Growth and melting of a Pb monolayer on Cu(111)

Abstract

The growth and thermal behavior of thin Pb films on Cu(111) up to 1 monolayer (ML) coverage was investigated by means of high resolution low energy electron diffraction (SPA-LEED). The lattice constant of the Pb monolayer depends on the coverage varying from 0.497 nm (0.5 ML) to 0.480 nm (1 ML) and is incommensurate. At a coverage of 0.7 ML the thermal expansion coefficient of this film is twice the Pb bulk value. The melting temperature of the film depends on the coverage, too, varying from 550 K (0.5 ML) to more than 700 K (>1ML) compared to the Pb bulk value of 601 K. The melting process was studied in detail by fitting the profiles with different models. Although not fully applicable a comparison has been tried with the predictions of the KTHNY theory (developed by Kosterlitz, Thouless, Halperin, Nelson and Young) of two-dimensional melting. Below the melting temperature the film can be described as a “floating solid” with a “quasi” long-range order.