Unveiling Lattice Dynamics and a Hidden Structural Transition in the 2D Ferromagnet AgVP$_2$Se$_6$ via Raman Spectroscopy

Thi Hai Yen Pham; Po-hao Chang; Aidan C. Malloy; Seng Huat Lee; Zhiqiang Mao; Maria F. Munoz; Riccardo Torsi; Angela R. Hight Walker; Igor Mazin; Patrick M. Vora

arXiv · 2609.33116

Unveiling Lattice Dynamics and a Hidden Structural Transition in the 2D Ferromagnet AgVP$_2$Se$_6$ via Raman Spectroscopy

Thi Hai Yen Pham·Po-hao Chang·Aidan C. Malloy·Seng Huat Lee·Zhiqiang Mao·Maria F. Munoz·Riccardo Torsi·Angela R. Hight Walker·Igor Mazin·Patrick M. Vora

View original source ↗Read source document ↗Export metadata ↓

Abstract

Transition-metal chalcophosphates (TMCs) are two-dimensional (2D) van der Waals materials supporting a broad range of electronic and magnetic properties. Among quaternary TMCs, AgVP$_2$Se$_6$ is a rare ferromagnet (FM) with a triangular V sublattice, in contrast to the antiferromagnetic (AFM) zigzag chains of the sulfide AgVP$_2$S$_6$. Prior Raman studies of AgVP$_2$Se$_6$ have been performed far above the Curie temperature ($T_C$), preventing studies of spin-phonon coupling. Here, we report polarization-resolved, temperature-dependent Raman and magneto-Raman spectroscopy of AgVP$_2$Se$_6$ single crystals grown by chemical vapor transport (CVT) and flux methods, across the FM transition, complemented by density functional theory (DFT) calculations of phonon, optical and magnetic properties. Cryogenic Raman spectra resolve up to thirty peaks, while angle-resolved measurements enable their symmetry assignment. Spectral differences between CVT- and flux-grown crystals are traced back to the presence of different interlayer stacking domains in these samples. Temperature- and field-dependent Raman spectra remain largely unchanged across $T_C$, suggesting weak spin-phonon coupling. However, pronounced spectral alterations near 100 K suggest a potential structural phase transition. Our DFT calculations further reconcile the large discrepancy between the transport activation gap ($\approx$ 0.325 eV) and optical absorption edge ($\approx$ 2.14 eV), attributing the former to a transition to the V-d upper Hubbard band that is optically dark but thermally accessible. We also suggest a theoretical explanation of the qualitative difference (reproduced by DFT calculations) between the magnetic properties of the sulfide and selenide. These results provide key information regarding the lattice dynamics of AgVP$_2$Se$_6$ and will guide future applications in spin-based electronics.

Explore related subjects

cond-mat.mtrl-sci

Keep this discovery

Explore connections, maps & timelines

BibTeX RIS

Thi Hai Yen Pham, Po-hao Chang, Aidan C. Malloy, Seng Huat Lee, Zhiqiang Mao, Maria F. Munoz, Riccardo Torsi, Angela R. Hight Walker, Igor Mazin, Patrick M. Vora. 2026-09-27. Unveiling Lattice Dynamics and a Hidden Structural Transition in the 2D Ferromagnet AgVP$_2$Se$_6$ via Raman Spectroscopy. https://arxiv.org/abs/2609.33116

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related papers

arXiv2026

Crystal Dislocations as Atomic Scale Ratchets

The symmetry of a system's response to external stimuli is a fundamental concept in physics and materials science. At the microscopic scale, breaking this symmetry to achieve a rectified response is exceptionally difficult to engineer and remains rare in nature. Conventional micromechanics models of crystalline solids often assume a symmetric response to applied stress, where reversing the load simply inverts the direction of defect velocity without altering its magnitude. In this work, we report an atomic-scale, geometry-rooted mechanism that breaks this symmetry. Molecular dynamics simulations of face-centered cubic nickel reveal that dislocations containing atomic-scale jogs exhibit asymmetric mobility under opposite applied stresses: reversing the loading direction triggers significantly higher drag. This asymmetry arises from the coupling of two internal variables with different transformation parity: a non-affine displacement of an atom at the jog core, and a strain-like tensor associated with the advance of the dislocation. Because jogs are ubiquitous structures in plastic deformation, this discovery challenges classical descriptions of plastic deformation mechanisms, with direct implications for cyclic creep, and opens new pathways for defect engineering to enhance fatigue resistance.

cond-mat.mtrl-sci↗

arXiv2026

Orbital-engineered px,y-kagome lattice in a halogen monolayer

Multi-orbital kagome lattices with explicit orbital degrees of freedom remain largely unexplored, as most experimentally realized systems rely on complex d-electron manifolds that are approximated by isotropic single-orbital models. Here, we overcome this limitation by realizing a px,y-orbital kagome lattice through deposition of a Br monolayer on Ag(111), where orbital filtering selectively suppresses the pz channel. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density-functional-theory calculations reveal a large-area, highly ordered kagome structure whose band dispersions quantitatively match the anisotropic px,y tight-binding model. To extract the intrinsic manifold from the substrate background, we construct an effective H-passivated model, which uncover the intrinsic electronic structure and reveals nontrivial topological characteristics of the px,y kagome manifold driven by first-order spin-orbit coupling effect. Our work establishes Br/Ag(111) as an experimentally accessible platform for multi-orbital kagome physics, extending the kagome paradigm from the conventional d-orbital regime to an orbitally engineered topological setting.

cond-mat.mtrl-sci↗

arXiv2026

From Automated Simulation to Autonomous Discovery: A Hierarchical Framework for Agentic Computational Materials Science

The convergence of large language models, materials-specific foundation models, and agentic artificial intelligence is reshaping the paradigm of computational materials discovery. While high-throughput computation, automated workflows, and data-driven modeling have greatly expanded the scale of materials exploration, the core scientific decision-making loop remains largely human-directed. Agentic AI introduces the possibility of systems that can autonomously reason about materials objectives, execute simulations, and refine strategies. However, the rapid emergence of such systems has created a critical need for a unified and operational framework to define, evaluate, and guide scientific autonomy in computational materials discovery. In this Perspective, we propose the Computational Materials Agent Autonomy Level (CMA-AL) framework, a hierarchical taxonomy defining six levels of autonomous agency in computational materials science: scripted excecutor, LLM-assisted operator, adaptive explorer, experiment-ready modeler, agentic digital twin, and self-extending intelligence. We further map emerging agentic systems onto the framework and identify key scientific and technological challenges toward higher autonomy. CMA-AL provides a common language for characterizing agentic computational materials discovery, evaluating the maturity of emerging systems, and guiding their evolution toward increasingly autonomous materials discovery.

cond-mat.mtrl-sci↗