Thursday, April 9, 2026
4:00 pm-5:00 pm
Chiral Electron’s Quantum Adventure in Topological Wonderland
A new class of quantum materials, known as Weyl semimetals, has emerged, in which electrons behave as massless particles with their spin locked to their direction of motion. This spin-momentum locking endows electrons with a handedness, or chirality, that governs their interactions with light and external electric or magnetic fields. When magnetism is introduced, as in magnetic Weyl semimetals, an additional degree of freedom arises, thereby opening new pathways for controlling and manipulating topological states. We investigate a mid-infrared chiral photoresponse in the magnetic Weyl semimetal PrAlGe, probing the excitation of chiral electrons in two Weyl bands near the Fermi level. A sharp transition in the chiral photoresponse is observed at the onset of ferromagnetic order below the Curie temperature. Unlike the broad ferromagnetic phase transition, this sharp transition signals a topological phase transition associated with a change in the sum of Chern numbers from zero to a nonzero value via chiral electron excitation across time-reversal-symmetry-broken Weyl bands. Because several Weyl bands lie close to the Fermi level, resolving their individual topological contributions requires low-energy polarimetry probes. To meet this need, we have developed a far-infrared polarimetry platform based on surface phonon polaritons operating in the terahertz gap. This approach enables sensitive low-energy polarimetric measurements and provides a powerful new tool for spectrally resolving Weyl physics and directly probing band-specific topological invariants.
