Enhanced stability of the square lattice of a classical bilayer Wigner crystal

The stability and melting transition of a single layer and a bilayer crystal consisting of charged particles interacting through a Coulomb or a screened Coulomb potential is studied using the Monte-Carlo technique. A new melting criterion is formulated which we show to be universal for bilayer as well as for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and 1/r^{12} repulsive inter-particle interactions. The melting temperature for the five different lattice structures of the bilayer Wigner crystal is obtained, and a phase diagram is constructed as a function of the interlayer distance. We found the surprising result that the square lattice has a substantial larger melting temperature as compared to the other lattice structures. This is a consequence of the specific topology of the defects which are created with increasing temperature and which have a larger energy as compared to the defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review

Measurement of cross sections and leptonic forward-backward asymmetries at the Z pole and determination of electroweak parameters

We report on the measurement of the leptonic and hadronic cross sections and leptonic forward-backward asymmetries at the Z peak with the L3 detector at LEP. The total luminosity of 40.8 pb −1 collected in the years 1990, 1991 and 1992 corresponds to 1.09·10 6 hadronic and 0.98·10 5 leptonic Z decays observed. These data allow us to determine the electroweak parameters. From the cross sections we derive the properties of the Z boson: assuming lepton universality. We obtain an invisible width of Γ inv =496.5±7.9 MeV which, in the Standard Model, corresponds to a number of light neutrino species of N v =2.981±0.050.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47894/1/10052_2005_Article_BF01574160.pd

Quantum hydrogen vibrational dynamics in LiH: new neutron measurements and variational Monte Carlo simulations

Hydrogen single-particle dynamics in solid LiH at T=20 K has been studied through the incoherent inelastic neutron scattering technique. A careful analysis of the scattering data has allowed for the determination of a reliable hydrogen-projected density of phonon states and, from this, of three relevant physical quantities: mean squared displacement, mean kinetic energy, and Einstein frequency. In order to interpret these experimental findings, a fully-quantum microscopic calculation has been carried out using the variational Monte Carlo method. The agreement achieved between neutron scattering data and Monte Carlo estimates is good. In addition, a purely harmonic calculation has been also performed via the same Monte Carlo code, but anharmonic effects in H dynamics were not found relevant. The possible limitations of the present semi-empirical potentials are finally discussed.Comment: RevTex, 19 pages, accepted for publication in PR