Theory of Scanning Tunneling Microscopy Probe of Impurity States in a   D-Wave Superconductor
  Scanning tunneling microscopy can provide a probe for the detailed study of quasiparticle states in high-Tc superconductors. We propose that it can also be used to acquire specific information about impurity-induced quasiparticle states and the superconducting order-parameter structure. In particular, the local density of states is found to be sensitive to impurity-induced resonances and to the symmetry of the order parameter. 
M.I. Salkola, A.V. Balatsky, and D.J. Scalapino