ANALYSIS OF STRUCTURAL PHASE-TRANSITIONS BY COMPUTER-SIMULATION

This paper presents the results from molecular-dynamics calculations on a lattice-dynamical system which undergoes a structural phase transition. The characteristics of the model system used for the calculations are described in detail; the model is two dimensional, has an antiferro-distortive structural change, and is in the category of displacive structural transitions. The following results are obtained from the molecular-dynamics calculations. First, some equilibrium properties are shown, including the static correlation function, to establish that the system has a second-order phase transition. Second, the results for the time-dependent order parameter or soft-mode correlation function are given. Near the transition the spectral function for this correlation function exhibits a very narrow and intense central peak, in addition to the soft-mode peak. The temperature dependences of the soft-mode frequency and the central-peak width are given. The results obtained here are very similar to experimental observations, particularly on SrTiO3, and they are used to argue that only intrinsic anharmonic mechanisms are needed to explain the origin of central peaks. Third, extensive results are given for the temperature and wave-vector dependence of the displacement correlation function, which give the wave-vector dependence of the central-peak characteristics. In a small temperature interval around the transition and in a small region of wave-vector space around but not including the soft-mode wave vector, the central peak is found to split so that the maximum is not exactly at zero frequency. Lastly, results for the energy-density correlation function are given for wave vectors around the soft-mode wave vector and for temperatures above the transition. Near the transition the corresponding spectral function is found to develop a pronounced high-frequency peak. © 1979 The American Physical Society.