A conventional STM can image a surface anywhere from once every 10 seconds to once every 3 minutes. At elevated temperatures surface features such as ad-atoms, ad-dimers, defects, and step edges hop from site to site tens to thousands of times more frequently. This disparity of rates severely limits a conventional STM's applicability in studying surface dynamics. However, when studying dynamics, the trajectory of a single feature is often of much greater interest than is the evolution of a relatively stable surface; the majority of scan time is wasted in gathering unnecessary information. Tracking is a method which solves this problem. Tracking makes use of an additional feedback loop to confine the scan range to the immediate vicinity of a feature. As the feature moves, the tip then "tracks" it across the surface. The result is that we are now able to see, say, a dimer's every hop from site to site. Through studying hop rates, we can now better measure barriers to different paths.
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