  We study the local density of states around potential scatterers in d-wave
superconductors, and show that quantum interference between impurity states is
not negligible for experimentally relevant impurity concentrations. The two
impurity model is used as a paradigm to understand these effects analytically
and in interpreting numerical solutions of the Bogoliubov-de Gennes equations
on fully disordered systems. We focus primarily on the globally particle-hole
symmetric model which has been the subject of considerable controversy, and
give evidence that a zero-energy delta function exists in the DOS. The
anomalous spectral weight at zero energy is seen to arise from resonant
impurity states belonging to a particular sublattice, exactly as in the
2-impurity version of this model. We discuss the implications of these findings
for realistic models of the cuprates.
