Constant-Temperature Molecular-Dynamics Simulation of the 2D Melting Transition of Rb and K
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CONSTANT-TEMPERATURE MOLECULAR-DYNAMICS SIMULATION OF THE 2D MELTING TRANSITION OF Rb AND K
2 1 J. D. FAN , ZHI1-XIONG CA1
I Department of Physics, Southern University and A & M College, Baton Rouge, LA 70813 2 Materials Science Division, Brookhaven National Laboratory, Upton, NY 11973
ABSTRACT The energy histogram method, introduced by Ferrenberg and Swendsen [Phys. Rev. Lett., 61, 2635, (1988) and 63, 1195, (1989)], was applied for the first time to the constant temperature molecular dynamics (MD) simulation of a two-dimensional (2D) system with incommensurate structures. We performed MD simulations for the stage-2 graphite intercalation compounds (GIC's) with Rb or K being the intercalants (Rb-GIC's and KGIC's). The temperature dependence of the specific heat, Cv, is calculated for various sizes up to 864 atoms. The melting temperature was found to be 158 K for Rb-GIC's and 119 K for KGIC's, respectively, which are in agreement with the experimental observations.
INTRODUCTION For over a decade the melting transitions of 2D systems have been intensively investigated both experimentally and theoretically and a valuable review has been given by Strandburg [3]. Computer simulation is one of the important techniques used in this field. The determination of the transition temperatures and critical properties at the transition were also done using both MC and MD simulation techniques in which a very large amount of computer time is needed for accumulating sufficient configuration information to finally calculate the temperature dependence of the specific heat, and hence to determine the transition temperature and the critical properties. An economical and more efficient method, called the energy histogram method, was, however, introduced by Ferrenberg and Swendsen [1, 2] and we will briefly describe it below. To our knowledge, this is the first time this method has been applied to an MD study of the 2D melting transition. The real systems we have studied are the stage-2 Rb and K graphite intercalation compounds (GIC's) in which alkali metal Rb and K atoms are intercalated in graphite forming a sandwich-like structure with a quasi 2D metal sheet between adjacent graphite layers separated by an empty (normal) graphite spacing. A variety of static and dynamic properties of different types of GIC's was extensively studied in the last decade by means of X-ray and neutron scattering experiments and mostly recently reviewed in a volume edited by Zabel and Solin [4]. The MD simulation method applied to these 2D alkali liquids by Fan et al. has been largely successful in reproducing both static and dynamic features of the data [5 - 9]. The determination of the order-disorder phase transition temperature of GIC's using the MD technique has not been published although it was attempted by Fan [5]. We intend to present the result of a more recent constant temperature MD simulation by means of the histogram method and compare it with the experimental observations. Our purpose here is to show the success of the MD simulation and
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