Czesław Oleksy

On phase transitions with bilinearly coupled order parameters

It is explicitly shown that, contrary to the common assertion, phase transitions driven by two bilinearly coupled order parameters can be continous, i.e., irreducibility of the representation involved is not necessary for the phase transition to be of second order in the Landau sense.

Theory of interfaces between discrete and continuous media — Application to transition metal surface states

Abstract A general theory of response functions for partly discrete and partly continuous composite systems is given. It is then used for the study of the influence of work function and tunneling on transition metal surface states. It is shown on a simple model that when account is taken of the work function, localized surface states may appear above the bulk band. A composite system formed out of a thin vacuum slab sandwiched between two identical transition metal surface (tunnel junction) is then considered. It is found that each surface state is split in two, having slightly different energies.

Diffusion in lattice Lorentz gases

The expressions for diffusion coefficients and for velocity autocorrelation functions of lattice Lorentz gases are derived both in the nearest scatterers and Boltzmann approximations. The results are obtained for linear chain, square, triangular, simple cubic, body centred cubic, face centred cubic and face centred hyper cubic lattices. The diffusion coefficients are compared with those from the effective medium approximation for the square lattice and with computer simulation results for triangular, simple cubic and body centred cubic lattices.

The velocity autocorrelation function for a random walk on percolation clusters

We apply the random Lorentz model to diffusion on percolation clusters on a square lattice. In this model a walker performs a random walk on cluster sites and can visit sites occupied by impurities which play the role of reflectors. The velocity autocorrelation function and the mean-square displacement of a random walker are calculated with high accuracy using a moment-propagation numerical technique. Exponents describing the decay of correlation function are calculated at the percolation threshold and above it.

Energy excitations in three-axial magnets with single-ion and exchange anisotropies

Abstract The energy spectra of three-axial ferromagnets like U 3 P 4 and ferrimagnets like U 3 Sb 4 are presented and their properties are discussed. In case of U 3 Sb 4 we find an interesting example of a magnetic structure which, due to the single-ion anisotropy, is unstable in the spin wave approximation. The k 2 dependence of magnon energies requires re-examination of the temperature dependence of the magnetisation and of the specific heat.