STM Studies of Individual Ti Impurity Atoms in Sr2RuO4 Barry Barker   Impurity atoms have long been known to perturb the electronic state of the surrounding material, producing well-known impurity bands, excess scattering, local electronic impurity-states, etc.  It has recently become possible to study impurity states directly with high resolution scanning tunneling microscopy and spectroscopy [1-3].  I will highlight some of the results obtained studying high temperature superconductors with such a method, then go on to describe the development of a next generation STM system to perform such studies at temperatures below 100 mK and at fields up to 8.5 Tesla.  I will then review the properties of Sr2RuO4, an unconventional low temperature superconductor.  New evidence gained through use of the ULT-STM in studies of Ti-doped Sr2RuO4 indicates multiple gap structures in the density of states with strong modifications near Ti atoms.  I will close by discussing the implications of these results in understanding the normal and superconducting properties of Sr2RuO4.   1E.W. Hudson et al., Science 285, 88 (1999). 2S.H. Pan et al., Nature 403, 746 (2000). 3E.W. Hudson et al., Nature 411, 920 (2001).