Real-Time Observation of Dynamic Spin Polarization in Copper Oxide by Carrier Scattering with Chiral Phonons.

Despite extensive experimental and theoretical efforts, the underlying mechanism of chiral-induced spin selectivity (CISS) remains elusive. Although one emerging theory is that CISS results from angular momentum exchange between electrons and chiral phonons, there has been no direct observation of spin-dependent electron-phonon scattering rates in a chiral environment. Toward this goal, ultrafast extreme ultraviolet circular dichroism (XUV-CD) was developed to measure spin-dependent thermalization and transport of hot carriers in chiral CuO. Results indicate that spin polarization emerges on the ultrafast time scale of carrier-phonon interactions and is preserved during phonon-mediated carrier hopping. These observations reveal that CISS does not operate entirely by a spin-filtering mechanism; rather, angular momentum exchange between carriers and a chiral lattice results in dynamic spin polarization.