Large quantum nonreciprocity in plasmons dragged by drifting electrons

Abstract

Collective plasmon modes, riding on top of drifting electrons, acquire a fascinating nonreciprocal dispersion characterized by $\omega_p(\bm{q}) \neq \omega_p(-\bm{q})$. The {\it classical} plasmonic Doppler shift arises from the polarization of the Fermi surface due to the applied DC bias voltage. Going beyond this paradigm, we predict a {\it quantum} plasmonic Doppler shift originating from the quantum metric of the Bloch wavefunction. We systematically compare the classical and quantum Doppler shifts by investigating the drift-induced nonreciprocal plasmon dispersion in generic quantum systems. We demonstrate quantum nonreciprocal plasmons in graphene and twisted bilayer graphene. We show that the quantum plasmonic Doppler shift dominates in \moire systems at large wavevectors, yielding plasmonic nonreciprocity up to 20\% in twisted bilayer graphene. Our findings demonstrate the supremacy of plasmonic quantum Doppler shift in \moire systems, motivating the design of innovative nonreciprocal photonic devices with potential technological implications.