Growth of an orientationally ordered incommensurate potassium overlayer and its order-disorder transition on the Cu(111) surface

Abstract

The evaporation of potassium onto the Cu(111) surface at 80 K results in the formation of an incommensurate orientationally ordered structure as observed by low-energy electron diffraction (LEED). The incommensurate structure appears when the potassium coverage is near 0.09, and can be characterized as a non-close-packed-hexagonal phase with an interatomic spacing continuously varying with concentration from 6.3 to 4.4 Å. As the temperature is increased, the incommensurate phase with larger K-K spacing (>5.2 Å) undergoes three transition stages as identified by LEED intensity and beam-width (full width at half maximum) measurements: (i) a better ordered hexagonal phase, (ii) a hexatic phase, and (iii) a disordered phase. The incommensurate phase with smaller K-K spacing (2 Å), however, undergoes only two transition stages: (i) a better ordered hexagonal phase and (ii) a disordered phase. The incommensurate overlayer may behave as a ‘‘two-dimensional solid.’’