Machine-learning approach to understanding ultrafast carrier dynamics in the three-dimensional Brillouin zone of PtBi2 - Research overview

Aug 6, 2026

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Machine-learning enabled time- and angle-resolved photoemission spectroscopy is used to study unoccupied electronic structure and ultrafast electron dynamics in the type-I Weyl semimetal PtBi2. By varying probe photon energy across a broad range, predicted Weyl points are identified within the three-dimensional band structure. The role of k⊥ broadening in normally unoccupied states is discussed. Electron dynamics across the three-dimensional Brillouin zone are further analyzed with k-means clustering, revealing distinct slowing near the Weyl points compared with regions close to the bulk Fermi surface.

LanguageEnglish

CategoryResearch & Report

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Pages11

Reading Time28 min

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Introduction

Dirac and Weyl semimetals

Motivation for TR-ARPES in 3D systems

Challenges from multidimensional 3D Brillouin zone data

Unsupervised clustering for trend discovery

Machine-learning approach to understanding ultrafast carrier dynamics in the three-dimensional Brillouin zone of PtBi2 Paulina Majchrzak , 1 Charlotte Sanders ,2, 3 Yu Zhang ,2 Andrii Kuibarov ,4 Oleksandr Suvorov ,4 Emma Springate ,2 Iryna Kovalchuk ,4, 5 Saicharan Aswartham ,4 Grigory Shipunov ,4 Bernd Büchner ,4, 6 Alexander Yaresko ,7 Sergey Borisenko ,4, 6 and Philip Hofmann 1 , * 1 Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark 2 Central Laser Facility, Research Complex at Harwell, STFC Rutherford Appleton Laboratory, OX11 0QX, Harwell, United Kingdom 3 School of Physics and Astronomy, University of St Andrews, St Andrews, KY16 9SS, United Kingdom 4 Leibniz IFW Dresden, 01069 Dresden, Germany 5 Kyiv Academic University, 03142 Kyiv, Ukraine 6 Würzburg-Dresden Cluster of Excellence ct.qmat, 01069 Dresden, Germany 7 Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany (Received 14 June 2024; revised 23 September 2024; accepted 2 December 2024; published 7 January 2025) Using time-and angle-resolved photoemission spectroscopy, we examine the unoccupied electronic structure and electron dynamics of the type-I Weyl semimetal PtBi2. Using the ability to change the probe photon energy over a wide range, we identify the predicted Weyl points in the unoccupied three-dimensional band structure and we discuss the effect of k⊥ broadening in the normally unoccupied states. We characterize the electron dynamics close to the Weyl points and in other parts of three-dimensional Brillouin zone using k-means, an unsupervised machine-learning technique. This reveals distinct differences—in particular, that the electron dynamics close to the Weyl points are slower than in Brillouin zone regions close to the bulk Fermi surface. DOI: 10.1103/PhysRevResearch.7.013025 I. INTRODUCTION Dirac semimetals (DSMs) and Weyl semimetals (WSMs) are three-dimensional (3D) topological solids with zerodimensional nodal points in the band structure close to the Fermi energy. These points realize Dirac and Weyl fermions in 3D. The unusual band topology results in fascinating transport properties: for instance, negative magnetoresistance is related to the chiral anomaly in WSMs [1–3] . The conelike band structure of DSMs and WSMs results in optical properties with some similarity to the 2D DSM graphene, such as the possibility of absorbing infrared radiation over a wide frequency range [4], with resulting applications in photodetection [5,6], as well as effects owed to the chiral nature of the Weyl fermions [7–9] . A key property with respect to photodetection is the ultrafast electron dynamics [10] . Optical studies ﬁnd similarities to graphene [11–15], where the Dirac point, if it occurs above the Fermi energy EF , can serve as a bottleneck for the relaxation of photoexcited carriers [16–20] . However, DSMs and WSMs are 3D materials, and optical experiments do not resolve the crystal momentum k, so it is not possible to conclusively *[Contact author: philip@phys.au.dk](Contact author: philip@phys.au.dk) Published by the American Physical Society under the terms of the Creative Commons Attribution 4 .0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. link the overall dynamics observed in those experiments toa particular part of the bulk Brillouin zone (BZ) . Time- and angle-resolved photoemission spectroscopy (TR-ARPES) has the potential to address this: It can probe unoccupied electronic states and reveal their ultrafast electron dynamics as a function of energy and k [19,21,22] . However, performing TR-ARPES on WSMs is highly challenging, due to the materials’ 3D nature [23–26] . The conventional approach to TR-ARPES uses a single photon energy as a probe, and, in doing so, essentially assumes a 2D character in the material. In the case of a WSM,

FAQs

How are unoccupied electronic states and electron dynamics probed in PtBi2?

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) is used to examine unoccupied electronic structure and ultrafast electron dynamics as functions of energy and momentum-related coordinates.

How are the Weyl points identified experimentally?

A broad range of probe photon energies is employed, enabling probing of the three-dimensional Brillouin zone and revealing the predicted Weyl points in the unoccupied band structure.

What role does k-means clustering play in the analysis?

k-means clustering is used to organize high-dimensional photoemission intensity data into momentum-energy-time trend groups, helping distinguish distinct electron-dynamics behavior across the 3D Brillouin zone.

Aug 6, 2026

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