arXiv subjects

Mengyuan Cui

Publications and source records attributed to Mengyuan Cui.

6 recordsLinked to original sources

arXiv2026

Quantum Vibronic Dynamics Shape Catalytically Relevant Au-Ligand Interfaces in Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters are versatile for photocatalysis and energy conversion because their electronic structure stems from strong metal-ligand interactions. However, these interactions are mostly discussed statically, leaving dynamic reorganization of Au-ligand interfaces under photoexcitation unclear. We investigate rod-shaped [Au25(PPh3)10(SC2H5)5Cl2]2+ using ultrafast transient-grating spectroscopy, two-dimensional electronic spectroscopy, ab initio calculations, and hierarchical equations-of-motion simulations. The multidimensional spectra resolve multiple electronic relaxation pathways and a hierarchy of coherent structural motions, from localized Au-ligand distortions to collective framework vibrations. Wavelet analysis reveals that high-frequency Au-ligand vibrations emerge immediately after excitation, whereas low-frequency collective modes appear later through interstate vibronic coupling, indicating sequential redistribution of structural coherence. Simulations reproduce the nonlinear response and identify the microscopic vibronic couplings responsible. The results show that photoexcitation drives continuous ultrafast reorganization of the Au-ligand bonding network, transiently reshaping interfacial electronic structure before thermalization. This work establishes dynamic Au-ligand interfaces as the microscopic link between excited-state energy flow and photochemical function in atomically precise nanoclusters.

physics.chem-ph↗

arXiv2026

Conical Intersections Enable Ultrafast Molecular Spin Control in a Chromium Complex

Molecular spintronics seeks to control spin states in single molecules for ultrafast switching and efficient information processing. Transition metal complexes are promising candidates for such applications due to their modular ligand fields, diverse spin configurations, and potential for spin-vibronic coupling that facilitates rapid spin dynamics. Chromium(III) complexes, in particular, offer long-lived emissive doublet states and chemical robustness, making them attractive for room-temperature spin control. Here we investigate the spin-state dynamics of tris(2,4-pentanedionato)chromium(III), [Cr(acac)3], a photochemically stable d3 complex with minimal vibrational congestion. Using ultrafast transient grating and two dimensional electronic spectroscopy with ~10 fs resolution, we directly probe vibrational and electronic dynamics associated with the 4T2 -> 2E intersystem crossing (ISC). These measurements reveal coherent vibrational modes implicated in mediating nonadiabatic spin transitions. Complementary theoretical modelling shows that vibronic coupling and spin orbit interactions promote the formation of multiple conical intersections, providing ultrafast channels for spin-flip dynamics. Metal-ligand bending and stretching modes serve as tuning and coupling coordinates, enabling ISC despite weak spin-orbit coupling in 3d transition metal. Our study provides mechanistic insight into spin-vibronic dynamics in Cr(III) complexes and establishes a design framework for achieving ultrafast molecular spin switching, advancing the development of optically addressable spin centres for future spintronic and quantum technologies.

physics.chem-ph↗

arXiv2026

Can Large Language Models Self-Correct in Medical Question Answering? An Exploratory Study

Large language models (LLMs) have achieved strong performance on medical question answering (medical QA), and chain-of-thought (CoT) prompting has further improved results by eliciting explicit intermediate reasoning; meanwhile, self-reflective (self-corrective) prompting has been widely claimed to enhance model reliability by prompting LLMs to critique and revise their own reasoning, yet its effectiveness in safety-critical medical settings remains unclear. In this work, we conduct an exploratory analysis of self-reflective reasoning for medical multiple-choice question answering: using GPT-4o and GPT-4o-mini, we compare standard CoT prompting with an iterative self-reflection loop and track how predictions evolve across reflection steps on three widely used medical QA benchmarks (MedQA, HeadQA, and PubMedQA). We analyze whether self-reflection leads to error correction, error persistence, or the introduction of new errors. Our results show that self-reflective prompting does not consistently improve accuracy and its impact is highly dataset- and model-dependent: it yields modest gains on MedQA but provides limited or negative benefits on HeadQA and PubMedQA, and increasing the number of reflection steps does not guarantee better performance. These findings highlight a gap between reasoning transparency and reasoning correctness, suggesting that self-reflective reasoning is better viewed as an analytical tool for understanding model behavior rather than a standalone solution for improving medical QA reliability.

cs.CL↗

arXiv2025

Excitonic Energy Transfer in Red Algal Photosystem I Reveals an Evolutionary Bridge between Cyanobacteria and Plants

Photosystem I converts light into chemical energy with near-unity quantum efficiency,yet its energy-transfer and charge-separation mechanisms remain debated. Evolution has diversified PSI architectures. The unicellular red algae Cyanidioschyzon merolae represents a key evolutionary intermediate,featuring a cyanobacterial-like monomeric core surrounded by three to five LHCR subunits. This hybrid organization provides a unique system to bridge mechanistic models across lineages. We applied two-dimensional electronic spectroscopy at ultralow temperatures to disentangle overlapping excitation pathways in C. merolae PSI. Cryogenic measurements suppressed thermal broadening, resolving five dynamical components: sub-picosecond equilibration acrossthe core-LHCR interface, subsequent population transfer into progressively lowerenergy manifolds, and slower feeding into red pools distributed across both core and antenna. On the longest timescales, a persistent ground-state bleach signifies excitons stabilised in terminal sinks. Notably, comparison of 8 K and 80 K spectra reveals that excitations are heterogeneously partitioned among multiple sinks at low disorder, whereas modest thermal activation promotes selective convergence into core-associated red chlorophylls. To interpret these dynamics, we employed atomistic excitonic Hamiltonians with time-nonlocal master equations, providing a quantitative framework for exciton migration and thermal redistribution. Together, these results demonstrate that C. merolae PSI broadens the kinetic funnel by distributing sinks across core and antenna, an evolutionary adaptation that extends spectral coverage whilst ensuring efficient trapping. These insights reconcile cyanobacterial and plant paradigms and illuminate how antenna expansion reshaped PSI function during the course of photosynthetic evolution.

physics.chem-ph↗

arXiv2024

The non-Abelian two-dimensional Toda lattice and matrix sine-Gordon equations with self-consistent sources

The non-Abelian two-dimensional Toda lattice and matrix sine-Gordon equations with self-consistent sources are established and solved. Two families of quasideterminant solutions are presented for the non-Abelian two-dimensional Toda lattice with self-consistent sources. By employing periodic and quasi-periodic reductions, a matrix sine-Gordon equation with self-consistent sources is constructed for the first time, for which exact solutions in terms of quasideterminants are derived.

nlin.SI↗

arXiv2024

The symmetric (2+1)-dimensional Lotka-Volterra equation with self-consistent sources

The symmetric (2+1)-dimensional Lotka-Volterra equation with self-consistent sources is constructed and solved by employing the source generation procedure, whose solutions are expressed in terms of pfaffians. As special cases of the pfaffian solutions, different types of explicit solutions are obtained, including dromions, soliton solutions and breather solutions.

nlin.SI↗