INTERACTION OF C WITH NI(100) - ATOM-RESOLVED STUDIES OF THE CLOCK RECONSTRUCTION

The interaction of C with Ni(100) has been studied by scanning tunneli ng microscopy (STM). The C atoms, imaged as almost-equal-to 0.3 angstr om deep depressions consistent with theoretical predictions, are at lo w C coverages located in the fourfold hollow sites, causing a small ra dial displacement of the surrounding Ni atoms. Above a local critical coverage, C induces a disorder-order transition to a reconstructed pha se, and the STM images reveal a coordinated 0.55 angstrom lateral cloc kwise-counterclockwise displacement of the topmost Ni atoms. A simple model for the driving force for this ''clock'' reconstruction is propo sed.