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Laurent Bellaiche

Publications and source records attributed to Laurent Bellaiche.

At least 19 recordsLinked to original sources
arXiv2026

Uncovering Antipolar Ordering and Pressure-Tunable Phases in Hexagonal LaN

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.

cond-mat.mtrl-sci↗
arXiv2026

Interaction between point defects and vertical inversion domain walls in wurtzite AlN

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.

cond-mat.mtrl-sci↗
arXiv2026

Robust control over polar skyrmion bubble density with a combined optical and electrical approach

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 $10^2\sim 10^4 \rm{bit}/μm^2$. 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.

cond-mat.mtrl-sci↗
arXiv2026

Systematic comparison of approximations and functionals in first-principle calculations of aluminum-based III-V ferroelectric nitrides

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.

cond-mat.mtrl-sci↗
arXiv2026

Light-Driven Ferroic Switching Enables Reversible Control of Hydrogen Adsorption Thermodynamics

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 TiGeSe$_3$, carrier-density-driven redistribution of transition-metal 3\textit{d} 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 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 AgBiP$_2$Se$_6$ and CuInP$_2$S$_6$, establishing ferroic order as an optically addressable knob for dynamic thermodynamic reconfiguration beyond static design.

cond-mat.mtrl-sci↗
arXiv2026

Domain-Wall Mediated Polarization Switching in Ferroelectric AlScN: Strain Relief and Field-Dependent Dynamics

While scandium-doped aluminum nitride (AlScN) exhibits robust ferroelectricity and excellent thermal stability, its utility is limited by an exceptionally high coercive field ($E_c$) for polarization switching. Unraveling the atomistic switching dynamics is therefore critical for tailoring $E_c$. Here, we combine density functional theory and machine-learning molecular dynamics to elucidate the polarization switching mechanisms in AlScN over various Sc concentrations and applied electric fields. We find that excessive lattice strain strictly prohibits collective polarization switching, but the pre-existing domain walls relieve strain and lead to a distinct switching dynamics -- dictating a field-dependent switching mechanism. At low electric fields, switching occurs via gradual domain-wall propagation consistent with the Kolmogorov-Avrami-Ishibashi model. In contrast, high fields stimulate additional nucleation, driving a rapid, homogeneous reversal process described by the simultaneous non-linear nucleation and growth model. These findings highlight the critical role of domain-wall dynamics and suggest domain engineering as a viable strategy to tailor coercive fields in AlScN and related ferroelectrics.

cond-mat.mtrl-sci↗
arXiv2026

Unified definition of ferroelectricity

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.

cond-mat.mtrl-sci↗
arXiv2026

Unravelling spontaneous Bloch-type skyrmion in centrosymmetric two-dimensional magnets

The realization of magnetic skyrmions in two-dimensional (2D) magnets holds great promise for both fundamental research and device applications. Despite recent progress, two-dimensional skyrmion hosts are still limited, due to the fact that most 2D magnets are centrosymmetric and thus lack Dzyaloshinskii-Moriya interaction (DMI). We show here, using a general analysis based on symmetry, that Bloch-type skyrmions can, in fact, be stabilized in 2D magnets, due to the interplay between in-plane component (dx) of second nearest-neighbor DMI and magnetic anisotropy. Its validity is demonstrated in the Cr2Ge2Te6 monolayer, which is also verified by recent experiments. Our work gives a clear direction for experimental studies of 2D magnetic materials to stabilize skyrmions and should greatly enrich the research on magnetic skyrmions in 2D lattices.

cond-mat.mtrl-sci↗
arXiv2026

Hidden Chiral Ferroelectricity in AgNbO$_3$ Perovskite

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.

cond-mat.mtrl-sci↗
arXiv2026

Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

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.

cond-mat.mtrl-sci↗
arXiv2026

Anisotropic magnon transport in an antiferromagnetic trilayer heterostructure: is BiFeO$_3$ an altermagnet?

Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which to control the stack by means of an electric field. We discuss the mechanism of the anisotropic spin transport based on the interaction between the antiferromagnetic order and the electric field. This allows us to manipulate and amplify the spin transport in such a confined geometry. Furthermore, lower crystal symmetric and suppression of the spin cycloid in ultrathin BiFeO$_3$ stabilizes a non-trivial antiferromagnetic state exhibiting symmetry-protected spin-split bands that provide the non-trivial sign inversion of the spin current, which is a characteristic of an altermagnet. This work provides an understanding of the anisotropic spin transport in complex antiferromagnetic heterostructures where ferroelectricity and altermagnetism coexist, paving the way for a new route to realize electric-field control of a novel state of magnetism.

cond-mat.mes-hall↗
arXiv2025

Electrically switchable non-relativistic Zeeman spin splittings in collinear antiferromagnets

Magnetic or electrical manipulation of electronic spin is elementary for spin-based logic, computing, and memory, where the latter is a low-power manipulation scheme. Rashba-like spin splittings stemming from spin-orbit interaction (SOI) enable electric-field manipulation of spin, but the relativistic SOI causes spin relaxations and yields dissipative transport of spin-encoded information. Recent works suggest the occurrence of electric-field switchable non-relativistic Zeeman spin splittings (NRZSSs) in collinear antiferromagnets -- allowing for electrical manipulation of spin in the non-relativistic regime; yet, a theory elucidating the mechanisms for these NRZSSs and guiding the materials discovery remains missing. Here, we develop such a theory by analyzing the symmetries of spin point groups. We highlight the linear magnetoelectric and bilinear piezomagnetoelectric mechanisms for NRZSSs that depend linearly on electric field and are electrically switchable. First-principles calculations further confirm that FeOOH and NaMnP showcase such NRZSSs. Our theory provides guidelines for discovering light-element collinear antiferromagnets with electrically switchable NRZSSs, which are promising for the design of high-performance spin-based devices.

cond-mat.mtrl-sci↗
arXiv2025

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↗
arXiv2025

On the origin of the unusual strain morphologies and polar Moiré patterns in twisted ferroelectrics

Density functional theory calculations are conducted to understand and reveal the origin of the complex shear strain morphology and of the polar Moiré topological pattern recently observed in twisted BaTiO$_3$ bilayers. Our first-principles calculations, along with an original analysis of them allowing the decomposition of forces into the acoustic and optical contributions, point out to the occurrence of forces mostly acting on the {\it acoustic-related} motions to produce the standing waves of the 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 Moiré 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. Furthermore, other forces, namely acting on the motions associated with the {\it optical phonons}, could also play a role in the formation of these polar vortices and antivortices, but at a smaller extent.

cond-mat.mtrl-sci↗
arXiv2025

Theory of polarization-switchable electrical conductivity anisotropy in nonpolar semiconductors

The anisotropic propagation of particles is a fundamental transport phenomenon in solid state physics. As for crystalline semiconductors, the anisotropic charge transport opens novel designing routes for electronic devices, where the electrical or magnetic manipulation of anisotropic resistance provides essential guarantees. Motivated by the concept of anisotropic magnetoresistance, we develop an original theory on the electrically manipulatable anisotropic electroresistance. We show that piezoelectrics and ferroelectrics may showcase polarization-dependent anisotropic electrical conductivities between two perpendicular directions and the electrical conductivity anisotropy (ECA) is switchable by flipping the polarization. By symmetry analysis, we identify several point groups hosting the polarization-switchable ECA. These point groups simultaneously enable polarization-reversal induced conductivity change along specific directions, akin to the tunnelling electroresistance in ferroelectric tunnel junctions. First-principles-based conductivity calculations predict that piezoelectric AlP and ferroelectric KH$_2$PO$_4$ are two good semiconductors having such exotic charge transport. Our theory can motivate the design of intriguing anisotropic electronic devices (e.g., anisotropic memristor and field effect transistor).

cond-mat.mtrl-sci↗
arXiv2025

Towards a deeper fundamental understanding of (Al,Sc)N ferroelectric nitrides

Density Functional Theory (DFT) calculations, within the virtual crystal alloy approximation, are performed, along with the development of a Landau-type model employing a symmetry-allowed analytical expression of the internal energy and having parameters being determined from first principles, to investigate properties and energetics of Al1-xScxN ferroelectric nitrides in their hexagonal forms. These DFT computations and this model predict the existence of two different types of minima, namely the 4-fold-coordinated wurtzite (WZ) polar structure and a 5-times paraelectric hexagonal phase (to be denoted as H5), for any Sc composition up to 40%. The H5 minimum progressively becomes the lowest energy state within hexagonal symmetry as the Sc concentration increases from 0 to 40%. Furthermore, the model points out to several key findings. Examples include the crucial role of the coupling between polarization and strains to create the WZ minimum, in addition to polar and elastic energies, and that the origin of the H5 state overcoming the WZ phase as the global minimum within hexagonal symmetry when increasing the Sc composition mostly lies in the compositional dependency of only two parameters, one linked to the polarization and another one being purely elastic in nature. Other examples are that forcing Al1-xScxN systems to have no or a weak change in lattice parameters when heating them allows to reproduce well their finite-temperature polar properties, and that a value of the axial ratio close to that of the ideal WZ structure does imply a large polarization at low temperatures but not necessarily at high temperatures because of the ordered-disordered character of the temperature-induced formation of the WZ state. Such findings should allow for a better fundamental understanding of (Al,Sc)N ferroelectric nitrides, which may be used to design efficient devices operating at low voltages.

cond-mat.mtrl-sci↗
arXiv2025

Electro-optic effects in some sliding ferroelectrics

Sliding ferroelectrics, which exhibit out-of-plane polarization arising from specific stacking rather than conventional ionic displacements, are new types of ferroelectrics whose underdeveloped physics needs to be explored. Here, we investigate the electro-optic (EO) response of these materials using first-principles calculations, focusing on ZrI$_{2}$ as a prototype. We reveal that, contrary to conventional ferroelectrics, the EO effect in ZrI$_{2}$ is dominated by its electronic contribution rather than the ionic one, which promises faster EO responses. Furthermore, both biaxial and uniaxial strains significantly enhance this response, and a universal-like linear relationship between the band gap and such response is discovered. We also report a large elasto-optic coefficient that is independent of biaxial strain. Similar large linear EO coefficients and properties are found in other sliding ferroelectrics, including different zirconium dihalides, as well as BN and BP bilayers. These findings highlight sliding ferroelectrics as highly promising candidates for ultrafast nonlinear optical devices and reveal unusual mechanisms.

cond-mat.mtrl-sci↗
arXiv2025

The nature of polar distortions in ferroelectrics

Polar distortion, the collective off-center displacements of atoms, is a fingerprint of a ferroelectric that governs its properties and functionalities. Since the 1970s, the concepts of proper, improper and triggered ferroelectrics have been established to shed light on a diversity of polar distortion mechanisms. Such concepts assign a single nature to polar distortion and are helpful to interpret how polar distortions occur in conventional ferroelectrics such as barium titanate. However, applying these concepts to complex ferroelectrics (e.g., polar orthorhombic hafnia) is notoriously challenging and can yield highly controversial arguments. Here we resolve this issue by developing a tailor-made graph theory for clarifying the nature of polar distortions in complex ferroelectrics, which emphasizes that polar distortions in such ferroelectrics usually exhibit multiple natures among proper, improper and triggered characteristics. We demonstrate the robustness of our theory by working with perovsktie superlattices and polar orthorhombic hafnia (i.e., two representative cases). We successfully identify the mixed proper-improper nature in perovsktite superlattices and reconcile the controversy on polar orthorhombic hafnia by confirming its mixed trigger-improper nature. Our work will definitely lead to a revisitation of concepts in ferroelectric physics and provide opportunities for discovering novel ferroelectrics and related phenomena.

cond-mat.mtrl-sci↗