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Signatures of Kramers-Weyl fermions in the charge density wave material (TaSe4)2I

Tue, 30 Sep 2025 15:00:00 GMT

Title: Signatures of Kramers-Weyl fermions in the charge density wave material (TaSe4)2I Author(s): Kim, Soyeun; McKay, Robert C.; Bielinski, Nina G.; Park, Junehu; Zhao, Chengxi; Lin, Meng Kai; Hlevyack, Joseph Andrew; Guo, Xuefei; Mo, Sung Kwan; Abbamonte, Peter M.; Chiang, Taichang; Schleife, André; Shoemaker, Daniel P.; Bradlyn, Barry; Mahmood, Fahad Abstract: The quasi-one-dimensional charge density wave (CDW) material (TaSe4)2I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle-resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe4)2I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. Our findings highlight the unique topological nature of (TaSe4)2I and provide a pathway for identifying KW fermions in other chiral materials.

Machine Learning Based on Digital Image Colorimetry Driven In Situ, Noncontact Plasma Etch Depth Prediction

Wed, 31 Dec 2025 15:00:00 GMT

Title: Machine Learning Based on Digital Image Colorimetry Driven In Situ, Noncontact Plasma Etch Depth Prediction Author(s): Kang, Minji; Kim, Seongho; Go, Eunseo; Paek, Donghyeon; Lim, Geon; Kim, Muyoung; Kim, Changmin; Kim, Soyeun; Jang, Sung Kyu; Bak, Moon Soo; Choi, Min Sup; Kang, Woo Seok; Kim, Jaehyun; Kim, Jaekwang; Kim, Hyeong-U Abstract: This study presents a noncontact, in situ framework for etch depth prediction in plasma etching using machine learning (ML) and digital image colorimetry (DIC). While conventional ex situ methods offer accuracy, they suffer from delays and contamination risks. To overcome these, two approaches are explored. First, etch depth is initially obtained through ellipsometry mapping and used to train an artificial neural network (ANN) based on process parameters (e.g., plasma power, pressure, and gas flow), achieving significantly lower mean squared error (MSE) than a linear baseline. This is extended with a Bayesian neural network (BNN) to capture uncertainty in the predictions. Second, it is demonstrated that red, green, and blue data from DIC alone can effectively predict etch depth without relying on process parameters. Together, these findings establish ML-DIC integration as a real-time, low-cost, and noninvasive alternative for plasma process monitoring.

Floquet-Bloch manipulation of the Dirac gap in a topological antiferromagnet

Fri, 28 Feb 2025 15:00:00 GMT

Title: Floquet-Bloch manipulation of the Dirac gap in a topological antiferromagnet Author(s): Bielinski, Nina; Chari, Rajas; May-Mann, Julian; Kim, Soyeun; Zwettler, Jack; Deng, Yujun; Aishwarya, Anuva; Roychowdhury, Subhajit; Shekhar, Chandra; Hashimoto, Makoto; Lu, Donghui; Yan, Jiaqiang; Felser, Claudia; Madhavan, Vidya; Shen, Zhi-Xun; Hughes, Taylor L.; Mahmood, Fahad Abstract: Floquet–Bloch manipulation, achieved by driving a material periodically with a laser pulse, is a method that enables the engineering of electronic and magnetic phases in solids by effectively modifying the structure of their electronic bands. However, the application of Floquet–Bloch manipulation in topological magnetic systems, particularly those with inherent disorder, remains largely unexplored. Here we realize Floquet–Bloch manipulation of the Dirac surface-state mass of the topological antiferromagnet MnBi2Te4. Using time- and angle-resolved photoemission spectroscopy, we show that opposite helicities of mid-infrared circularly polarized light result in substantially different Dirac mass gaps in the antiferromagnetic phase, despite the equilibrium Dirac cone being massless. We explain our findings in terms of a Dirac fermion with a random mass. Our results underscore Floquet–Bloch manipulation as a powerful tool for controlling topology, even in the presence of disorder, and for uncovering properties of materials that may elude conventional probes. © The Author(s), under exclusive licence to Springer Nature Limited 2025.