The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.     

The Fisher-Widom line for systems with low melting temperature

We present Monte Carlo results for the pair distribution function of three simple fluid models, with pairwise interactions, which have low triple point temperatures and mimic some aspects of Na and Hg liquid metals. The results are then used to get the direct correlation function, by numerical solution of the Ornstein-Zernike equation, and to characterize the decay modes of any density distribution towards the bulk fluid. The Fisher-Widom line is obtained from the crossing of the two lowest inverse decay lengths, associated to monotonic and to oscillatory decay modes. For the pair potential models with a soft repulsive core, the Fisher-Widom line appears well below the critical temperature and has positive slope of the temperature with respect to the density, contrary to previous results for the Lennard-Jones and square-well potentials which had located that line quite close to T _c and with negative slope.     

Isotropic graphite melting under pulsed heating

Experiments on pulsed heating (few microseconds) of graphite with measurements of the liquid carbon resistivity are described. It is confirmed that heating in water at atmospheric pressure do not allow production and study of liquid carbon; in the best case, the liquid state region beginning is achieved. Heating in sapphire tubes results in pulsed pressure (to ten of kbar) as expanding graphite bears against the tube wall. This increasing (during few microseconds) pressure makes it possible to study the carbon liquid state in a limited volume. Isochoric heating resulted in the possibilities ofmeasuring the liquid carbon resistivity at high specific energies (to ～32 kJ/g) and high pressures. Such measurements are extremely expensive at stationary studies.     

The two-dimensional liquid-vapor coexistence line of methane adsorbed on graphite

We measured the specific heat of two-dimensional films of CH₄ adsorbed on graphite for several coverages and we observed broad peaks at the liquid-vapor transition. The critical temperature is estimated to be 70 ± 1 K, somewhat lower than reported in previous measurements. At intermediate coverages the liquid-vapor boundary is very flat indicating that it could be described by a 2D Ising model. We measured also for some coverages the triple temperature at 56.2 K and the commensurate-incommensurate transition at about 50 K, both in reasonable agreement with previous measurements.     

The melting line of the WCA lennard-jones reference system

Abstract

The Melting line of the WCA L-J reference system, i.e. the part of the L-J potential representing the repulsive forces, is determined by means of Monte Carlo calculations. The Gibbs free energy of the fluid is calculated by thermodynamic integration and that of the solid by using the Einstein-crystal method of Frenkel and Ladd (1984).     

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10⁻¹⁰ s. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 661–665 (25 November 1997)     

Thermal melting and ablation dynamics on a femtosecond laser-heated highly-oriented pyrolytic graphite surface

Time-resolved optical reflection microscopy studies demonstrate spatiotemporal dynamics of melting and ablation of graphite surface molten by single IR femtosecond laser pulses, which are revealed by monitoring picosecond oscillations of the probe reflectivity modulated by transient acoustic reverberations in the surface melt. Temporal periods and amplitudes of the reverberations are affected through transient variations of melt thickness and acoustic impedance by melting, thermal expansion, spallation and fragmentation processes, thus enabling quantitative evaluation of their contributions and basic parameters.     

Disorder driven melting of the vortex line lattice

The 3D XY model with random in-plane couplings is simulated to model the phase diagram of a disordered type II superconductor as a function of temperature T and randomness strength p for fixed applied magnetic field. As p increases to a critical p(c), the first order vortex lattice melting line turns parallel to the T axis, continuing down to low temperatures, rather than ending at a critical point. Above p(c) preliminary results suggest the absence of a phase coherent vortex glass.