The lattice dynamics of systems with several identical atoms per unit surface cell: a unit cell spectral density

In studies of the lattice dynamics of surfaces in which there are many identical atoms in the unit cell, for example a reconstructed surface, there is a problem in presenting the results. The problem arises because each atom in the unit cell has a different spectral density and it is not, in general, clear which of the many densities should be presented. We propose here that in such studies a “unit cell spectral density”, one which replaces the collection of densities for individual atoms with a single density, should be used. The relationship of this unit cell spectral density to experimental phonon measurements is also discussed.     

Quasiclassical negative magnetoresistance of a 2D electron gas: interplay of strong scatterers and smooth disorder

We study the quasiclassical magnetotransport of noninteracting fermions in two dimensions moving in a random array of strong scatterers (antidots, impurities, or defects) on the background of a smooth random potential. We demonstrate that the combination of the two types of disorder induces a novel mechanism leading to a strong negative magnetoresistance, followed by the saturation of the magnetoresistivity rho(xx)(B) at a value determined solely by the smooth disorder. Experimental relevance to the transport in semiconductor heterostructures is discussed.     

Optical phonons in ionic crystals with a diatomic unit cell

Optical phonons in an ionic crystal with two atoms per unit cell have been considered. The equation of relative motion of oppositely charged sublattices has been derived with the use of the concept of the self-consistent electromagnetic field. The frequency corresponding to transverse optical phonons and the expression for the frequency of the eigenmodes of the ionic crystal in terms of the physical characteristics of the crystal have been obtained from the condition of electrostatic equilibrium. The calculated values of the latter frequency agree with the experimental data.