaking effects, and perform Monte Carlo simulations. Using the latter, we study the dependences of magnetic properties on temperature and strain in gadolinium iron garnet thin-film systems, subjected to a uniaxial strain. To further advance our understanding of the magnetic behavior, we also consider a simple analytical model, which is based on the N & eacute;el molecular field theory for ferrimagnetics and incorporates magnetic exchange coupling constants taken from the first-principles calculations. The case of uniaxial strains, considered in this work, is compared to the behavior of biaxially-strained thin films, studied in a previous work, and the theoretical results of this work are also compared with available experimental data on rare-earth iron garnets subjected to an external strain. The implications of the obtained results for use of layered rare-earth iron garnets-based materials for magnetic technologies are also discussed.

Gyrotropic effects, including natural optical activity (NOA) and the nonlinear anomalous Hall effect (NAHE), are crucial for advancing optical and transport devices. We explore these effects in the BaTiS3 system, a quasi-one-dimensional crystal that exhibits giant optical anisotropy. (Niu et al. Nat. Photonics 12, 392 (2018); Zhao et al. Chem. Mater. 34, 5680 (2022)). In the P63cm phase which is stable under room temperature, we predict two distinct strain-induced phase transitions: a symmetry-lowering transition from the P63cm to P63 phase under tensile strain, which enhances NOA and enables optical rotation; and an isostructural insulator-to-polar Weyl semimetal (WSM) transition under compressive strain, which activates the NAHE and exhibits a strain-induced sign reversal. The low-temperature P21 phase also transforms into a P212121 phase under enough compressive strains with such phase transition exhibiting a large NOA. All these results highlight BaTiS3 as a viable candidate for novel ferroelectrics, optical and transport devices with strain enhanced or activated gyrotropic properties.

We predict an antipolar instability in hexagonal LaN using first-principles density functional theory. Starting from a nonpolar hexagonal phase, we identify competing polar and antipolar zone-center phonon instabilities. Condensation of the polar and antipolar modes stabilizes, respectively, dynamically stable wurtzite (WZ) phase and an hexagonal antipolar (AP) phase which is characterized by alternating local polarization and zero net macroscopic polarization within the unit cell. At ambient conditions, the AP phase is metastable with respect to the WZ phase, and a finite energy barrier exists between these phases, suggesting a possible polarization-switching pathway via the AP intermediate state. The energy barrier between the WZ and AP phases decreases with increasing pressure, indicating enhanced tunability between polar and antipolar states. The sublattice polarization increases with pressure in the AP phase, while it decreases in the WZ phase. We further find that, with increasing pressure, the rock-salt and tetragonal phases of LaN become more stable than the hexagonal phases (AP and WZ). Consequently, the realization of the AP phase is more favorable in the low-pressure regime, where hexagonal phases remain energetically competitive. These results demonstrate pressure-driven competition between polar and antipolar phases in LaN and point toward antiferroelectric-like behavior in this binary nitride system.

AgNbO_{3} is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group R3, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses-including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics-can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO_{3} but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.

Liquid crystals (LCs) provide a canonical framework to understand intermediate phases between liquids and crystals, such as nematic and smectic order, and the rich phenomenology that emerges when such orders are subjected to boundaries, confinement, and elastic frustration. Here, we reveal a hidden mesoscale liquid-crystal-like organization of spin cycloids in a room-temperature magnetoelectric antiferromagnet BiFeO3. It is observed that ferroelectric domain walls act as anchoring surfaces, locally selecting cycloid propagation vectors analogous to surface induced alignment in LCs. Interestingly, confinement imposed by multiple domain walls stabilizes coherent smectic-like cycloidal order in between domain walls through “order by confinement”, while geometrical frustration at domain boundaries induces non-local Helfrich-Hurault–type elastic instabilities in cycloid smectics. Building on these insights, we show that surface-energy engineering dramatically enhances the long range orientational coherence of spin cycloids, enabling robust non-local magnon transport in the ultra-thin limit. These findings establish self-organization as a guiding principle for engineering novel magnetic textures and emergent spin functionalities, opening new opportunities for spintronic and magnonic technologies.

Achieving ultra-high dielectric tunability with robust temperature and frequency stability poses a key challenge for next-generation microwave electronics and telecommunications devices. Likewise, the integration of such materials with silicon is critical for scalability, yet it remains a complex task. This work addresses these challenges by engineering high-quality, lead-free Ba1- xSrxTiO3 (BST; x = 0.2-0.8) epitaxial thin films. Through systematic control of composition and epitaxial strain, we have experimentally revealed the coexistence of cubic, tetragonal, rhombohedral, and orthorhombic phases, forming a mixed-phase state analogous to a morphotropic phase boundary (MPB). This phase coexistence results in exceptional dielectric properties, including ultra-high tunability (∼91%) and a high breakdown electric field (∼800 kV/cm) at room temperature (10 kHz). The films exhibit good thermal (from 330 to 473 K) and frequency (10 kHz-1 MHz) stability. The robust dielectric tunability being associated with a diffuse-phase transition at higher strontium concentrations, arising from dipole dispersion, leading to relaxor-like behavior. Theoretical studies using effective-Hamiltonian approaches confirm the emergence of the MPB-like state and its role in enhanced dielectric permittivity and tunability. Finally, integration of these BST thin films onto silicon is demonstrated, highlighting the potential for scalability. These findings bridge the gap between material innovation and industrial implementation.

The interplay between epitaxial strain, magnetic compensation, Curie temperature, and anisotropy in ferrimagnetic insulators remains a central question with both fundamental and technological significance. Here, we present a comparative theoretical study of strain effects on Gd3Fe5O12 (GIG) thin films grown along [001] and [111] orientations. Using density functional theory and an ab initio-based effective Hamiltonian, we study the evolution of the magnetic compensation temperature (TM), Curie temperature (TC), and perpendicular magnetic anisotropy (PMA) under epitaxial strain. Our results show a linear enhancement of both TM and TC under compressive strain, with (001)-oriented films exhibiting a notably higher strain sensitivity than (111)-oriented counterparts. PMA was also found to have a nearly linear strain dependence with a strong orientation dependence, and to reverse when Gd 4 f electrons were treated as core states-hinting at a possible strong temperature dependence of PMA. Our results offer clarity to experimental variability in TM and provide a microscopic basis for tailoring magnetic compensation, Curie temperature and anisotropy in rare-earth-based spintronics.

ABSTRACT The Rashba effect is of central importance for spintronic and quantum technologies, and its controllability enables efficient and tunable spin manipulation. Using first‐principles calculations and molecular dynamics with machine‐learned force fields, we reveal a laser‐pulse‐triggered reversible phase transition from a nonpolar phase lacking spin splitting to a polar phase characterized by Rashba spin splitting in monolayer . This nonpolar‐to‐polar phase transition provides an ultrafast route for switching spin states between a spin‐splitting‐vanishing state and Rashba‐like splitting state. The transition is mainly associated with resonant excitation of infrared‐active shear phonon modes and their subsequent nonlinear coupling to Raman‐active modes under a THz light field. Our findings establish an effective strategy for manipulating Rashba‐like spin splitting in two‐dimensional systems and highlight the potential of ultrafast optical control in spintronics and phase transitions.

AgNbO_3 is a lead-free perovskite with considerable potential for energy storage and optoelectronic applications, yet its low-temperature crystal structure has remained controversial. In this Letter, we revisit its low-energy structural landscape using a systematic first-principles structural search based on symmetry-adapted phonon-mode theory. We uncover a previously unreported chiral ferroelectric phase with space group R3, which exhibits a large spontaneous polarization and a low polarization switching barrier, enabling polarization reversal under electric fields. Crucially, the structural chirality of this phase is intrinsically locked to the ferroelectric polarization, allowing electrical control of the chiral handedness. Consequently, chiral optical responses–including circular dichroism, circular photogalvanic effect, optical activity, and second-order nonlinear optics–can be reversibly switched by an external electric field. These results not only clarify the complex low-temperature structural behavior of AgNbO_3 but also establish a rare purely inorganic platform for electric-field-tunable chirality, opening a pathway toward ultrafast, electrically controlled chiral optoelectronics.