Ultrafast carrier dynamics throughout the three-dimensional Brillouin zone of the Weyl semimetal PtBi$_2$

TL;DR

This study uses advanced photoemission spectroscopy and machine learning to explore unoccupied electronic states and electron dynamics in the Weyl semimetal PtBi$_2$, revealing spatially dependent ultrafast behaviors.

Contribution

It identifies the three-dimensional Weyl points in unoccupied states and characterizes their electron dynamics using $k$-means clustering, a novel approach in this context.

Findings

Faster electron dynamics in bulk Fermi surface regions

Identification of Weyl points in unoccupied states

Distinct dynamical behaviors near Weyl points

Abstract

Using time- and angle-resolved photoemission spectroscopy, we examine the unoccupied electronic structure and electron dynamics of the type-I Weyl semimetal PtBi$_2$. 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 $k_\perp$ broadening in the normally unoccupied states. We characterise the electron dynamics close to the Weyl point and in other parts of three-dimensional Brillouin zone using $k$-means, an unsupervised machine learning technique. This reveals distinct differences -- in particular, dynamics that are faster in the parts of the Brillouin zone that host most of the bulk Fermi surface than in parts close to the Weyl points.