Cherie Kagan

Humboldt Fellow as of January 2025

Overview of my Research Project:

Topological insulators and Weyl semimetals are quantum materials whose properties arise from strong spin–orbit coupling. Unlike conventional conductors or semiconductors, they host surface and nodal states with helical spin textures, where an electron’s spin direction is locked to its momentum. This unique relationship between spin and charge enables direct control of electronic states with light, opening opportunities to study new physical phenomena and to design next-generation optoelectronic and spintronic devices.

In topological insulators, carefully tuning the polarization and energy of light allows us to separate the contributions of spin-degenerate bulk bands from those of spin-locked surface states. This capability provides a powerful window into the band structure, as well as the carrier and spin dynamics that govern these materials. It also enables us to probe the creation and stability of chiral excitons — bound electron–hole pairs whose optical properties reflect the underlying spin–momentum locking.

Both topological insulators and Weyl semimetals also exhibit the circular photogalvanic effect, where excitation with circularly polarized light generates spin-polarized currents. This effect directly links light polarization to spin and charge transport, offering a way to convert optical excitation into functional spin currents. By exploring and enhancing these responses, we aim to reveal fundamental mechanisms of light–matter interaction in topological systems and to advance their potential as platforms for novel photonic and spin-based technologies.

As an awardee of the Alexander von Humboldt Research Award, I am excited to pursue these questions with the Topological Quantum Chemistry group led by Felser at the Max Planck Institute for Chemical Physics of Solids. I plan to use a combination of optical spectroscopies and optoelectronic measurements to investigate chiral excitation, emission, and spin-dependent conductivity in topological insulators and Weyl semimetals. These spectroscopies are carried out across a wide range of timescales and temperatures, with excitation energies spanning the visible to mid-infrared — above and below the bulk band gap in topological insulators and near the Weyl nodal points in semimetals. We will characterize crystals with different structures as we tune the Fermi level through differential doping, explore electrostatically gated thin layers in phototransistor platforms, and employ thermal gating via integrated optical absorbers.