J. Oitmaa

Lattice dynamics of YBa2Cu4O8

Abstract We report results for phonon dispersion curves and phonon density of states for the material YBa2Cu4O8, based on a shell-model description. We also present a group theoretical analysis of the q=0 modes and obtain good agreement with recently reported Raman Measurements.

Nonlinear excitations in a diatomic chain

Abstract We have set up field equations for a diatomic chain of atoms, including nonlinear single site potentials. Some particular solitary wave solutions are found for the case of equal displacement fields. These solutions propagate with a fixed characteristic velocity. Some preliminary molecular dynamics results are also reported.

Lattice dynamics of YBa 2 Cu 3 O 6+ x ( x =0,1)

We report shell model based calculations of the lattice dynamics of YBa2Cu306 and YBa2Cu307. Two models are proposed for YBa2Cu306, the model parameters being obtained by a least squares fit to recent neutron scattering results. The parameters obtained are then used, with modification, to model the lattice dynamics of YBa2Cu307. Since this material has significant carrier concentration, even in the normal state we have used both screened and unscreened versions of the shell model. The screened model gives a better overall description of the lattice dynamics of YBa2Cu307. The phonon dispersion curves along the symmetry directions and the weighted phonon density of states for both YBa2Cu306 and YBa2Cu307 are presented.

Dynamics and statistical mechanics of a diatomic φ4 chain

Abstract We examine the dynamics and the statistical mechanics of a nonlinear diatomic model for a solid which may undergo a displacive structural phase transition. In addition to the familiar linear (phonon) oscillations, nonlinear (kink soliton) vibrations are found which behave as elementary excitations of the system. The nonlinear kinks give rise to a central peak in the dynamical structure factor. The analytic results obtained are supported by constant temperature molecular dynamics simulations of the lattice.

Lattice Dynamics of YBa2Cu3O6+x (x = 0·25, 0·5, 0.75)

The lattice dynamics of YBa2Cu306+x with partially filled 0(4) sites is investigated using a supercell method. The model used is the lattice-dynamical shell model, which incorporates the short-range interactions and the long-range Coulomb potential as well as the polarisabilities of ions. Varying the oxygen content produces small changes throughout the spectrum of normal modes, as well as major changes in those modes with large 0(4) amplitudes. Selected phonon dispersion curves are presented, with emphasis on the variation of frequency with oxygen concentration. The nature of new modes, which appear with an increase in oxygen concentration, is investigated. The overall phonon density of states is also presented.

Classical free‐energy densities for harmonic chains: A pedagogic application of the transfer integral technique

The classical free‐energy density for both a monatomic and a diatomic chain of particles with harmonic coupling between nearest neighbors is calculated using (i) the transfer integral technique and (ii) real‐space renormalization. It is shown that the results agree with the classical limit of the well‐known quantum‐mechanical free‐energy densities for these chains.