arXiv subjects

Tao E. Li

Publications and source records attributed to Tao E. Li.

At least 19 recordsLinked to original sources

arXiv2026

Autonomous Quantum Transport Measurements of 2D Semiconductors by an AI Agent

Artificial-intelligence (AI) agents are beginning to enter experimental laboratories, automating experiments and accelerating scientific discovery. Herein, we introduce an AI-driven workflow in which an AI agent performs multi-step, multi-day quantum transport measurements end-to-end. Specifically, given brief instructions, the agent starts by planning the multi-step measurements, then safely operates the cryogenic instruments, analyzes the data, and concludes with a final report. We demonstrate this AI workflow on multiple monolayer and bilayer MoS2 devices. Through autonomous measurement campaigns lasting up to six days, the agent determined the conduction-band spin-orbit coupling energy in monolayer MoS2, and mapped a layer- and valley-resolved phase diagram in bilayer MoS2. This experimental workflow is implemented through the FermiLink agent harness, which emphasizes instrumental safety and the reliability of the measurement and analysis. The framework is general and can be readily adapted to other types of experiments, representing a step toward self-driving laboratories.

cond-mat.mes-hall↗

arXiv2026

Nonlinear Freezing of Vibrational Polariton Transport via Mesoscale Simulations

Two-dimensional real-space imaging of vibrational polariton transport in planar Fabry--Pérot microcavities is numerically simulated via the mesoscale cavity molecular dynamics approach, which self-consistently propagates $\sim\!2\times10^4$ realistic molecular simulation cells on a two-dimensional grid coupled to the same number of cavity modes. Beyond the well-known polariton ballistic-to-diffusive turnover in the linear response regime, these atomistic simulations reveal a nonlinear freezing mechanism of vibrational polariton transport, i.e., under strong pumping of the upper polariton, the initially ballistically propagating upper polariton completely freezes and localizes energy to molecules at specific locations. This mechanism originates from pump-induced breaking of the in-plane translation symmetry: significant molecular excitations at the pulse hot spot broaden the polariton density of states, thus funneling population to the $k_{\parallel}\rightarrow 0$ band edge with vanishing group velocities. Overall, our work provides numerical insights for exploring novel mechanisms of vibrational polaritons.

physics.chem-ph↗

arXiv2026

Nuclear-Electronic Quantum Dynamics in a Plasmonic Nanocavity

Plasmonic nanocavities are a promising platform for strong light-matter coupling and enhanced spectroscopies at the single-molecule level. These nanoscale environments are challenging to model due to their strongly multimodal character and short cavity lifetimes. Herein, we study the effects of these environments using real-time nuclear-electronic orbital time-dependent density functional theory (RT-NEO-TDDFT) coupled to multiple classical cavity modes in a manner that includes cavity loss. In RT-NEO-TDDFT, the quantum mechanical densities of all electrons and specified nuclei, typically protons, are propagated in real time. We show that a cavity with many modes at different frequencies can be used to probe and modify the nuclear-electronic quantum dynamics of chemical systems. Ultrafast excited-state proton transfer reactions can be probed through the time- and energy-resolved cavity emission of a multimode cavity. Under strong coupling conditions, the cavity can modify the dynamics, in some cases suppressing proton transfer and exhibiting Rabi-like oscillations of the cavity emission due to polariton formation. Utilizing the spectral density for an experimentally relevant nanoparticle-on-mirror single-molecule cavity, we show that an excited-state proton transfer system can evolve into resonance with the cavity even when initially out of resonance with the dominant cavity peak. In this case, tuning the dominant cavity peak to be resonant with the electronic transition leads to polariton formation for a small collection of molecules. The RT-NEO framework with multimode cavities enables the efficient simulation of chemical reactions in physically realistic electromagnetic environments, providing fundamental insights into the dynamics and associated spectroscopic signatures.

physics.chem-ph↗

arXiv2026

FermiLink: A Unified Agent Framework for Multidomain Autonomous Scientific Simulations

Artificial-intelligence (AI) agent frameworks have been developed for autonomous scientific simulations, but most current agent frameworks are tailored to a single or a small set of software packages. Herein, FermiLink, a unified and extensible open-source agent framework is introduced for multidomain scientific simulations. Its key design principle is the separation of package knowledge bases from simulation workflows, so that simulation workflows in FermiLink, from figure-level simulations to full-paper-level research on high-performance computing clusters, operate uniformly among supported packages via a four-layer progressive disclosure mechanism. Using OpenAI Codex as the agent provider, the capabilities of FermiLink are demonstrated across approximately 50 scientific software packages spanning nine research domains from physics to engineering. Systematic benchmarks on 132 real-world figure-level reproduction tasks with 44 packages show that FermiLink reproduces 74 (56.1%) of published figures with simulations, among which 30 achieve high-fidelity agreement and 35 reach qualitative agreement with the target figures. A smaller set of human expert-guided reproduction benchmarks with 10 packages further highlights the importance of expert insights for improving the simulation fidelity. Beyond reproduction, a single-blinded study demonstrates that FermiLink can produce research-grade results on unpublished polariton physics problems when provided with sufficiently detailed research objectives and source code, even in the absence of external documentation or tutorials. Overall, FermiLink provides a scalable research infrastructure that may accelerate the path from scientific questions to computational results across diverse domains.

physics.chem-ph↗

arXiv2026

Two-dimensional IR-Raman spectroscopy of vibrational polaritons: Role of dipole surfaces

Nonlinear spectroscopy provides a unique perspective to understand time-resolved molecular dynamics under vibrational strong coupling (VSC). Herein, equilibrium-nonequilibrium cavity molecular dynamics simulations are performed to compute the two-dimensional (2D) infrared-infrared-Raman (IIR) spectroscopy of liquid water under VSC. In conventional computational chemistry practices, accurate molecular spectra are often constructed by using an advanced molecular dipole or polarizability model to post-process molecular dynamics trajectories evolved under a computationally efficient potential. By contrast, this work highlights the necessity of employing a consistent dipole surface model in both CavMD simulations and spectroscopic post-processing. While utilizing inconsistent dipole models only mildly influences the linear polariton spectrum, it severely distorts 2D spectra in wide frequency regions. With a consistent dipole-induced-dipole model, compared to the outside-cavity molecular 2D-IIR spectrum, the cavity 2D-IIR spectrum splits the OH stretch band to a pair of polariton branches along only the IR (not Raman) axis, while fading molecular signals at other frequency regions. This work provides the foundation for employing direct CavMD simulations to construct 2D spectra of realistic molecules under VSC.

physics.chem-ph↗

arXiv2026

MaxwellLink: A unified framework for self-consistent light-matter simulations

A major challenge in light-matter simulations is bridging the disparate time and length scales of electrodynamics and molecular dynamics. Current computational approaches often rely on heuristic approximations of either the electromagnetic (EM) or the material component, hindering the exploration of complex light-matter systems. Herein, MaxwellLink -- a modular, open-source Python framework -- is developed for the massively parallel, self-consistent propagation of classical EM fields interacting with a large heterogeneous molecular ensemble. The package utilizes a robust TCP/UNIX socket interface to couple EM solvers with a wide range of molecular drivers. In this initial release, MaxwellLink supports EM solvers spanning from single-mode cavities to full-feature three-dimensional finite-difference time-domain (FDTD) engines, and molecules described by multilevel open quantum systems, force-field and first-principles molecular dynamics, and nonadiabatic real-time Ehrenfest dynamics. With the socket-based architecture, users can seamlessly switch between levels of theory of either the EM solver or molecules without modifying the counterpart. Moreover, the EM engine and molecular drivers scale independently across multiple high-performance computing (HPC) nodes, facilitating large-scale simulations previously inaccessible to existing numerical schemes. The versatility and accuracy of this code are further demonstrated through applications including superradiance, radiative energy transfer, vibrational strong coupling in Bragg resonators, and plasmonic heating of molecular gases. By providing a unified, extensible engine, MaxwellLink potentially offers a powerful platform for exploring emerging phenomena across the research fronts of spectroscopy, quantum optics, plasmonics, and polaritonics.

physics.comp-ph↗

arXiv2025

FDTD with Auxiliary Bath Fields for Condensed-Phase Polaritonics: Fundamentals and Implementation

Understanding condensed-phase polariton experiments requires accurately accounting for both realistic cavity geometries and the interplay between polaritons and material dark modes arising from microscopic molecular interactions. The finite-difference time-domain (FDTD) approach numerically propagates classical Maxwell's equations in the time domain, offering a versatile scheme for modeling polaritons in realistic cavities. However, the simple dielectric functions routinely used in FDTD often fail to describe molecular details. Consequently, standard FDTD calculations, to date, cannot accurately describe processes involving the complex coupling between polaritons and dark modes, such as polariton relaxation, transport, and condensation. For more faithful simulations of the energy flow between polaritons and dark modes, herein, local bath degrees of freedom coupled to the material polarization are explicitly included in FDTD to describe the dark-mode dynamics. This method -- FDTD with auxiliary bath fields (FDTD-Bath) -- is implemented in the open-source MEEP package by adding a Lorentz-Bath material susceptibility, where explicit bath modes are coupled to conventional Lorentz oscillators. With this Lorentz-Bath susceptibility, linear polariton spectra and Rabi-splitting-dependent polariton relaxation rates in planar Fabry--Pérot cavities are reproduced more accurately than those with the conventional Lorentz susceptibility. Supported by a user-friendly Python interface and efficient MPI parallelism, the FDTD-Bath approach implemented in MEEP is ready to model a wide range of polariton phenomena involving realistic cavity geometries.

physics.optics↗

arXiv2025

Light-Matter Entanglement in Real-Time Nuclear-Electronic Orbital Polariton Dynamics

Molecular polaritons are hybrid light-matter states that enable the exploration of potential cavity-modified chemistry. The development of dynamical, first-principles approaches for simulating molecular polaritons is important for understanding their origins and properties. Herein, we present a hierarchy of first-principles methods to simulate the real-time dynamics of molecular polaritons in the strong coupling regime. These methods are based on real-time time-dependent density functional theory (RT-TDDFT) and the corresponding real-time nuclear-electronic orbital (RT-NEO) approach, in which specified nuclei are treated quantum mechanically on the same level as the electrons. The hierarchy spans semiclassical, mean-field-quantum, and full-quantum approaches to simulate polariton dynamics under both electronic strong coupling and vibrational strong coupling. In the semiclassical approaches, the cavity mode is treated classically, whereas in the full-quantum approaches, the cavity mode is treated quantum mechanically with propagation of a joint molecule-mode density matrix. The semiclassical and full-quantum approaches produce virtually identical Rabi splittings and polariton peak locations for the systems studied. However, the full-quantum approaches allow exploration of molecule-mode quantum entanglement in the real-time dynamics. Although the degree of light-matter entanglement is relatively small in the systems considered, the oscillations of the von Neumann entropy reveal an entanglement Rabi splitting that differs from the Rabi splitting computed from the time-dependent dipole moment. These results suggest that a classical treatment of the cavity mode may provide an excellent description of polariton dynamics for macroscopic observables such as the Rabi splitting, but novel physics may be detectable by considering molecule-mode entanglement.

physics.chem-ph↗

arXiv2025

Selective Excitation of IR-Inactive Modes via Vibrational Polaritons: Insights from Atomistic Simulations

Vibrational polaritons, hybrid light-matter states formed between molecular vibrations and infrared (IR) cavity modes, provide a novel approach for modifying chemical reaction pathways and energy transfer processes. For vibrational polaritons involving condensed-phase molecules, the short polariton lifetime raises debate over whether pumping polaritons may produce different effects on molecules compared to directly exciting the molecules in free space or under weak coupling. Here, for liquid methane under vibrational strong coupling, classical cavity molecular dynamics simulations show that pumping the upper polariton (UP) formed by the asymmetric bending mode of methane can sometimes selectively excite the IR-inactive symmetric bending mode. This finding is validated when the molecular system is described using both empirical force fields and machine-learning potentials, also in qualitative agreement with analytical theory of polariton energy transfer rates based on Fermi's golden rule calculations. Additionally, our study suggests that polariton-induced energy transfer to IR-inactive modes reaches maximal efficiency when the UP has significant contributions from both photons and molecules, underscoring the importance of light-matter hybridization. As IR-inactive vibrational modes are generally inaccessible to direct IR excitation, our study highlights the unique role of polariton formation in selectively controlling IR-inactive vibrations. Since this polariton-induced process occurs after the polariton decays, it may impact IR photochemistry on a timescale longer than the polariton lifetime, as observed in experiments.

physics.chem-ph↗

arXiv2025

Magnetic Interactions between Nanoscale Domains in Correlated Liquids

The formation of nanoscale domains (NDs) in correlated liquids and the emerging collective magnetic properties have been suggested as key mechanisms governing ion transport under external magnetic fields (eMFs). However, the molecular-level understanding of these magnetic field-driven phenomena and the interaction between these domains remain elusive. To this end, we introduce a simplified model of a solvated nanoparticle (NP) that consists of localized magnetic domains at their surfaces to represent groups of paramagnetic ions, forming NDs, whose effective magnetic dipole moments are at least one order of magnitude greater than the individual ions. We use classical density functional theory (cDFT) to estimate the effective interactions between these localized magnetic NPs (LMNPs). Our findings indicate that, unlike individual ions, magnetic dipole interactions of NDs in the LMNP model can indeed compete with the electrostatic, van der Waals, and hydration interactions. Depending on the direction of eMF, the cDFT effective interactions between two LMNPs turn out to become more attractive or repulsive, which may play a critical role in ion separation and nucleation processes. This indicates that the cDFT interaction barrier heights can be significantly affected by the magnetic dipole interactions and the barrier heights tend to increase as the size of LMNPs increases.

cond-mat.soft↗

arXiv2025

Polariton-induced Purcell effects via a reduced semiclassical electrodynamics approach

Recent experiments have demonstrated that polariton formation provides a novel strategy for modifying local molecular processes when a large ensemble of molecules is confined within an optical cavity. Herein, a numerical strategy based on coupled Maxwell--Schrödinger equations is examined for simulating local molecular processes in a realistic cavity structure under collective strong coupling. In this approach, only a few molecules, referred to as quantum impurities, are treated quantum mechanically, while the remaining macroscopic molecular layer and the cavity structure are modeled using dielectric functions. When a single electronic two-level system embedded in a Lorentz medium is confined in a two-dimensional Bragg resonator, our numerical simulations reveal a polariton-induced Purcell effect: the radiative decay rate of the quantum impurity is significantly enhanced by the cavity when the impurity frequency matches the polariton frequency, while the rate can sometimes be greatly suppressed when the impurity is near resonance with the bulk molecules forming strong coupling. Additionally, this approach demonstrates that the cavity absorption of light exhibits Rabi-splitting-dependent suppression due to the inclusion of a realistic cavity structure. Our simulations also identify a fundamental limitation of this approach -- an inaccurate description of polariton dephasing rates into dark modes. This arises because the dark-mode degrees of freedom are not explicitly included when most molecules are modeled using simple dielectric functions. As the polariton-induced Purcell effect alters molecular radiative decay differently from the Purcell effect under weak coupling, this polariton-induced effect may facilitate understanding the origin of polariton-modified photochemistry under electronic strong coupling.

physics.chem-ph↗

arXiv2024

Lagrangian Formulation of Nuclear-Electronic Orbital Ehrenfest Dynamics with Real-time TDDFT for Extended Periodic Systems

We present a Lagrangian-based implementation of Ehrenfest dynamics with nuclear-electronic orbital (NEO) theory and real-time time-dependent density functional theory (RT-TDDFT) for extended periodic systems. In addition to a quantum dynamical treatment of electrons and selected protons, this approach allows for the classical movement of all other nuclei to be taken into account in simulations of condensed matter systems. Furthermore, we introduce a Lagrangian formulation for the traveling proton basis approach and propose new schemes to enhance its application for extended periodic systems. Validation and proof-of-principle applications are performed on electronically excited proton transfer in the o-hydroxybenzaldehyde molecule with explicit solvating water molecules. These simulations demonstrate the importance of solvation dynamics and a quantum treatment of transferring protons. This work broadens the applicability of the NEO Ehrenfest dynamics approach for studying complex heterogeneous systems in the condensed phase.

physics.chem-ph↗

arXiv2024

i-PI 3.0: a flexible and efficient framework for advanced atomistic simulations

Atomic-scale simulations have progressed tremendously over the past decade, largely due to the availability of machine-learning interatomic potentials. These potentials combine the accuracy of electronic structure calculations with the ability to reach extensive length and time scales. The i-PI package facilitates integrating the latest developments in this field with advanced modeling techniques, thanks to a modular software architecture based on inter-process communication through a socket interface. The choice of Python for implementation facilitates rapid prototyping but can add computational overhead. In this new release, we carefully benchmarked and optimized i-PI for several common simulation scenarios, making such overhead negligible when i-PI is used to model systems up to tens of thousands of atoms using widely adopted machine learning interatomic potentials, such as Behler-Parinello, DeePMD and MACE neural networks. We also present the implementation of several new features, including an efficient algorithm to model bosonic and fermionic exchange, a framework for uncertainty quantification to be used in conjunction with machine-learning potentials, a communication infrastructure that allows deeper integration with electronic-driven simulations, and an approach to simulate coupled photon-nuclear dynamics in optical or plasmonic cavities.

physics.chem-ph↗

arXiv2024

Mesoscale Molecular Simulations of Fabry-Pérot Vibrational Strong Coupling

Developing theoretical frameworks for vibrational strong coupling (VSC) beyond the single-mode approximation is crucial for a comprehensive understanding of experiments with planar Fabry-Pérot cavities. Herein, a generalized cavity molecular dynamics (CavMD) scheme is developed to simulate VSC of a large ensemble of realistic molecules coupled to an arbitrary 1D or 2D photonic environment. This approach is built upon the Power-Zienau-Woolley Hamiltonian in the normal mode basis and uses a grid representation of the molecular ensembles to reduce the computational cost. When simulating the polariton dispersion relation for a homogeneous distribution of molecules in planar Fabry-Pérot cavities, our data highlight the importance of preserving the in-plane translational symmetry of the molecular distribution. In this homogeneous limit, CavMD yields the consistent polariton dispersion relation as analytic theory, i.e., incorporating many cavity modes with varying in-plane wave vectors ($k_{\parallel}$) produces the same spectrum as the system with a single cavity mode. Furthermore, CavMD reveals that the validity of the single mode approximation is challenged when nonequilibrium polariton dynamics are considered, as polariton-polariton scattering occurs between modes with nearest neighbor $k_{\parallel}$. The procedure for numerically approaching the macroscopic limit is also demonstrated with CavMD by increasing the system size. Looking forward, our generalized CavMD approach may facilitate understanding vibrational polariton transport and condensation.

physics.chem-ph↗

arXiv2024

Vibrational Polaritons with Broken In-Plane Translational Symmetry

Vibrational polaritons form in a planar Fabry-Perot microcavity when a vibrational mode of a layer of molecules is near resonant with an infrared cavity mode. Herein, dispersion relations of vibrational polaritons are studied when the molecular density distribution breaks the macroscopic translational symmetry along the cavity mirror plane. Both perturbative theory and numerical calculations show that, if a homogeneous in-plane molecular distribution is modulated by sinusoidal fluctuations, in addition to a pair of upper and lower polariton branches, a discrete number of side polariton branches may emerge in the polariton dispersion relation. Moreover, for a periodic Gaussian molecular in-plane density distribution, only two, yet significantly broadened polariton branches exist in the spectra. This polariton linewidth broadening is caused by the scattering between cavity modes at neighboring in-plane frequencies due to the symmetry breaking, which is distinguished from known origins of polariton broadening such as the homogeneous broadening of molecules, the cavity loss, or the large energetic disorder of molecules. Associated with the broadened polariton branches, under the periodic Gaussian in-plane inhomogeneity, a significant number of the VSC eigenstates contain a non-zero contribution from the cavity photon mode at zero in-plane frequency, blurring the distinction between the bright and the dark modes. Looking forward, our theoretical investigation should facilitate the experimental exploration of vibrational polaritons with patterned in-plane molecular density distributions.

physics.chem-ph↗

arXiv2024

Theory of Supervibronic Transitions via Casimir Polaritons

A remote energy transfer pathway from electronic to vibrational degrees of freedom is identified inside an infrared optical cavity under vibrational strong coupling conditions. This mechanism relies on the dynamical Casimir effect, whereby real infrared photons are generated due to a sudden electronic transition of anisotropic molecules. Moreover, the formation of vibrational polaritons enables the excited photon energy to be transferred to the vibrational degrees of freedom before any dissipation occurs. Both analytic solutions and numerical simulations reveal that the magnitude of this electronic to vibrational energy transfer depends quadratically on the number of molecules and resonantly on the vibration-cavity detuning. During this "supervibronic" transition process, because the vibrational energy gain per molecule can be meaningful in the macroscopic limit, this process may potentially be observed using conventional vibrational strong coupling devices.

quant-ph↗

arXiv2023

First-Principles Approach for Coupled Quantum Dynamics of Electrons and Protons in Heterogeneous Systems

The coupled quantum dynamics of electrons and protons is ubiquitous in many dynamical processes involving light-matter interaction, such as solar energy conversion in chemical systems and photosynthesis. A first-principles description of such nuclear-electronic quantum dynamics requires not only the time-dependent treatment of nonequilibrium electron dynamics but also that of quantum protons. Quantum mechanical correlation between electrons and protons adds further complexity to such coupled dynamics. Here we extend real-time nuclear-electronic orbital time-dependent density functional theory (RT-NEO-TDDFT) to periodic systems and perform first-principles simulations of coupled quantum dynamics of electrons and protons in complex heterogeneous systems. The process studied is electronically excited state intramolecular proton transfer of o-hydroxybenzaldehyde in water and at a silicon (111) semiconductor-molecule interface. These simulations illustrate how environments such as hydrogen-bonding water molecules and an extended material surface impact the dynamical process on the atomistic level. Depending on how the molecule is chemisorbed on the surface, excited state electron transfer from the molecule to the semiconductor surface can inhibit ultrafast proton transfer within the molecule. This work elucidates how heterogeneous environments influence the balance between the quantum mechanical proton transfer and excited electron dynamics. The periodic RT-NEO-TDDFT approach is applicable to a wide range of other photoinduced heterogeneous processes.

physics.chem-ph↗

arXiv2023

Electronic Born-Oppenheimer Approximation in Nuclear-Electronic Orbital Dynamics

Within the nuclear-electronic orbital (NEO) framework, the real-time NEO time-dependent density functional theory (RT-NEO-TDDFT) approach enables the simulation of coupled electronic-nuclear dynamics. In this approach, the electrons and quantum nuclei are propagated in time on the same footing. A relatively small time step is required to propagate the much faster electronic dynamics, thereby prohibiting the simulation of long-time nuclear quantum dynamics. Herein, the electronic Born--Oppenheimer (BO) approximation within the NEO framework is presented. In this approach, the electronic density is quenched to the ground state at each time step, and the real-time nuclear quantum dynamics is propagated on an instantaneous electronic ground state defined by both the classical nuclear geometry and the nonequilibrium quantum nuclear density. Because the electronic dynamics is no longer propagated, this approximation enables the use of an order-of-magnitude larger time step, thus greatly reducing the computational cost. Moreover, invoking the electronic BO approximation also fixes the unphysical asymmetric Rabi splitting observed in previous semiclassical RT-NEO-TDDFT simulations of vibrational polaritons even for small Rabi splitting, instead yielding a stable, symmetric Rabi splitting. For intramolecular proton transfer in malonaldehyde, both RT-NEO-Ehrenfest dynamics and its BO counterpart can describe proton delocalization during the real-time nuclear quantum dynamics. Thus, the BO RT-NEO approach provides the foundation for a wide range of chemical and biological applications.

physics.chem-ph↗