Among the phenomena observed by STM are the effects of individual impurity atoms. Scattering by impurity atoms should create local electronic states characteristic of d-wave superconductivity. I will discuss experiments on the high-Tc superconductor Bi2Sr2CaCu2O8-d (Bi-2212) in which these states were discovered , measurements of their properties at deliberately introduced Ni and Zn impurity atoms [2,3], and the physical implications of these observations.
Finally, I will introduce newly emerging evidence of an exotic electronic state bound to the vortex cores in Bi-2212 . These states break continuous translational and rotational symmetries and exhibit a ‘checkerboard’ pattern, with a periodicity of four unit cells (4a0) and an orientation parallel to the Cu-O bonds. Their existence may be related to the recently discovered spin structure of the HTSC vortex, which exhibits an 8a0 periodic spin density modulation with wavevector parallel to the Cu-O bonds. The implications of these observations for the mechanism of HTSC will be reviewed.
This project was carried out in collaboration with K. M. Lang and V. Madhavan, J. Hoffman (Berkeley), E. W. Hudson (NIST Gaithersburg) S. H. Pan (Boston University), H. Eisaki (Stanford University) and S. Uchida (Tokyo University).
 E. W. Hudson et al, Nature 411 920 (2001).
 J. Hoffman et al submitted.