Unusual Microwave Response of Dirac Quasiparticles in Graphene

TL;DR

This paper explores how microwave measurements reveal the unique dynamical properties of Dirac quasiparticles in graphene, including effects of chemical potential, gaps, and magnetic fields on conductivity responses.

Contribution

It demonstrates that microwave response is an effective probe of Dirac quasiparticle dynamics in graphene, highlighting novel lineshapes and magnetic field effects.

Findings

Intraband response exhibits a cusp at zero frequency at small chemical potential.

Response becomes Drude-like with increasing chemical potential, with width proportional to μ.

At large magnetic fields, conductivities become B-independent but remain nonzero, showing Landau level structure.

Abstract

Recent experiments have proven that the quasiparticles in graphene obey a Dirac equation. Here we show that microwaves are an excellent probe of their unusual dynamics. When the chemical potential is small the intraband response can exhibit a cusp around zero frequency $\Omega$ and this unusual lineshape changes to Drude-like by increasing the chemical potential $|\mu|$, with width also increasing linearly with $\mu$. The interband contribution at T=0 is a constant independent of $\Omega$ with a lower cutoff at $2 \mu$. Distinctly different behavior occurs if interaction-induced phenomena in graphene cause an opening of a gap $\Delta$. At large magnetic field $B$, the diagonal and Hall conductivities at small $\Omega$ become independent of $B$ but remain nonzero and show structure associated with the lowest Landau level. This occurs because in the Dirac theory the energy of this level, $E_0 = \pm \Delta$, is field independent in sharp contrast to the conventional case.