arXiv subjects

Jiaxuan Wu

Publications and source records attributed to Jiaxuan Wu.

12 recordsLinked to original sources

arXiv2026

Terahertz oscillation of $180^{\circ}$ domain walls in ferroelectric membranes

A fundamentally intriguing yet not well understood topic in the field of ferroelectrics is the collective excitation of domain walls (DWs), with potential applications to DW-based nanoelectronic and optoelectronic devices. Here we use dynamical phase-field simulations to identify the collective modes of an Ising-type $180^{\circ}$ DW in a uniaxially strained BaTiO3 membrane. The membrane, which concurrently functions as a cavity for polarization and acoustic waves, permits cavity-enhanced resonant excitation of polarization waves. The simulation reveals an unconventional DW sliding mode that exhibits a bulk-polarization-charge-driven nonzero resonant frequency in the sub-terahertz (THz) regime with a dynamically changing internal structure during sliding. These features differ from the previously reported zero-frequency DW sliding mode or the surface-polarization-charge-driven DW sliding mode. The effect of strain on the frequency of this unconventional DW sliding mode and other previously known THz DW eigenmodes is investigated by dynamical phase-field simulations and interpreted by eigenmode analysis or analytical calculation in a simplified one-dimensional (1D) system. These results provide new insights into the high-frequency dynamics of ferroelectric DWs and suggest opportunities for realizing strain control of phonon-DW resonance, and more broadly, for discovering and controlling unconventional DW modes in conventional domain patterns with applications to reconfigurable THz and optical devices.

cond-mat.mtrl-sci↗

arXiv2026

Strong coupling between coherent ferrons and cavity acoustic phonons

Coherent ferrons, the quanta of polarization waves, can potentially be hybridized with many other quasiparticles for achieving novel control modalities in quantum communication, computing, and sensing. Here, we theoretically demonstrate a new hybridized state resulting from the strong coupling between fundamental-mode (wavenumber is zero) coherent ferrons and cavity bulk acoustic phonons. Using a van der Waals ferroelectric CuInP2S6 membrane as an example, we predict an ultra-strong ferron-phonon coupling at room temperature, where the coupling strength g_c reaches over 10% of the resonant frequency ω_0. We also predict an in-situ bistable electric-field control of mode-specific ferron-phonon hybridization via ferroelectric switching. We further show that CuInP2S6 allows for reaching the fundamentally intriguing but challenging deep strong coupling regime (i.e., g_c/ω_0>1) near the ferroelectric-to-paraelectric phase transition. Our findings establish the theoretical basis for exploiting coherent ferron as a new contender for hybrid quantum system with strong and highly tunable coherent coupling

cond-mat.mtrl-sci↗

arXiv2026

Multimode magnon-phonon cavity driven by symmetry-locked strain fields

Hybrid magnon-phonon cavities with precise control knobs are highly sought after for coherent energy and signal transduction in solid-state platforms. While strain offers a powerful means to tune magnonic characteristics, extending strain engineering into magnon-phonon hybridization has remained elusive. Moreover, implementing controllable strain at the meso- or nanoscale poses a formidable challenge, as spatial inhomogeneity of strain fields often leads to enhanced damping and reduced coherence, thereby hindering device integration and scalability. Here, we present an epitaxial La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) heterostructure that exhibits strong coupling between the Kittel magnon and acoustic phonon. Leveraging the emergence of structural domains when STO undergoes a cubic-to-tetragonal phase transition, we create anisotropic local strains at the interface. Remarkably, the anisotropic local strain of less than 0.1 % drives the pronounced splitting of the magnon into three branches. Each branch independently hybridizes with acoustic phonons, forming a matrix of magnon-phonon avoided crossings that underpins multimode transduction and programmable networks in frequency and magnetic field space. An analytical model reveals that the split magnon branches are deterministically locked to the three main crystalline axes, enabling robust, orientation-selective control of the hybridized magnon-phonon spectrum against spatial inhomogeneity. Our results establish designed local strain as an exceptionally sensitive trigger for multimode magnon-phonon hybridization in magnetoelastic oxide heterostructures, and highlight local strain engineering as a viable strategy for designing tunable hybrid magnonic and phononic devices.

cond-mat.mtrl-sci↗

arXiv2026

Strain patterning of flexomagnetism

Flexomagnetism, the coupling of magnetic ordering to strain gradients, provides access to novel symmetry-broken magnetic phases that cannot be accessed via uniform strain. However, flexomagnetism is hard to understand because it is extremely difficult to control a spatially varying strain. Here, we develop a top-down strategy to pattern transverse strain gradients using helium ion implantation through a lithographically defined mask. Using epitaxial films of the antiferromagnetic nodal line semimetal GdAuGe, we demonstrate that transverse strain gradients $\partial \varepsilon_{zz}/\partial x$ induce near-room-temperature ferromagnetic response, compared to the retained para or antiferromagnetism for homogeneously strained GdAuGe. We spatially correlate the magnetic response with the regions of largest strain gradient, via magnetic force microscopy and nanobeam x-ray diffraction, respectively, to confirm the flexomagnetic response. Our approach opens new avenues for the precise control of magnetic phases in thin films of quantum materials via a patterned strain gradient.

cond-mat.mtrl-sci↗

arXiv2026

Gilbert Damping Parameters of Epitaxially-Stabilized Iron Gallium Thin Films from Ferromagnetic Resonance

Iron gallium (FeGa) alloys are excellent rare-earth-free magnetostrictors. Through epitaxial stabilization, the disordered A2 alloy can be extended from 19% to 30% gallium resulting in a magnetostrictive coefficient almost twice than that which is seen in rare earth magnetostrictors like SmFe2. In a composite magnetoelectric structure, this makes epitaxially-stabilized iron gallium a key material for energy-efficient beyond CMOS technologies. The energy dissipation and speed of magnetoelectric switching, however, is affected by the magnetic resonance frequency and damping. Here we report the evolution of the ferromagnetic resonance and key materials parameters (magnetic anisotropy, magnetic damping, and magnetostriction coefficient) for 70 nm thick epitaxially-stabilized single crystal A2 FeGa films beyond 19% Ga. Using flip chip ferromagnetic resonance (1-14 GHz), we find that the Gilbert damping parameter spans the range of 0.09-0.16 and decreases as the Ga concentration increases. This correlates an increasing magnetoelastic coupling with a reduction in the Gilbert damping. We find that the effective damping is a mix of contributions from the intrinsic magnon-phonon scattering and other scattering/dissipation mechanisms, with the latter being dominant especially at high Ga composition. Our results provide insight into the mechanism of magnetic relaxation in metastable high magnetostriction materials and potential switching behavior of composite magnetoelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Inhibitory Attacks on Backdoor-based Fingerprinting for Large Language Models

The widespread adoption of Large Language Model (LLM) in commercial and research settings has intensified the need for robust intellectual property protection. Backdoor-based LLM fingerprinting has emerged as a promising solution for this challenge. In practical application, the low-cost multi-model collaborative technique, LLM ensemble, combines diverse LLMs to leverage their complementary strengths, garnering significant attention and practical adoption. Unfortunately, the vulnerability of existing LLM fingerprinting for the ensemble scenario is unexplored. In order to comprehensively assess the robustness of LLM fingerprinting, in this paper, we propose two novel fingerprinting attack methods: token filter attack (TFA) and sentence verification attack (SVA). The TFA gets the next token from a unified set of tokens created by the token filter mechanism at each decoding step. The SVA filters out fingerprint responses through a sentence verification mechanism based on perplexity and voting. Experimentally, the proposed methods effectively inhibit the fingerprint response while maintaining ensemble performance. Compared with state-of-the-art attack methods, the proposed method can achieve better performance. The findings necessitate enhanced robustness in LLM fingerprinting.

cs.CR↗

arXiv2025

ImF: Implicit Fingerprint for Large Language Models

Training large language models (LLMs) is resource-intensive and expensive, making protecting intellectual property (IP) for LLMs crucial. Recently, embedding fingerprints into LLMs has emerged as a prevalent method for establishing model ownership. However, existing fingerprinting techniques typically embed identifiable patterns with weak semantic coherence, resulting in fingerprints that significantly differ from the natural question-answering (QA) behavior inherent to LLMs. This discrepancy undermines the stealthiness of the embedded fingerprints and makes them vulnerable to adversarial attacks. In this paper, we first demonstrate the critical vulnerability of existing fingerprint embedding methods by introducing a novel adversarial attack named Generation Revision Intervention (GRI) attack. GRI attack exploits the semantic fragility of current fingerprinting methods, effectively erasing fingerprints by disrupting their weakly correlated semantic structures. Our empirical evaluation highlights that traditional fingerprinting approaches are significantly compromised by the GRI attack, revealing severe limitations in their robustness under realistic adversarial conditions. To advance the state-of-the-art in model fingerprinting, we propose a novel model fingerprint paradigm called Implicit Fingerprints (ImF). ImF leverages steganography techniques to subtly embed ownership information within natural texts, subsequently using Chain-of-Thought (CoT) prompting to construct semantically coherent and contextually natural QA pairs. This design ensures that fingerprints seamlessly integrate with the standard model behavior, remaining indistinguishable from regular outputs and substantially reducing the risk of accidental triggering and targeted removal. We conduct a comprehensive evaluation of ImF on 15 diverse LLMs, spanning different architectures and varying scales.

cs.CL↗

arXiv2025

EditMF: Drawing an Invisible Fingerprint for Your Large Language Models

Training large language models (LLMs) is resource-intensive and expensive, making protecting intellectual property (IP) for LLMs crucial. Recently, embedding fingerprints into LLMs has emerged as a prevalent method for establishing model ownership. However, existing back-door-based methods suffer from limited stealth and efficiency. To simultaneously address these issues, we propose EditMF, a training-free fingerprinting paradigm that achieves highly imperceptible fingerprint embedding with minimal computational overhead. Ownership bits are mapped to compact, semantically coherent triples drawn from an encrypted artificial knowledge base (e.g., virtual author-novel-protagonist facts). Causal tracing localizes the minimal set of layers influencing each triple, and a zero-space update injects the fingerprint without perturbing unrelated knowledge. Verification requires only a single black-box query and succeeds when the model returns the exact pre-embedded protagonist. Empirical results on LLaMA and Qwen families show that EditMF combines high imperceptibility with negligible model's performance loss, while delivering robustness far beyond LoRA-based fingerprinting and approaching that of SFT embeddings. Extensive experiments demonstrate that EditMF is an effective and low-overhead solution for secure LLM ownership verification.

cs.CR↗

arXiv2025

Unconventional spin texture driven by higher-order spin-orbit interactions

Spin splitting and the resulting spin texture are central to emerging spintronic applications. In non-centrosymmetric non-magnetic materials containing heavy elements, spin textures are typically governed by low-order, momentum-dependent spin-orbit interactions, such as Rashba spin-orbit interaction with linear or cubic order in crystal momentum. In this work, we use \textit{ab initio} calculations to reveal a previously unidentified spin texture in the conduction bands of a prototypical ferroelectric nitride LaWN$_3$. In addition to the usual $Γ$-centered vortex, we find six new vortices and anti-vortices located at non-high-symmetry points near the Brillouin zone center. Furthermore, by combining group-theoretical analysis and $\textbf{k}\cdot\textbf{p}$ perturbation modeling, we show that, constrained by the $C_{3v}$ point group to which ferroelectric LaWN$_3$ belongs, a 7th-order Weyl spin-orbit interaction is essential to reproduce the unconventional spin structure observed in first-principles calculations. We also find that weak electron doping of LaWN$_3$ leads to a Fermi surface whose spin-arrow contour exhibits an unusual epicycloid pattern--a distinctive signature that is experimentally accessible. Our work demonstrates that higher-order spin-orbit interactions are more than perturbative corrections. They can play a dominant role in shaping the spin texture of non-centrosymmetric materials. Our results open up new avenues for designing spintronic devices that exploit multi-chiral spin textures beyond the conventional spin-orbit paradigm.

cond-mat.mtrl-sci↗

arXiv2024

Generative Text Steganography with Large Language Model

Recent advances in large language models (LLMs) have blurred the boundary of high-quality text generation between humans and machines, which is favorable for generative text steganography. While, current advanced steganographic mapping is not suitable for LLMs since most users are restricted to accessing only the black-box API or user interface of the LLMs, thereby lacking access to the training vocabulary and its sampling probabilities. In this paper, we explore a black-box generative text steganographic method based on the user interfaces of large language models, which is called LLM-Stega. The main goal of LLM-Stega is that the secure covert communication between Alice (sender) and Bob (receiver) is conducted by using the user interfaces of LLMs. Specifically, We first construct a keyword set and design a new encrypted steganographic mapping to embed secret messages. Furthermore, to guarantee accurate extraction of secret messages and rich semantics of generated stego texts, an optimization mechanism based on reject sampling is proposed. Comprehensive experiments demonstrate that the proposed LLM-Stega outperforms current state-of-the-art methods.

cs.CL↗

arXiv2022

Dynamical structural instability and its implication on the physical properties of infinite-layer nickelates

We use first-principles calculations to find that in infinite-layer nickelates $R$NiO$_2$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R$ = La-Sm. For late lanthanide elements $R$ = Eu-Lu, an imaginary phonon frequency appears at $A=(π,π,π)$ point. For those infinite-layer nickelates, condensation of this phonon mode into the $P4/mmm$ structure leads to a more energetically favorable $I4/mcm$ structure that is characterized by an out-of-phase rotation of "NiO$_4$ square". Special attention is given to two borderline cases: PmNiO$_2$ and SmNiO$_2$, in which both the $P4/mmm$ structure and the $I4/mcm$ structure are local minima and the energy difference between the two structures can be fine-tuned by epitaxial strain. Compared to the $P4/mmm$ structure, $R$NiO$_2$ in the $I4/mcm$ structure has a substantially reduced Ni $d_{x^2-y^2}$ bandwidth, a smaller Ni $d$ occupancy, a "cleaner" Fermi surface with a lanthanide-$d$-derived electron pocket suppressed at $Γ$ point, and a decreased critical $U_{\textrm{Ni}}$ to stabilize long-range antiferromagnetic ordering. All these features imply enhanced correlation effects and favor Mott physics. Our work reveals the importance of structure-property relation in infinite-layer nickelates, in particular, the spontaneous "NiO$_4$ square" rotation provides a tuning knob to render $R$NiO$_2$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.

cond-mat.mtrl-sci↗

arXiv2022

Pressure-induced superconductivity reentrant in transition metal dichalcogenide TiSe2

Through either elements intercalation or application of pressure, transition metal dichalcogenide 1T-TiSe2 exhibits superconductivity in proximity to a charge density wave (CDW) quantum critical point (QCP), thus providing an ideal avenue to study the correlation between the two symmetry-breaking exotic quantum electronic states. We report herein that, in addition to the well-known superconducting dome that emerges within the low pressure range of 2 - 4 GPa and peaks with the maximal Tc of about 1.8 K, the pressure induces another separate superconducting transition starting around 15 GPa with a substantially higher Tc that reaches 5.6 K at about 21.5 GPa. The high-pressure X-ray diffraction and Raman spectroscopy measurements unveil that the superconductivity reentrant is caused by a first-order structural phase transition (from P-3m1 space group to Pnma space group), which is also supported by the density functional theory calculation. A comparative theoretical calculation also reveals that the conventional phonon-mediated mechanism can account for the superconductivity of 1T-TiSe2 under low pressure, while the electron-phonon coupling of 4O-TiSe2 under high pressure is too weak to induce the superconductivity with a Tc as high as 5.6 K. This implies that the emergent superconductivity in the 4O-TiSe2 may have an unconventional origin. Our finding would open a new window toward the discovery of more exotic quantum states in transition metal dichalcogenides via high pressure.

cond-mat.supr-con↗