S. Lapinskas

On oxygen ordering in YBa2Cu3O7−x

The phase diagram of YBa 2 Cu 3 O 7− x is obtained in the lattice-gas model. It is shown that at 0≤ x ≤1 and superconducting temperatures two-phase regions do not occur. The O-Cu-O chains at the low temperatures consist of the considerably long, occupied and empty chain segments whose length depends on x and temperature.

Cell tripled structure of YBa2Cu3O6+xforx>0.5

Abstract A model considering weak long-range attractive interactions as an addition to the two-dimensional anisotropic next-nearest neighbour interactions is proposed to describe the occurence of the cell tripled phase in YBa2Cu3O6+x. The phase diagram of oxygen ordering is obtained at low temperatures and x>0.5.

Oxygen ordering and superconducting temperature of YBa2Cu3O6+x

Abstract An analysis of the relation between the superconducting temperature T c and the oxygen ordering in the basal CuO x plane of the high-temperature superconductor YBa 2 Cu 3 O 6+ x is presented. Superconductivity in the CuO 2 planes of a non-stoichiometric compound is assumed to appear locally, just in the neighbourhood of the regions with O-Cu-O chains. Bulk superconductivity occurs when these regions form a percolation network characterized by the minimal width d of a percolation path with T c depending on d . The evaluation of d by the cluster field method allows one to obtain quantitative agreement of the calculated and experimental T c ( x ) dependences.

Theoretical study of the oxygen-induced missing-row reconstruction on the Rh(110) surface

A microscopic model based on the Blume-Emery-Griffiths model is proposed to describe the missing-row reconstructions on the Rh(110) surface induced by oxygen. The phase diagram of the model at finite temperatures has been calculated employing the Monte Carlo and cluster variation methods. We find overall agreement between our results and the experimental data on the missing-row reconstruction for the ORh(110) surface as well as for other fcc (110) surfaces covered by atoms adsorbed in the threefold fcc hollow sites.

Diffusion of adatoms: Comparison of the path probability and Monte Carlo method for H/Fe(110)

Abstract The tracer, D t , and chemical, D c , diffusion coefficients of hydrogen atoms adsorbed on Fe(110) are calculated for the disordered phase by means of the path probability method of Kikuchi. The correlation functions, used for calculation of diffusion, are obtained by the cluster variation method for a lattice-gas model with repulsive nearest-neighbor and one (of two) triple interactions, a set of interactions which gives a good fit to the experimental phase diagram. We also calculated the concentration dependence of D t , D c , and the correlation factor, f , by the Monte Carlo method to make the comparison between the path probability and Monte Carlo method in the calculation of the kinetic characteristics.

On the first-order phase transitions in YBa2Cu3O6+x

Abstract We investigate, using the cluster-variation method (CVM), the existence of first-order phase transitions in the ASYNNNI model for YBa 2 Cu 3 O 6+ x . We find that the regions of phase separation between the tetragonal and orthorhombic phases and between the two orthorhombic phases in the ( T , x ) phase diagram shrink with increasing size of the basic cluster. The phase diagram, calculated in the 12-point approximation, demonstrates the best known agreement between the CVM and the Monte Carlo results. The first-order phase transitions occur in the model for YBa 2 Cu 3 O 6+ x when the strain of the tetragonal-to-orthorhombic transition is accounted for.

Magnetization and compensation temperature of transition-metal–rare-earth multilayers in a model with long-range interactions

Abstract A model is proposed to describe the behavior of magnetization and compensation temperature in transition-metal–rare-earth multilayers. Long-range exponentially decreasing ferromagnetic (antiferromagnetic) interactions are considered between the same (different) kind of atoms. The magnetization and compensation temperature are shown to decrease with increasing single layer thickness. Good agreement is obtained with experimental data on Tb/Co system.

Order-disorder phase transition in bismuth-based high-temperature superconductors

Abstract A model for a structural phase transition, determined by an ice-rule-type of Bi–O bonding in the BiO plane, is proposed. A first-order transition is obtained at higher temperatures. The entropy, specific heat, discontinuity of the order parameter and phase diagram, useful for the description of substitutions of Bi by other atoms, are calculated. We suggest an explanation for the modulation unrelated to extra (non-stoichiometric) oxygen within the BiO plane.

Monte Carlo study of oxygen thermal desorption from YBa2Cu3O6+x compound

Abstract A Monte Carlo study of oxygen thermal desorption from the free edges of a basal CuO plane of YBa 2 Cu 3 O 6 + x compound is performed. Perfectly ordered copper-oxygen chains with the imposed nearest and next-nearest neighbour oxygen-oxygen interactions and energy barriers for oxygen diffusion and desorption are the starting conditions of the simulation. The characteristic peak related to the destruction of an ordered OI structure is found. The experimental data can be described if the surface reaction is considered as rate-limiting and readsorption of oxygen is introduced in the calculation.

On ergodic relaxation time in the three-dimensional Ising model

We have studied the dynamical decay of the autocorrelation function of the 3D Ising model for different sizes L = 20–52 of spin cluster-cubes. The behaviour of the longest, ergodic relaxation time, τe, of a finite domain below the phase transition temperature Tc was mostly considered for two types of phase transition dynamics. A study of the scaling properties of τe demonstrates a negligible difference between the types of dynamics used, but a considerable difference for different boundary conditions. In contrast to the known result for periodic boundary conditions (τe ~ L z exp [const(Lєν)2], where z and ν are the dynamical and correlation length exponents, respectively, and є = 1 – T/Tc), the ergodic relaxation time for open boundary conditions is proportional to Lz exp [const(Lєν)2k] with coeffcient k for lattices explored in this work slightly decreasing with L in between 1.65 and 1.58. This result implies that only the lattices of sizes close to or exceeding L = 300 with open boundary conditions might have ergodic relaxation times similar to those with perodic boundary conditions.