The processes involved in the solid-to-liquid phase transition (original) (raw)

The detailed study of crystals and the dynamics of the melting process in two and three dimensions is a fundamental and interesting research topic, which is important for increasing our knowledge of solid state physics. In natural crystals, structure information can be obtained principally by Bragg-scattering of neutrons, electrons or photons on the crystal, followed by an analysis in Fourier space. Dynamical aspects cannot be investigated in these systems. Recently, a new crystalline system was discovered whose properties are such that the melting transition can be investigated in great detail-the 'plasma crystal'. This article presents the results of such an investigation and shows evidence for the existence of intermediate phases between the solid, liquid and gaseous phases. The observed 'structured' phase transition may be specific for plasma crystals but, alternatively, it may indicate the existence of intermediate stages in the melting transition more generally. 'Plasma crystals' are a special state of colloidal plasmas, that is, plasmas enriched (or doped) with microspheres [l-3]. Under certain conditions such plasmas are found to spontaneously self-organize into a regular, ordered 'crystalline structure'. The interaction which gives rise to this effect is the Coulomb force. Particles embedded in a plasma are continuously charged and discharged due to the incidence of electrons and ions [4-71. Since the impact rate of the (much faster) electrons is higher than that of the ions, the particles charge up negatively. The plasma rearranges itself in the electrostatic field of the particle; the net effect is to 'shield' the particle over a characteristic length, the Debye length, A. The electrostatic potential of the particles is then a 'screened Coulomb potential' with the characteristic range A,. If the particle density is sufficiently high, neighbouring particles may interest and form macroscopic lattices. The lattice constants, A, are of the Hubertus M. Thomas Studied physics at Cologne University in Germany. He obtained his doctorate from the Ludwig-Maximillians Universitv in Munich in 1996. Since 1994 he has been a sdentific employee of the Max-Planck Institute for extraterrestrial physics and currently works on space experiments. His research interest is plasma physics, in particular, dusty plasmas.