Signatures of adatom effects in the quasiparticle spectrum of Li-doped graphene

We study the spectral function and quasiparticle scattering in Li-decorated graphene (Li@graphene) with an atomistic $T$-matrix formalism and uncover adatom-induced spectral effects which shed light on experimentally observed angle-resolved photoemission spectroscopy (ARPES) features. From transport studies, alkali adatoms are known to introduce charged-impurity scattering limiting the carrier mobility. Here, we demonstrate that Li adatoms furthermore give rise to a low-energy impurity band centered at the $\mathrm{\ensuremath{\Gamma}}$ point which originates from the hybridization between the atomic $2s$ state of the Li adatoms and graphene ``surface'' states. We show that the impurity band is strongly dependent on the concentration ${c}_{\mathrm{Li}}$ of Li adatoms, and aligns with the Li-induced Fermi level on the Dirac cone at ${c}_{\mathrm{Li}}\ensuremath{\sim}8%$ (${E}_F\ensuremath{\approx}1.1$ eV). Finally, we show that adatom-induced quasiparticle scattering increases dramatically at energies above $\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ close to the van Hove singularity in the graphene density of states, giving rise to a large linewidth broadening on the Dirac cone with a concomitant downshift and a characteristic kink in the conduction band. Our findings are highly relevant for future studies of ARPES, transport, and superconductivity in adatom-doped graphene.