Surface melting or non-melting

A430 Surface Science 189/190 (1987) 645-648 North-Holland. Amsterdam

645

S U R F A C E MELTING B E H A V I O U R OF A u ( I l I ) P. C A R N E V A L I

I B M European Center for Scientific and Engineering Computing, Via Giorgione 15~), 00147 Rome, ltalv F. E R C O L E S S I

a n d E. T O S A T Y I

International S(hool for Advanced Studies, Strada Costiera I I, 34014 Trieste. Italy Received 1 April 1987; accepted for publication 4 May 1987 The melting behaviour of a metal surface is of considerab[e experimental interest, but has not so far been studied theoretically at a quantitative level. We have undertaken a molecular dynamics study of Au(111) based on the empirical but thoroughly tested "glue'" many-body force scheme. Using a slab geometry, we consider both reconstructed and unreconstructed surfaces. The reconstructed surface (which has a lower surface energy) remains stable in its "crystalline" form up to the bulk melting temperature T.,. It further survives as a metastable state for another ~ 15{/ K above Tm, Hence, we conclude that microscopic surface melting is absent in this case. The two outermost layers of the unreconstructed surface "melt'" - 1 0 0 K below Tm. This state of the surface remains stable up to T,,~. and it further survives as a metastable state again as high as - 100 K above Tm. Within the severe limits imposed by our finite-size fluctuations, we observe no other stable surface melting behaviour. These results for A u ( l l l ) are contrasted with those for Lennard-Jones cry'sial surfaces.

Surface Science 189/I90 (19871 649-655 North-Holland, Amsterdam

649

N E W E V I D E N C E FOR S E G R E G A T I O N OF PI ON P I ~ N i s o ( l l l ) S U R F A C E F R O M F O R W A R D SCATI'ERING PHOTOELECTRON DIFFRACTION Y. J U G N E T ,

N.S. PRAKASH,

G. GRKNET,

Tran Minh

ALLOY

DUC

lnstitut de Physique Nuel~aire de l~von, UniPersit~ Claude Bernard L~on 1, 43 Boule~,ard du iI noPembre 1918. 69622 Villeurbanne Cedex, France and H.C. POON

The Blackett Laboratory, Imperial College, lxmdon S W7 2BZ, UK Received 30 March 1987; accepted for publication 15 April 1987 It has been shown thal on PtsoNis{~{lll), PI segregates to the surface and forms the uppermost layer. X-ray photoelectron diffraction experiments appear to be useful for investigating this segregation. We found that Ni 2p azimuthal plots exhibit stronger anisotropy than Pt 4f does. This is new evidence for Pt segregation at the surface. The results are interpreted in terms of multiple forward scattering. The calculations have been done for different Pt surface concentrations in order to estimate the degree of segregation.