Unconventional quasiparticle lifetime in undoped graphene

TL;DR

This paper investigates the quasiparticle decay rate in undoped graphene at low energies, revealing a cubic energy dependence due to soft phonon modes, which affects transport properties.

Contribution

It demonstrates how many-body effects induce soft phonon modes that dominate quasiparticle decay in undoped graphene at low energies.

Findings

Quasiparticle decay rate scales as the cube of energy at low energies.

Soft out-of-plane phonon modes significantly influence quasiparticle decay.

Transport properties exhibit a crossover from linear to slower decay regimes.

Abstract

We address the question of how small can the quasiparticle decay rate be at low energies in undoped graphene, where kinematical constraints are known to prevent the decay into particle-hole excitations. For this purpose, we study the renormalization of the phonon dispersion by many-body effects, which turns out to be very strong in the case of the out-of-plane phonons at the K point of the spectrum. We show that these evolve into a branch of very soft modes that provide the relevant channel for quasiparticle decay, at energies below the scale of the optical phonon modes. In this regime, we find that the decay rate is proportional to the cube of the quasiparticle energy. This implies that a crossover should be observed in transport properties from the linear dependence characteristic of the high-energy regime to the much slower decay rate due to the soft phonon modes.