Phonon induced pseudospin rotation in graphene

Pseudospin, a degree of freedom in a two-level quantum system of two sublattices, is a basic quantum number that poses a fundamental symmetry prone to the electronic nature of graphene. We propose an extended Hamiltonian that involves the SU(2) rotation of the pseudospin for electron-phonon scattering, which has a significant advantage over the conventional second-quantized polar coupling. A phonon satellite accompanied by an electron linearly pumped to the upper Dirac cone corresponds to a rotated pseudospin within a scheme of the time-resolved photoemission spectroscopy (TRPES), which is found to carry nonzero angular momenta and induce nonvanishing Berry curvatures through the dichroic mode of TRPES. A phonon bringing the pseudospin rotation is identified to be an elliptically polarized one, which compensates for the angular momenta delivered to the pseudospin. This is an indication of the chiral phonon excitation without the circularly polarized pulse pumping. Our findings suggest that the controlled generation of phonon builds up a new roadmap for engi-neering the Berry curvature and topology in Dirac semiconductors.

Automated summary

Pseudospin, a fundamental quantum number in graphene, can be changed by the rotation of phonons. By observing the phonon satellite accompanying the linearly pumped electron to the upper Dirac cone, we found that pseudospin exhibits rotation, nonzero angular momentum, and Berry curvature. We also identified the phonon responsible for pseudospin rotation as an elliptically polarized phonon, which can induce chiral phonon excitation without circularly polarized pumping.