Dynamical charge and pseudospin currents in graphene and possible Cooper pair formation

TL;DR

This paper investigates charge and pseudospin currents in graphene using quantum kinetic equations, revealing anomalous currents, universal conductivities under magnetic fields, and conditions conducive to Cooper pair formation.

Contribution

It introduces a regularization-free approach to calculate currents in graphene with SU(2) structure, highlighting anomalous pseudospin currents and potential for superconductivity.

Findings

Currents have only anomalous parts, no quasiparticle contribution.

Optical conductivity matches experimental data with impurity scattering.

Conditions for effective attractive Coulomb interaction and Cooper pairing are identified.

Abstract

Based on the quantum kinetic equations for systems with SU(2) structure, regularization-free density and pseudospin currents are calculated in graphene realized as the infinite mass-limit of electrons with quadratic dispersion and a proper spin-orbit coupling. Correspondingly the currents possess no quasiparticle part but only anomalous parts. The intraband and interband conductivities are discussed with respect to magnetic fields and magnetic domain puddles. It is found that the magnetic field and meanfield of domains can be represented by an effective Zeeman field. For large Zeeman fields the dynamical conductivities become independent of the density and are universal in this sense. The different limits of vanishing density, relaxation, frequency, and Zeeman field are not interchangeable. The optical conductivity agrees well with the experimental values using screened impurity scattering and an effective Zeeman field. The universal value of Hall conductivity is shown to be modified due to the Zeeman field. The pseudospin current reveals an anomaly since a quasiparticle part appears though it vanishes for particle currents. The density and pseudospin response functions to an external electric field are calculated and the dielectric function is discussed with respect to collective excitations. A frequency and wave-vector range is identified where the dielectric function changes sign and the repulsive Coulomb potential becomes effectively attractive allowing Cooper pairing.