Nesting Symmetries and Diffusion in Disordered d-Wave Superconductors

The low-energy density of states (DOS) of disordered 2D d-wave superconductors is extremely sensitive to details of both the disorder model and the electronic band structure. Using diagrammatic methods and numerical solutions of the Bogoliubov--de Gennes equations, we show that the physical origin of this sensitivity is the existence of a novel diffusive mode with momentum close to (pi,pi) which is gapless only in systems with a global nesting symmetry. We find that in generic situations the DOS vanishes at the Fermi level. However, proximity to the highly symmetric case may nevertheless lead to observable nonmonotonic behavior of the DOS in the cuprates