Ionic Overlayers on Corrugated Surfaces II: Melting
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IONIC OVERLAYERS ON CORRUGATED SURFACES II: MELTING Hyangsuk Seong", S.D. Mahanti*, Surajit Sen*, and Tahir 'agin*" "Department of Physics and Astronomy and the Center for Fundamental Materials Research, Michigan State University, East Lansing, MI 48824 "'Molecular Simulations Inc., Suite 540, 199 S. Robles Avenue, Pasadena, CA 91101 ABSTRACT Using molecular dynamics simulation we have investigated the melting properties of a 2dimensional system of ions interacting through a screened Coulomb potential in the presence of a substrate. We focus on the role of the substrate potential in the melting transition. The physical system we study is the stage-2 graphite intercalation compound RbC 2 4 and a related system RbC24.57 whose intercalant density differs from the former by a small amount. Our results reveal the nature of melting processes characteristic of periodic domain wall solids. INTRODUCTION The melting properties of two-dimensional (2D) solids continue to be of theoretical and experimental interest since the dislocation- and disclination-mediated melting ideas put forth by Kosterlitz and Thouless[1], Nelson and Halperin [2], and Young [3]. In real physical systems such as physisorbed overlayers on graphite (or other) substrates [4] or intercalation compounds [5]-[8] one has to deal with the effects of the substrate corrugation potential on the melting transition. In addition, weak interlayer interactions may also affect the nature of this phase transition. In this paper we investigate the melting properties of a real physical system namely RbbC 24 , a stage-2 graphite intercalation compound (GIC) which has been extensively studied both experimentally [5, 6] and theoretically [7, 8] during the past several years. In paper I (to be referred to as I) of this series we discussed the ground state structure of this system obtained from molecular dynamics (MD) simulation using a potential which successfully reproduced the liquid structure data [8]. We found that the same potential described the ground state structure of the solid very well. In fact the ground state structure could be represented rather well by a periodic domain wall (PDW) structure with some defects [9]. In this paper we discuss how this PDW solid with defects (to be referred to as System II) melts with increasing temperature and how its melting properties differ from that of an ordinary triangular solid (in the absence of a corrugation potential) (to be referred to as System I). Also to isolate the effects of defects we study a third system (System III) whose ground state has a perfect PDW structure. MODEL AND SYSTEMS The model consists of Rb ions interacting through a screened Coulomb potential and moving in 2D. The substrate potential is that appropriate for a GIC and we assume that the effects of the graphite host and other intercalant layers can be represented by a single particle potential V, given by Vi(r) = -2K[2 cos(2irx/a) cos(27ry/-VFa) + cos(47ry/V/ a)]. (1) In Eq. (1), KI determines the strength of the corrugation potential and a is the g
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