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First-Principles Study of Apparent Barrier Height

We have analyzed apparent barrier height measured by scanning tunneling microscopy (STM), using calculations within the density functional theory including the effects of electric fields and currents. In particular, we have compared the apparent barrier height of an Al(100) surface containing a vacancy cluster in the second layer with that of a surface without defects. We have found that the site with the defect, appearing as hillocks in the STM topographic image, is imaged as dark spots in the apparent barrier height image. These results can be attributed to two factors, the difference of the density of states (DOS) near EF and the difference of the decay rate of electron density around the sample surface.