arXiv subjects

Fan Fei

Publications and source records attributed to Fan Fei.

At least 19 recordsLinked to original sources

arXiv2026

A Poromechanics-Based Framework for Fully Coupled Reactive Transport and Geomechanics in Porous Rocks

Mineral precipitation and dissolution in rocks alter pore structure, stress, and hydraulic properties, producing tightly coupled chemo-hydro-mechanical processes that are central to subsurface systems. Yet, existing continuum-scale simulators lack a generalizable, mathematically tractable, and physically grounded chemo-mechanics formulation for modeling these coupled processes. To address this gap, this work presents a poromechanics-based framework for chemo-mechanics coupling and integrates it into coupled reactive transport and geomechanics simulations. Building upon classical poromechanics theory, the framework provides distinct descriptions for the stress states of the host rock, pore fluid, and pore minerals, enabling a rigorous and flexible treatment of their individual behavior and mechanical interactions during precipitation and dissolution. As demonstrated by numerical examples, the poromechanics-based method captures the expected mechanical response by translating pore-scale mineral growth into mineralization pressure. By accounting for mineral compressibility, the method also supports more physically realistic representations of deformation and induced stress. Comparisons with the eigenstrain method further highlight the unique capability of the poromechanics-based approach to model effective stress evolution and fracture development without explicitly representing pore geometry or other microstructural features. Overall, this framework offers a versatile, physics-based predictive approach to modeling coupled chemo-mechanical processes in reactive porous rocks and supports future reservoir-scale analyses of engineered subsurface systems.

physics.geo-ph↗

arXiv2026

Lift3D-VLA: Lifting VLA Models to 3D Geometry and Dynamics-Aware Manipulation

Recently, Vision-Language-Action (VLA) models have demonstrated strong generalization across diverse tasks. However, effective robotic manipulation in physical environments fundamentally requires geometric understanding and spatial reasoning. While some VLA approaches attempt to incorporate 3D information, they are constrained by limited data availability and geometric information loss in current 3D encoding pipelines, and fail to jointly capture 3D geometry and temporally structured actions in dynamic environments. To address these limitations, we introduce Lift3D-VLA, a unified VLA framework that equips models with explicit 3D point cloud reasoning and enables temporally coherent action generation. First, building upon our previous work Lift3D, an enhanced 2D model-lifting strategy is proposed to geometrically align 3D points with pretrained 2D positional embeddings. This design enables direct point-cloud encoding within the VLA vision encoder while minimizing spatial information loss. Based on explicit 3D inputs, we propose Geometry-Centric Masked Autoencoding (GC-MAE), a dual-objective self-supervised framework that reconstructs the current point cloud while predicting its future geometric evolution. This formulation allows the 2D vision encoder to internalize both 3D structure and physical dynamics. To fully exploit 3D representations, we further design layer-wise temporal action modeling, which leverages multiple layers of the LLM to collaboratively predict action chunks, enabling temporally consistent predictions. Across 22 simulated tasks and 8 real-world manipulation tasks, Lift3D-VLA achieves 10.8% and 11.1% higher mean success rates on MetaWorld and RLBench than the best-performing prior VLA methods, and outperforms the strongest real-world baseline by 4 percentage points, while exhibiting stronger generalization to out-of-distribution perturbations.

cs.RO↗

arXiv2026

Strain engineering of ultrafast magnetism in the room-temperature vdW ferromagnet Fe3GaTe2

Controlling ultrafast magnetic dynamics is critical to understanding nonequilibrium spin interactions and advancing high-speed spintronics. However, a lack of efficient in situ tuning strategies leaves most ultrafast studies largely dependent on the intrinsic properties of the individual materials. Here we demonstrate continuous strain tuning of both the equilibrium magnetic response and ultrafast demagnetization dynamics in the room-temperature van der Waals ferromagnet Fe3GaTe2. Applying up to 4.2% uniaxial tensile strain increases the coercive field from nearly zero to 100 Oe, consistent with an enhancement of the effective perpendicular magnetic anisotropy. Time-resolved magneto-optical Kerr effect measurements further reveal strain-accelerated ultrafast demagnetization, with 1.2% tensile strain reducing the characteristic demagnetization time by approximately 20%. Remarkably, strain accesses an accelerated demagnetization regime that cannot be reached simply by increasing pump fluence in the unstrained sample. Combined with first-principles calculations, our results resolve that the applied strain modifies the spin-lattice energy transfer, leading to the observed accelerated demagnetization. These findings establish mechanical strain as an effective route for on-demand control of ultrafast magnetic dynamics while reducing the required optical energy by reconfiguring the magnetic energy landscape and associated spin-relaxation pathways.

cond-mat.mtrl-sci↗

arXiv2026

ViLL-E: Video LLM Embeddings for Retrieval

Video Large Language Models (VideoLLMs) excel at video understanding tasks where outputs are textual, such as Video Question Answering and Video Captioning. However, they underperform specialized embedding-based models in Retrieval tasks, such as Text-toVideo Retrieval and Moment Retrieval. We introduce ViLL-E (Video-LLM-Embed), a unified VideoLLM architecture endowed with a novel embedding generation mechanism that allows the model to "think longer" for complex videos and stop early for easy ones. We train this model with a three-stage training methodology combining generative and contrastive learning: initial large-scale pre-training with video-caption pairs; followed by continual training on a smaller, detailed-caption dataset; and concluding with task-specific fine-tuning on a novel multi-task dataset covering Video QA, Temporal Localization, Video Retrieval, and Video-Text Matching. Our model significantly improves temporal localization (on avg. 7% over other VideoLLMs) and video retrieval (up to 4% over dual encoder models), achieving performance comparable to state-of-the-art specialized embedding models while remaining competitive on VideoQA tasks. Furthermore, our joint contrastive-generative training unlocks new zero-shot capabilities, significantly outperforming state-of-the-art methods in composed video retrieval (+5% over SotA) and retrieval from long text (+2% over SotA).

cs.CV↗

arXiv2026

Electrically switchable ferron upconversion in a van der Waals ferroelectric

Nonlinear phononics provides a powerful ultrafast route to control lattice excitations, enabling access to hidden quantum orders, phononic computing, and quantum transduction. However, dynamic control of anharmonic phonon interactions remains limited, as these interactions are typically fixed by the equilibrium crystal lattice and lack external tunability. Emergent ferrons in ferroelectrics, which are collective oscillations of the spontaneous electric polarization, may offer a promising platform to overcome this limitation by combining intrinsic phononic nonlinearity with direct electrical control of the ferroelectric order parameter. Here we report electrically controllable nonlinear ferron upconversion in the van der Waals ferroelectric NbOI2. We show that resonant THz excitation of a 3.1 THz ferron drives coherent upconversion to a 7.0 THz optical phonon. Using two-dimensional THz spectroscopy, we directly resolve off-diagonal coupling features and establish the nonlinear upconversion pathway. Supported by first-principles calculations and analytical modeling, we identify the microscopic origin as a cubic anharmonic lattice coupling. Importantly, in situ electric-field switching enables nonvolatile control of both the ferron dynamics and the associated upconversion process. The phase reversal and hysteretic behavior across the coercive fields establish that the ferron-mediated nonlinear phononic interaction is strongly dependent on the underlying ferroelectric order parameter. These results introduce ferron upconversion as a new and universal regime of nonlinear phononics in ferroelectrics and establish an electrically programmable platform for coherent lattice control, paving the way for ferronic information processing and quantum phononic transduction.

cond-mat.mtrl-sci↗

arXiv2026

Modular Safety Guardrails Are Necessary for Foundation-Model-Enabled Robots in the Real World

The integration of foundation models (FMs) into robotics has accelerated real-world deployment, while introducing new safety challenges arising from open-ended semantic reasoning and embodied physical action. These challenges require safety notions beyond physical constraint satisfaction. In this paper, we characterize FM-enabled robot safety along three dimensions: action safety (physical feasibility and constraint compliance), decision safety (semantic and contextual appropriateness), and human-centered safety (conformance to human intent, norms, and expectations). We argue that existing approaches, including static verification, monolithic controllers, and end-to-end learned policies, are insufficient in settings where tasks, environments, and human expectations are open-ended, long-tailed, and subject to adaptation over time. To address this gap, we propose modular safety guardrails, consisting of monitoring (evaluation) and intervention layers, as an architectural foundation for comprehensive safety across the autonomy stack. Beyond modularity, we highlight possible cross-layer co-design opportunities through representation alignment and conservatism allocation to enable faster, less conservative, and more effective safety enforcement. We call on the community to explore richer guardrail modules and principled co-design strategies to advance safe real-world physical AI deployment.

eess.SY↗

arXiv2025

No Redundancy, No Stall: Lightweight Streaming 3D Gaussian Splatting for Real-time Rendering

3D Gaussian Splatting (3DGS) enables high-quality rendering of 3D scenes and is getting increasing adoption in domains like autonomous driving and embodied intelligence. However, 3DGS still faces major efficiency challenges when faced with high frame rate requirements and resource-constrained edge deployment. To enable efficient 3DGS, in this paper, we propose LS-Gaussian, an algorithm/hardware co-design framework for lightweight streaming 3D rendering. LS-Gaussian is motivated by the core observation that 3DGS suffers from substantial computation redundancy and stalls. On one hand, in practical scenarios, high-frame-rate 3DGS is often applied in settings where a camera observes and renders the same scene continuously but from slightly different viewpoints. Therefore, instead of rendering each frame separately, LS-Gaussian proposes a viewpoint transformation algorithm that leverages inter-frame continuity for efficient sparse rendering. On the other hand, as different tiles within an image are rendered in parallel but have imbalanced workloads, frequent hardware stalls also slow down the rendering process. LS-Gaussian predicts the workload for each tile based on viewpoint transformation to enable more balanced parallel computation and co-designs a customized 3DGS accelerator to support the workload-aware mapping in real-time. Experimental results demonstrate that LS-Gaussian achieves 5.41x speedup over the edge GPU baseline on average and up to 17.3x speedup with the customized accelerator, while incurring only minimal visual quality degradation.

cs.AR↗

arXiv2025

PacTure: Efficient PBR Texture Generation on Packed Views with Visual Autoregressive Models

We present PacTure, a novel framework for generating physically-based rendering (PBR) material textures for an untextured 3D mesh from a text description. Existing 2D generation-based texturing approaches either generate textures sequentially from different views, resulting in long inference times and globally inconsistent textures, or adopt multi-view generation with cross-view attention to enhance global consistency, which, however, limits the resolution for each view. In response to these weaknesses, we first introduce view packing, a novel technique that significantly increases the effective resolution for each view during multi-view generation, without imposing additional inference cost. Unlike UV mapping, it preserves the spatial proximity essential for image generation and maintains full compatibility with current 2D generative models. To further reduce the inferencing cost, we enable fine-grained control and multi-domain generation within the next-scale prediction autoregressive framework, creating an efficient multi-view PBR generation backbone. Extensive experiments show that PacTure outperforms state-of-the-art methods in both quality and efficiency.

cs.CV↗

arXiv2025

A phase-field model for quasi-dynamic rupture nucleation and propagation of in-plane faults

Computational modeling of faulting processes is an essential tool for understanding earthquake mechanics but remains challenging due to the structural and material complexities of fault zones. The phase-field method has recently enabled unified modeling of fault propagation and off-fault damage; however, its capability has been restricted to simplified anti-plane settings. In this study, we extend the phase-field method to in-plane faulting by introducing two key advancements: (i) the incorporation of enhanced fault kinematics and pressure-dependent shear strength for a more accurate representation of fault behavior, and (ii) a revised fault propagation criterion that explicitly accounts for the coupling between shear strength and normal stress. The proposed formulation is verified against standard discontinuous approaches to quasi-dynamic fault rupture under in-plane conditions and validated using experimental observations and numerical data on fault nucleation and propagation. Simulations incorporating structural complexities and material heterogeneities demonstrate the robustness and versatility of the phase-field model, establishing it as a powerful tool for investigating the interactions between fault zone properties and earthquake processes.

physics.geo-ph↗

arXiv2025

Roadmap on Atomic-scale Semiconductor Devices

Spin states in semiconductors provide exceptionally stable and noise-resistant environments for qubits, positioning them as optimal candidates for reliable quantum computing technologies. The proposal to use nuclear and electronic spins of donor atoms in silicon, introduced by Kane in 1998, sparked a new research field focused on the precise positioning of individual impurity atoms for quantum devices, utilising scanning tunnelling microscopy and ion implantation. This roadmap article reviews the advancements in the 25 years since Kane's proposal, the current challenges, and the future directions in atomic-scale semiconductor device fabrication and measurement. It covers the quest to create a silicon-based quantum computer and expands to include diverse material systems and fabrication techniques, highlighting the potential for a broad range of semiconductor quantum technological applications. Key developments include phosphorus in silicon devices such as single-atom transistors, arrayed few-donor devices, one- and two-qubit gates, three-dimensional architectures, and the development of a toolbox for future quantum integrated circuits. The roadmap also explores new impurity species like arsenic and antimony for enhanced scalability and higher-dimensional spin systems, new chemistry for dopant precursors and lithographic resists, and the potential for germanium-based devices. Emerging methods, such as photon-based lithography and electron beam manipulation, are discussed for their disruptive potential. This roadmap charts the path toward scalable quantum computing and advanced semiconductor quantum technologies, emphasising the critical intersections of experiment, technological development, and theory.

quant-ph↗

arXiv2024

Colossal terahertz emission with ultrafast tunability based on van der Waals ferroelectric NbOI$_2$

Terahertz (THz) technology is critical for quantum material physics, biomedical imaging, ultrafast electronics, and next-generation wireless communications. However, standing in the way of widespread applications is the scarcity of efficient ultrafast THz sources with on-demand fast modulation and easy on-chip integration capability. Here we report the discovery of colossal THz emission from a van der Waals (vdW) ferroelectric semiconductor NbOI$_2$. Using THz emission spectroscopy, we observe a THz generation efficiency an order of magnitude higher than that of ZnTe, a standard nonlinear crystal for ultrafast THz generation. We further uncover the underlying generation mechanisms associated with its large ferroelectric polarization by studying the THz emission dependence on excitation wavelength, incident polarization and fluence. Moreover, we demonstrate the ultrafast coherent amplification and annihilation of the THz emission and associated coherent phonon oscillations by employing a double-pump scheme. These findings combined with first-principles calculations, inform new understanding of the THz light-matter interaction in emergent vdW ferroelectrics and pave the way to develop high-performance THz devices on them for quantum materials sensing and ultrafast electronics.

cond-mat.mtrl-sci↗

arXiv2024

Crack opening calculation in phase-field modeling of fluid-filled fracture: A robust and efficient strain-based method

The phase-field method has become popular for the numerical modeling of fluid-filled fractures, thanks to its ability to represent complex fracture geometry without algorithms. However, the algorithm-free representation of fracture geometry poses a significant challenge in calculating the crack opening (aperture) of phase-field fracture, which governs the fracture permeability and hence the overall hydromechanical behavior. Although several approaches have been devised to compute the crack opening of phase-field fracture, they require a sophisticated algorithm for post-processing the phase-field values or an additional parameter sensitive to the element size and alignment. Here, we develop a novel method for calculating the crack opening of fluid-filled phase-field fracture, which enables one to obtain the crack opening without additional algorithms or parameters. We transform the displacement-jump-based kinematics of a fracture into a continuous strain-based version, insert it into a force balance equation on the fracture, and apply the phase-field approximation. Through this procedure, we obtain a simple equation for the crack opening which can be calculated with quantities at individual material points. We verify the proposed method with analytical and numerical solutions obtained based on discrete representations of fractures, demonstrating its capability to calculate the crack opening regardless of the element size or alignment.

physics.geo-ph↗

arXiv2024

Analog Quantum Simulator of a Quantum Field Theory with Fermion-Spin Systems in Silicon

Simulating fermions coupled to spin degrees of freedom, relevant for a range of quantum field theories, represents a promising application for quantum simulators. Mapping fermions to qubits is challenging in $2+1$ and higher spacetime dimensions, and mapping bosons demands substantial quantum-computational overhead. These features complicate the realization of mixed fermion-boson quantum systems in digital quantum computers. We propose a native fermion-(large-)spin analog quantum simulator by utilizing dopant arrays in silicon. Specifically, we show how to use a dynamical lattice of coupled nuclear spins and conduction-band electrons to realize a quantum field theory: an extended Jackiw-Rebbi model involving coupled fermions and quantum rotors. We demonstrate the feasibility of observing dynamical mass generation and a confinement-deconfinement quantum phase transition in 1+1 dimensions on this platform, even in the presence of strong long-range Coulomb interactions. Furthermore, we employ finite-temperature Hartree-Fock-Bogoliubov simulations to investigate the dynamics of mass generation in two-dimensional square and honeycomb arrays, showing that this phenomenon can be simulated with realistic experimental parameters. Our findings reveal two distinct phases, and demonstrate robustness against the addition of Coulomb interactions. Finally, we discuss experimental signatures of the phases through transport and local charge sensing in dopant arrays. This study lays the foundation for quantum simulations of quantum field theories exhibiting fermions coupled to spin degrees of freedom using donors in silicon.

quant-ph↗

arXiv2024

Spin-mechanical coupling in 2D antiferromagnet CrSBr

Spin-mechanical coupling is vital in diverse fields including spintronics, sensing and quantum transduction. Two-dimensional (2D) magnetic materials provide a unique platform for investigating spin-mechanical coupling, attributed to their mechanical flexibility and novel spin orderings. However, studying spin-mechanical coupling in 2D magnets presents challenges in probing mechanical deformation and thermodynamic properties change at nanoscale. Here we use nano opto-electro-mechanical interferometry to mechanically detect the phase transition and magnetostriction effect in multilayer CrSBr, an air-stable antiferromagnets with large magnon-exciton coupling. The transitions among antiferromagnetism, spin-canted ferromagnetism and paramagnetism are visualized by optomechanical frequency anomalies. Nontrivial magnetostriction coefficient 2.3x10^(-5) and magnetoelastic coupling strength on the order of 10^6 J/m^3 have been found. Moreover, we demonstrate the substantial tunability of the magnetoelastic constant by nearly 50% via gate-induced strain. Our findings demonstrate the strong spin-mechanical coupling in CrSBr and paves the way for developing sensitive magnetic sensing and efficient quantum transduction at atomically thin limit.

cond-mat.mes-hall↗

arXiv2024

VidLA: Video-Language Alignment at Scale

In this paper, we propose VidLA, an approach for video-language alignment at scale. There are two major limitations of previous video-language alignment approaches. First, they do not capture both short-range and long-range temporal dependencies and typically employ complex hierarchical deep network architectures that are hard to integrate with existing pretrained image-text foundation models. To effectively address this limitation, we instead keep the network architecture simple and use a set of data tokens that operate at different temporal resolutions in a hierarchical manner, accounting for the temporally hierarchical nature of videos. By employing a simple two-tower architecture, we are able to initialize our video-language model with pretrained image-text foundation models, thereby boosting the final performance. Second, existing video-language alignment works struggle due to the lack of semantically aligned large-scale training data. To overcome it, we leverage recent LLMs to curate the largest video-language dataset to date with better visual grounding. Furthermore, unlike existing video-text datasets which only contain short clips, our dataset is enriched with video clips of varying durations to aid our temporally hierarchical data tokens in extracting better representations at varying temporal scales. Overall, empirical results show that our proposed approach surpasses state-of-the-art methods on multiple retrieval benchmarks, especially on longer videos, and performs competitively on classification benchmarks.

cs.CV↗

arXiv2023

A phase-field model for hydraulic fracture nucleation and propagation in porous media

Many geo-engineering applications, e.g., enhanced geothermal systems, rely on hydraulic fracturing to enhance the permeability of natural formations and allow for sufficient fluid circulation. Over the past few decades, the phase-field method has grown in popularity as a valid approach to modeling hydraulic fracturing because of the ease of handling complex fracture propagation geometries. However, existing phase-field methods cannot appropriately capture nucleation of hydraulic fractures because their formulations are solely energy-based and do not explicitly take into account the strength of the material. Thus, in this work, we propose a novel phase-field formulation for hydraulic fracturing with the main goal of modeling fracture nucleation in porous media, e.g., rocks. Built on the variational formulation of previous phase-field methods, the proposed model incorporates the material strength envelope for hydraulic fracture nucleation through two important steps: (i) an external driving force term, included in the damage evolution equation, that accounts for the material strength; (ii) a properly designed damage function that defines the fluid pressure contribution on the crack driving force. The comparison of numerical results for two-dimensional (2D) test cases with existing analytical solutions demonstrates that the proposed phase-field model can accurately model both nucleation and propagation of hydraulic fractures. Additionally, we present the simulation of hydraulic fracturing in a three-dimensional (3D) domain with various stress conditions to demonstrate the applicability of the method to realistic scenarios.

physics.geo-ph↗

arXiv2022

Extended Barton-Bandis model for rock joints under cyclic loading: Formulation and implicit algorithm

In this paper, the Barton-Bandis model for rock joints is extended to cyclic loading conditions, without any new material parameter. Also developed herein is an algorithm for implicit numerical solution of the extended Barton-Bandis model, which can also be used for the original Barton-Bandis model for which an implicit algorithm has been unavailable. To this end, we first cast the Barton-Bandis model into an incremental elasto-plastic framework, deriving an expression for the elastic shear stiffness being consistent with the original model formulation. We then extend the model formulation to cyclic loading conditions, incorporating the dependence of shear stress and dilation on the joint position and the shearing direction. The extension is achieved by introducing a few state-dependent variables which can be calculated with the existing material parameters. For robust and accurate utilization of the model, we also develop an implicit algorithm based on return mapping, which is unconditionally stable and guarantees satisfaction of the strength criterion. We verify that the proposed model formulation and algorithm produces the same results as the original Barton-Bandis model under monotonic shearing conditions. We then validate the extended Barton-Bandis model against experimental data on natural rock joints under cycling loading conditions. The present work thus enables the Barton-Bandis model, which has been exceptionally popular in research and practice, to be applicable to a wider range of problems in rock mechanics and rock engineering.

physics.geo-ph↗

arXiv2022

A phase-field model for quasi-dynamic nucleation, growth, and propagation of rate-and-state faults

Despite its critical role in the study of earthquake processes, numerical simulation of the entire stages of fault rupture remains a formidable task. The main challenges in simulating a fault rupture process include complex evolution of fault geometry, frictional contact, and off-fault damage over a wide range of spatial and temporal scales. Here, we develop a phase-field model for quasi-dynamic fault nucleation, growth, and propagation, which features two standout advantages: (i) it does not require any sophisticated algorithms to represent fault geometry and its evolution; and (ii) it allows for modeling fault nucleation, propagation, and off-fault damage processes with a single formulation. Built on a recently developed phase-field framework for shear fractures with frictional contact, the proposed formulation incorporates rate- and state-dependent friction, radiation damping, and their impacts on fault mechanics and off-fault damage. We show that the numerical results of the phase-field model are consistent with those obtained from well-verified approaches that model the fault as a surface of discontinuity, without suffering from the mesh convergence issue in the existing continuous approaches to fault rupture (e.g. the stress glut method). Further, through numerical examples of fault propagation in various settings, we demonstrate that the phase-field approach may open new opportunities for investigating complex earthquake processes that have remained overly challenging for the existing numerical methods.

physics.geo-ph↗