arXiv subjects

Long-Qing Chen

Publications and source records attributed to Long-Qing Chen.

At least 37 records · Page 2Linked to original sources

arXiv2026

Quantum Saturation of the Electro-Optic Effect

Future quantum computing architectures require electro-optic materials that maintain a strong, stable performance at cryogenic temperatures. In conventional electro-optic materials, large electro-optic coefficients are often confined to narrow temperature windows near structural phase transitions, where small changes in temperature lead to large changes in the electro-optic response. Using thermodynamic analysis, phase-field simulations, experimental growth and cryogenic optical measurements we show that quantum fluctuations can be harnessed to overcome this trade-off. By tuning the ferroelectric phase boundaries down to 0 K, quantum fluctuations induce a saturation regime in which a large electro-optic response becomes nearly temperature-independent below 25 K. We demonstrate that the phase boundaries can be tuned through either strain in BaTiO3 or through chemical composition in Ba1-xCaxTiO3, leading to a large, temperature insensitive, cryogenic electro-optic effect comparable to bulk BaTiO3 at room temperature; the performance exceeds BaTiO3-on-Si by over an order of magnitude. These findings establish a general design principle for engineering high-performance electro-optic materials for cryogenic applications.

cond-mat.mtrl-sci↗

arXiv2026

Chemical heterogeneity at conducting ferroelectric domain walls

Natural interfaces in ferroic oxides have developed into versatile playgrounds for studying electronic correlation effects in 2D systems. The microscopic origin of the emergent local electronic properties is often debated, however, as quantitative atomic-scale characterization remains challenging. A prime example is enhanced conductivity at ferroelectric domain walls, attributed to mechanisms ranging from local band gap reduction to point defect accumulations. Here, we resolve the microscopic mechanisms for domain wall conduction in the ferroelectric model system BiFeO3, by combining transport measurements with atom probe tomography to quantify the local chemical composition and correlate it with the electrical properties. Significant chemical variations along the walls are observed, demonstrating an outstanding chemical flexibility at domain walls, which manifest in spatially varying physical properties. The results give a unifying explanation for the diverse electronic behavior observed and establish the fundamental notion that multiple conduction mechanisms can coexist within individual domain walls.

cond-mat.mtrl-sci↗

arXiv2026

A composite electron-lattice order: electronic nematicity of 2DEG and polarization density waves at a near-ferroelectric interface

We consider a two-dimensional electron gas (2DEG) formed at a near-ferroelectric interface and strongly coupled to polar phonons. Through a self-consistent microscopic many-body calculation, we show that the coupled system stabilizes a composite electron-lattice ordered state in which the lattice polarization spontaneously forms a polarization density wave (PDW), accompanied by an electronic stripe order in the 2DEG. This intertwined order partially reconstructs the electronic spectrum and generates a twofold quasiparticle anisotropy, giving rise to electronic nematicity at the single-particle level. However, under strong external electric fields, the nematic response becomes dominated by the collective sliding dynamics of the composite order: the sliding motion overwhelms the quasiparticle anisotropy and produces a strongly enhanced nematic signal with higher-order angular harmonics. The theory offers a natural explanation for several anomalous transport and anisotropic responses recently observed at the KTaO$_3$ (111) interface. We also estimate the mean-field transition temperature of this emergent ordered state, obtaining good agreement with experiments, and analyze its evolution with several tuning parameters. The proposed composite order, along with the field-induced crossover from quasiparticle-driven to sliding-dominated nematicity, provides a distinct mechanism of nematicity arising from many-body effects and collective dynamics in critical electron-boson systems, with applicability beyond ferroelectric platforms.

cond-mat.str-el↗

arXiv2026

Thermodynamics of Ferroelectric and Optical Properties in KNbO3

Potassium niobate (KNbO3) is a prototypical perovskite ferroelectric with large electro-optic and nonlinear optical responses, high optical damage thresholds and a rich sequence of temperature-driven phase transformations, making it a promising platform for tunable photonic devices. In this work, we develop a thermodynamic model for the coupled ferroelectric and optical properties of KNbO3. By separating the total polarization into lattice and electronic contributions, the model provides a unified description of both the anisotropic ferroelectric and optical properties. The thermodynamic coefficients are determined by fitting to experimental measurements of the spontaneous polarization, dielectric susceptibilities, lattice parameters, and refractive indices. Without any further fitting, the model quantitatively predicts the temperature dependence of the electro-optic and piezoelectric coefficients in close agreement with experimental measurements. By utilizing the electronic polarization equation of motion, the model further captures the optical dispersion in the near infrared to visible spectrum. This work provides the thermodynamic foundation for future studies of coupled ferroelectric and optical phenomena in KNbO3.

cond-mat.mtrl-sci↗

arXiv2026

Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

Epitaxial strain is a powerful means to engineer emergent phenomena in thin films and heterostructures. Here, we demonstrate that KTaO3, a cubic perovskite in bulk form, can be epitaxially strained into a highly tunable ferroelectric. KTaO3 films grown commensurate to SrTiO3 (001) substrates experience an in-plane strain of -2.1 % that transforms the cubic structure into a tetragonal polar phase with transition temperature of 475 K, consistent with our thermodynamic calculations. We show that the Curie temperature and the spontaneous electric polarization can be system- atically controlled with epitaxial strain. Scanning transmission electron microscopy reveals cooperative polar displacements of the potassium columns with respect to the neighboring tantalum columns at room temperature. Optical second-harmonic generation results are described by a tetragonal polar point group (4mm), indicating the emergence of a global polar ground state. We observe a ferroelectric hysteresis response, using metal-insulator-metal capacitor test structures. The results demon- strate a robust intrinsic ferroelectric state in epitaxially strained KTaO3 thin films.

cond-mat.mtrl-sci↗

arXiv2026

A Structure-Preserving Scheme for the Time-Dependent Ginzburg-Landau Model with BCS Gap Coupling

We propose a structure-preserving scheme for a hybrid model that couples the time-dependent Ginzburg-Landau (TDGL) equation of superconducting vortex dynamics and the nonlinear Bardeen-Cooper-Schrieffer (BCS) gap equation. This formulation is consistent with the classical TDGL equation in the near-critical temperature, while extending the applicability of the existing TDGL model to regimes beyond the critical temperature. The resulting system poses significant computational challenges due to its nonlinear and coupled structure. To achieve stable and reliable simulations of the vortex dynamics and accompanying morphological transitions, we develop a maximum bound preserving, energy-stable implicit-explicit (IMEX) scheme. The structure-preserving properties of the scheme are rigorously established, ensuring long-time stability and physical consistency. Through two- and three-dimensional simulations, the hybrid model successfully captures the temporal and spatial formation and alignment of vortices and the suppression of superconductivity under increasing magnetic fields, demonstrating both the accuracy and robustness of the proposed computational approach.

math.NA↗

arXiv2025

Tip-Based Proximity Ferroelectric Switching and Piezoelectric Response in Wurtzite Multilayers

Proximity ferroelectricity is a novel paradigm for inducing ferroelectricity, where a non-ferroelectric polar material, which is unswitchable with an external field below the dielectric breakdown field, becomes a practically switchable ferroelectric in direct contact with a thin switchable ferroelectric layer. Here, we develop a Landau-Ginzburg-Devonshire approach to study the proximity effect of local piezoelectric response and polarization reversal in wurtzite ferroelectric multilayers under a sharp electrically biased tip. Using finite element modeling we analyze the probe-induced nucleation of nanodomains, the features of local polarization hysteresis loops and coercive fields in the Al1-xScxN/AlN bilayers and three-layers. Similar to the wurtzite multilayers sandwiched between two parallel electrodes, the regimes of "proximity switching" (where the multilayers collectively switch) and the regime of "proximity suppression" (where they collectively do not switch) are the only two possible regimes in the probe-electrode geometry. However, the parameters and asymmetry of the local piezo-response and polarization hysteresis loops depend significantly on the sequence of the layers with respect to the probe. The physical mechanism of the proximity ferroelectricity in the local probe geometry is a depolarizing electric field determined by the polarization of the layers and their relative thickness. The field, whose direction is opposite to the polarization vector in the layer(s) with the larger spontaneous polarization (such as AlN), renormalizes the double-well ferroelectric potential to lower the steepness of the switching barrier in the "otherwise unswitchable" polar layers. Tip-based control of domains in otherwise non-ferroelectric layers using proximity ferroelectricity can provide nanoscale control of domain reversal in memory, actuation, sensing and optical applications.

cond-mat.mtrl-sci↗

arXiv2025

Colossal Cryogenic Electro-Optic Response Through Metastability in Strained BaTiO$_{3}$ Thin Films

The search for thin film electro-optic (EO) materials that can retain superior performance under cryogenic conditions has become critical for quantum computing. Barium titanate thin films show large linear EO coefficients in the tetragonal phase at room temperature, which is severely degraded down to ~200 pm V$^{-1}$ in the rhombohedral phase at cryogenic temperatures. There is immense interest in manipulating these phase transformations and retaining superior EO properties down to liquid helium temperature. Utilizing the thermodynamic theory of optical properties, a large low-temperature EO response is designed by engineering the energetic competition between different ferroelectric phases, leading to a low-symmetry monoclinic phase with a massive EO response. The existence of this phase is demonstrated in a strain-tuned BaTiO$_{3}$ thin film that exhibits a linear EO coefficient of 2516 +/- 100 pm V$^{-1}$ at 5 K, which is an order of magnitude higher than the best reported performance thus far. Importantly, the EO coefficient increases by 100x during cooling, unlike the conventional films, where it degrades. Further, at the lowest temperature, significant higher order EO responses also emerge. These results represent a new framework for designing materials with property enhancements by stabilizing highly tunable metastable phases with strain. Copyright 2025 The Author(s). Advanced Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. (A. Suceava, S. Hazra, A. Ross, et al. "Colossal Cryogenic Electro-Optic Response Through Metastability in Strained BaTiO3 Thin Films." Adv. Mater. (2025): e07564. https://doi.org/10.1002/adma.202507564)

cond-mat.mtrl-sci↗

arXiv2025

Symmetry-designed BiFeO3 single domain spin cycloid for efficient spintronics

Deterministic control of coupled ferroelectric and antiferromagnetic orders remains a central challenge in multiferroics, limiting their integration into functional magnetoelectrics and magnonic-devices. (111)pc BiFeO3 with a robust single spin cycloid, offers direct magnetoelectric-coupling and a platform for efficient spin transport, yet multi-magnetic domains and ferroelectric-fatigue have prevented reproducible control. Here, we show that anisotropic-compressive in-plane strain stabilizes a single antiferromagnetic domain with unique spin-cycloid vector, by breaking the symmetry of the (111)pc plane. Epitaxial BiFeO3 films grown on orthorhombic NdGaO3 (011)o [(111)pc] substrates impose the required anisotropic in-plane strain and stabilizes single antiferromagnetic domain, as confirmed through direct imaging with scanning NV microscopy and non-resonant-x-ray-magnetic-scattering. Remarkably, these engineered films exhibit deterministic and non-volatile 180° switching of ferroelectric and single antiferromagnetic domains over 1,000 cycles. The monodomain state also enables anisotropic and threefold enhanced magnon transport with reduced scattering. Thus, symmetry-designed (111)pc monodomain BiFeO3 offers a robust platform for advanced magnetoelectric and magnonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Proximity Ferroelectricity in Compositionally Graded Structures

Proximity ferroelectricity is a novel paradigm for inducing ferroelectricity in a non-ferroelectric polar material such as AlN or ZnO that are typically unswitchable with an external field below their dielectric breakdown field. When placed in direct contact with a thin switchable ferroelectric layer (such as Al1-xScxN or Zn1-xMgxO), they become a practically switchable ferroelectric. Using the thermodynamic Landau-Ginzburg-Devonshire theory, in this work we performed the finite element modeling of the polarization switching in the compositionally graded AlN-Al1-xScxN, ZnO-Zn1-xMgxO and MgO-Zn1-xMgxO structures sandwiched in both a parallel-plate capacitor geometry as well as in a sharp probe-planar electrode geometry. We reveal that the compositionally graded structure allows the simultaneous switching of spontaneous polarization in the whole system by a coercive field significantly lower than the electric breakdown field of unswitchable polar materials. The physical mechanism is the depolarization electric field determined by the gradient of chemical composition "x". The field lowers the steepness of the switching barrier in the otherwise unswitchable parts of the compositionally graded AlN-Al1-xScxN and ZnO-Zn1-xMgxO structures, while it induces a shallow double-well free energy potential in the MgO-like regions of compositionally graded MgO-Zn1-xMgxO structure. Proximity ferroelectric switching of the compositionally graded structures placed in the probe-electrode geometry occurs due to nanodomain formation under the tip. We predict that a gradient of chemical composition "x" significantly lowers effective coercive fields of the compositionally graded AlN-Al1-xScxN and ZnO-Zn1-xMgxO structures compared to the coercive fields of the corresponding multilayers with a uniform chemical composition in each layer.

cond-mat.mtrl-sci↗

arXiv2025

Theory of terahertz pulse transmission through ferroelectric nanomembranes

An analytical model is developed to predict the temporal evolution of the lattice polarization in ferroelectric nanomembranes upon the excitation by a terahertz (THz) electromagnetic pulse of an arbitrary waveform, and the concurrent transmission of the THz pulse in both the linear and the nonlinear regimes. It involves the use of the perturbation method to solve the equation of motion for the lattice polarization in both unclamped and strained ferroelectric nanomembranes within the framework of Landau-Ginzburg-Devonshire theory. The model is applicable to perovskite oxides such as BaTiO3 and SrTiO3, wurtzite Al1-xScxN, and trigonal LiNbO3. Our analytical model provides a theoretical basis for determining the thermodynamic and kinetic parameters of ferroelectric materials through THz transmission experiment. The calculation results also suggest an approach to reversing the chirality of a circularly polarized THz pulse by harnessing the resonant polarization-photon coupling in ferroelectrics. This capability of chirality reversal, along with the high tunability from a strain applied along any arbitrarily oriented in-plane axis, provides new opportunities for THz wave modulation without relying on complex metasurface designs.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz-field activation of polar skyrons

Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discovered subterahertz collective modes, dubbed 'skyrons', which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. Momentum-resolved time-domain measurements of diffuse scattering revealed an avoided crossing in the dispersion relation of skyrons. We further demonstrated that the amplitude and dispersion of skyrons can be controlled by sample temperature and electric-field bias. Atomistic simulations and dynamical phase-field modeling provided microscopic insights into the three-dimensional crystallographic and polarization dynamics. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures.

cond-mat.mtrl-sci↗

arXiv2025

The Influence of Electric Field on the Anisotropic Dispersion of the Flexocoupling Induced Phonons and Ferrons in Van der Waals Ferrielectrics

As has been shown recently, the influence of the flexoelectric coupling (shortly "flexocoupling") on the fluctuations of electric polarization and elastic strains can lead to the principal changes of the dispersion law of soft optical and acoustic phonons (shortly "flexophonons") and ferrons (shortly "flexoferrons") in van der Waals ferrielectrics. Analytical results, derived in the framework of Landau-Ginzburg-Devonshire approach, revealed that the dispersion of flexophonons and flexoferrons is strongly anisotropic and should depend on the magnitude and direction of applied electric field. In this work we study the influence of applied electric field on the anisotropic dispersion of the flexophonons and flexophonons in a uniaxial van der Waals ferrielectric CuInP2S6. We reveal that the frequency of acoustic flexophonons and flexoferrons tends to zero at nonzero wavevectors under increase of applied electric field. We relate the changes with a possible appearance of a spatially modulated incommensurate polar phase induced by the flexocoupling in external field. The critical strength of flexocoupling is determined by the magnitude of electric field and direction of the wavevector. This allows us to propose a method for estimating the strength of flexocoupling in van der Waals ferrielectrics, providing that the frequency of the acoustic flexophonon is zeroing at the threshold value of the electric field. Since the flexoelectric coefficients are poorly known in van der Waals ferrielectrics, obtained analytical results can be useful for their flexo-engineering.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamic stabilization and electronic effects of oxygen vacancies at BiFeO$_3$ neutral ferroelectric domain walls

Enhanced conductivity at ferroelectric domain walls in BiFeO$_3$ has been widely observed, yet the microscopic origins of this effect, including electronic contributions from domain-wall defects, are incompletely understood at the atomistic level. Here, we carry out first-principles simulations to quantify the thermodynamic stability and electronic impact of oxygen vacancies at charge-neutral 71$^\circ$, 109$^\circ$, and 180$^\circ$ domain walls of BiFeO$_3$. We find that vacancies are energetically favored at domain walls by up to 0.3 eV compared to the bulk, leading to orders-of-magnitude increase in vacancy equilibrium concentration. The corresponding formation energy landscapes are discontinued and explained by local bond weakening. The vacancies induce localized electronic intragap states corresponding to small polarons, which promote thermally activated n-type conduction in the low-current regime, and their tendency to aggregate facilitate Schottky emission in the high-current regime. Our results provide a quantitative foundation for interpreting domain-wall conduction, offer guidance for defect engineering in ferroelectrics, and provide important information to phase-field simulations of defect-domain wall interactions in a ferroelectric domain structure.

cond-mat.mtrl-sci↗

arXiv2025

Flexocoupling-induced phonons and ferrons in van der Waals ferroelectrics

The contribution of flexoelectric coupling to the long-range order parameter fluctuations in ferroics can be critically important to the ferron dispersion and related polar, pyroelectric and electrocaloric properties. Here we calculate analytically the dispersion relations of soft optic and acoustic flexocoupling-induced phonons and ferrons by incorporating the flexoelectric coupling, damping, and higher elastic gradients in the Landau-Ginzburg-Devonshire free energy functional using the van der Waals uniaxial ferrielectric CuInP2S6 as an example. We analyze the changes in the flexocoupling-induced phonon and ferron spectra arising from the appearance of spatially modulated phases induced by the flexoelectric coupling. We show that the free energy landscape of CuInP2S6 determines the specific features of its phonon spectra and ferron dispersion. We also discuss the contributions of optic and acoustic flexocoupling-induced ferrons to the pyroelectric and electrocaloric responses of CuInP2S6 at low temperatures.

cond-mat.mtrl-sci↗

arXiv2025

A Thermodynamic Theory of Proximity Ferroelectricity

Proximity ferroelectricity has recently been reported as a new design paradigm for inducing ferroelectricity, where a non-ferroelectric polar material becomes a ferroelectric by interfacing with a thin ferroelectric layer. Strongly polar materials, such as AlN and ZnO, which were previously unswitchable with an external field below their dielectric breakdown fields, can now be switched with practical coercive fields when they are in intimate proximity to a switchable ferroelectric. Here, we develop a general Landau-Ginzburg theory of proximity ferroelectricity in multilayers of non-ferroelectrics and ferroelectrics to analyze their switchability and coercive fields. The theory predicts regimes of both "proximity switching" where the multilayers collectively switch, as well as "proximity suppression" where they collectively do not switch. The mechanism of the proximity ferroelectricity is an internal electric field determined by the polarization of the layers and their relative thickness in a self-consistent manner that renormalizes the double-well ferroelectric potential to lower the steepness of the switching barrier. Further reduction in the coercive field emerges from charged defects in the bulk that act as nucleation centers. The application of the theory to proximity ferroelectricity in Alx-1ScxN/AlN and Zn1-xMgxO/ZnO bilayers is demonstrated. The theory further predicts that multilayers of dielectric/ferroelectric and paraelectric/ferroelectric layers can potentially result in induced ferroelectricity in the dielectric or paraelectric layers, resulting in the entire stack being switched, an exciting avenue for new discoveries. This thawing of "frozen ferroelectrics", paraelectrics and potentially dielectrics, promises a large class of new ferroelectrics with exciting prospects for previously unrealizable domain-patterned optoelectronic and memory technologies.

cond-mat.mtrl-sci↗

arXiv2025

Colossal enhancement of spin transmission through magnon confinement in an antiferromagnet

Since Felix Bloch's introduction of the concept of spin waves in 1930, 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. In this letter, we demonstrate highly efficient magnon transport in an LaFeO$_3$/BiFeO$_3$/LaFeO$_3$ 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, its non-volatility enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamics-Inspired High-Entropy Oxide Synthesis

High-entropy oxide (HEO) thermodynamics transcend temperature-centric approaches, spanning a multidimensional landscape where oxygen chemical potential plays a decisive role. Here, we experimentally demonstrate how controlling the oxygen chemical potential coerces multivalent cations into divalent states in rock salt HEOs. We construct a preferred valence phase diagram based on thermodynamic stability and equilibrium analysis, alongside a high throughput enthalpic stability map derived from atomistic calculations leveraging machine learning interatomic potentials. We identify and synthesize seven equimolar single-phase rock salt compositions that accommodate multivalent Mn, Fe, or both, as confirmed by X-ray diffraction and fluorescence. X-ray absorption fine structure spectra reveal predominantly divalent cations. Ultimately, we introduce oxygen chemical potential overlap as a key complementary descriptor predicting HEO stability and synthesizability. Although we focus on rock salt HEOs, our methods are chemically and structurally agnostic, providing a broadly adaptable framework for navigating HEOs thermodynamics and enabling a broader compositional range with contemporary property interest.

cond-mat.mtrl-sci↗