We address the problem of Dirac fermions interacting with longitudinal phonons. A gap in the spectrum of fermions leads to the emergence of the Chern--Simons excitations in the spectrum of phonons. We study the effect of those excitations on observab
le quantities: the phonon dispersion, the phonon spectral density, and the Hall conductivity.
We investigate the finite-size scaling behavior of the conductivity in a two-dimensional Dirac electron gas within a chiral sigma model. Based on the fact that the conductivity is a function of system size times scattering rate, we obtain a two-param
eter scaling flow toward a finite fixed point. The latter is the minimal conductivity of the infinite system. Depending on boundary conditions, we also observe unstable fixed points with conductivities much larger than the experimentally observed values, which may account for results found in some numerical simulations. By including a spectral gap we extend our scaling approach to describe a metal-insulator transition.
The conductivity of an electron gas can be alternatively calculated either from the current--current or from the density--density correlation function. Here, we compare these two frequently used formulations of the Kubo formula for the two--dimension
al Dirac electron gas by direct evaluations for several special cases. Assuming the presence of weak disorder we investigate perturbatively both formulas at and away from the Dirac point. While to zeroth order in the disorder amplitude both formulations give identical results, with some very strong assumptions though, they show significant discrepancies already in first order. At half filling we evaluate all second order diagrams. Virtually none of the topologically identical diagrams yield the same corrections for both formulations. We conclude that a direct comparison of conductivities of disordered system calculated in both formulas is not possible.
We investigate the scaling properties of the recently acquired fermionic non--linear $sigma$--model which controls gapless diffusive modes in a two--dimensional disordered system of Dirac electrons beyond charge neutrality. The transport on large sca
les is governed by a novel renormalizable nonlocal field theory. For zero mean random gap, it is characterized by the absence of a dynamic gap generation and a scale invariant diffusion coefficient. The $beta$ function of the DC conductivity, computed for this model, is in perfect agreement with numerical results obtained previously.
