Molecular-dynamics simulations suggest that in 2D screened Coulomb solids melt when the r.m.s. displacement of particles in local co-ordinate systems exceeds ∼ 10% of the lattice period, close to the Lindemann criterion in 3D. This criterion is over-estimated in the co-operative motion model, but under-estimated in the Einstein model, because during melting a 2D solid is a mixture of both ordered and disordered phases.

No abstract

We introduce a simple empirical rule wherein the pairing interaction in superconductors is cancelled when normal and umklapp phonon scattering coexist. Superconductivity then arises solely from the residual umklapp contribution. As a result the deduced electron-phonon interactions in niobium, tantalum, lead and aluminum become virtually identical in the normal and superconducting states. Transition temperatures calculated under the rule are accurate within a few per cent when compared with experimental data. Features of the Matthias relations are also explained. The high T c so far predicted for metallic hydrogen is probably overly optimistic.

No abstract

No abstract