# Ultrafast Momentum Imaging of Pseudospin-Flip Excitations in Graphene

## TL;DR

This study investigates the ultrafast momentum-dependent hot carrier dynamics in graphene, revealing pseudospin-related anisotropies, azimuthal thermalization, and doping effects using advanced photoemission spectroscopy.

## Contribution

It provides a detailed microscopic understanding of carrier relaxation processes in graphene, highlighting the role of pseudospin and substrate effects in hot carrier dynamics.

## Key findings

- Azimuth-dependent electron distribution initially forms after excitation.
- Thermalization occurs azimuthally at longer times, influenced by substrate and doping.
- Differences observed between electron and hole dynamics in n-doped graphene.

## Abstract

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photo-excited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization, and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy and in-plane momentum. We first show that the rapidly-established quasi-thermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photo-excited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photo-control experiments and optoelectronic device applications.