Ewald method for simulating Yukawa systems

Ab initio study of deuterium in the dissociating regime: sound speed and transport properties

The sound speed and the transport properties of dense hydrogen (deuterium) are computed from local spin-density approximation molecular-dynamics simulations in the dissociating regime. The sound speed c(s) is evaluated from the thermodynamical differentiation of the equation of state in the molecular phase and is in very good agreement with recent experiments. The diffusion constant D and the viscosity eta are extracted from simulations performed at V=6, 4, and 2.7 cm(3)/mole, corresponding, respectively, for deuterium at rho=0.672, 1.0, and 1.5 g/cm(3) in a range of temperatures 1000 K<T<50,000 K. In the dissociated regime, the diffusion coefficient is well predicted by one-component plasma formulas using a renormalized coupling parameter recently proposed by Murillo [M. S. Murillo, Phys. Rev. B 62, 4115 (2000)]. The behavior of the shear viscosity in the dissociated regime is more complex and exhibits a crossover between atomic and screened plasma formulation. A comparison with recent molecular-dynamics simulations of Yukawas systems shows that the inverse of the screening length must lie between 1 and 2, in nearest-neighbor radius units, as suggested by the results on the diffusion.

Statistical mechanics of vacancy and interstitial strings in hexagonal columnar crystals

Columnar crystals contain defects in the form of vacancy-interstitial loops or strings of vacancies and interstitials bounded by column "heads" and "tails." These defect strings are oriented by the columnar lattice and can change size and shape by movement of the ends and by forming kinks along the length. Hence an analysis in terms of directed living polymers [S. A. Safran, Statistical Thermodynamics of Surfaces, Interfaces, and Membranes (Addison-Wesley, Reading, MA, 1994), Sec. 8] is appropriate to study their size and shape distribution, volume fraction, etc. If the entropy of transverse fluctuations overcomes the string line tension in the crystalline phase, a string proliferation transition occurs, leading to a supersolid phase [E. Frey, D. R. Nelson, and D. S. Fisher, Phys. Rev. B 49, 9723 (1994); see also J. Prost, Liq. Cryst. 8, 123 (1990)]. We estimate the wandering entropy and examine the behavior in the transition regime. We also calculate numerically the line tension of various species of vacancies and interstitials in a triangular lattice for power-law potentials as well as for a modified Bessel function interaction between columns such as occurs in the case of flux lines in type-II superconductors or long polyelectrolytes in an ionic solution. We find that the centered interstitial is the lowest-energy defect for a very wide range of interactions; the symmetric vacancy is preferred only for extremely short interaction ranges.