Based on an extended Hubbard model, we present calculations of both the local
(i.e., single-site) and spatially-averaged differential tunneling conductance
in d-wave superconductors containing nonmagnetic impurities in the unitary
limit. Our results show that a random distribution of unitary impurities of any
concentration can at most give rise to a finite zero-bias conductance (with no
peak there) in spatially-averaged non-STM type of tunneling, in spite of the
fact that local tunneling in the immediate vicinity of an isolated impurity
does show a conductance peak at zero bias, whereas to give rise to even a small
zero-bias conductance peak in the former type of tunneling the impurities must
form dimers, trimers, etc. along the [110] directions. In addition, we find
that the most-recently-observed novel pattern of the tunneling conductance
around a single impurity by Pan et al. [Nature (London) 403,746 (2000)] can be
explained in terms of a realistic model of the tunneling configuration which
gives rise to the experimental results reported there. The key feature in this
model is the blocking effect of the BiO and SrO layers which exist between the
tunneling tip and the CuO_2 layer being probed.Comment: 9 pages, 7 ps-figures, to appear in Phys. Rev. B (Sep. 1, 2000);
typos corrected, references added, figure 6 changed to expand the explanation
on recent experimental measurements by S.H. Pan et al. [Nature (London) 403,
746 (2000)
