E.E. Tornau

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.

New superstructures on the YBa2Cu3O7−x phase diagram

Abstract The conditions which give rise to the superstructure with reduced wave vector ( 1 3 , 0, 0) are investigated. The next-nearest interchain interaction of oxygen atoms is responsible for the stability of this phase. The consideration of this interaction leads to the decrease of the phase with ( 1 2 , 0, 0) on the phase diagram of YBa2Cu3O7−x.

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.

Monte Carlo study of surface reconstruction in Ge(001)

Abstract A Monte Carlo study of the phase transitions on Ge(001) is carried out using the Ising model with axial next nearest neighbours interaction. It is shown that the transition between high and low temperature phases occurs via an intermediate modulated phase for definite relations of the interaction parameters. The period of the modulated phase depends on the temperature. The low temperature phase is found to be a c(4 × 2) structure. A phase diagram is obtained.

Modelling of Y and Cu substitution in : the effect on the oxygen ordering and superconducting temperature

The gradual destruction of the 60 K plateau in the dependence of due to the effects of substitutions of aluminium for the basal-plane copper and rare earths for yttrium is investigated. The calculations are performed using the percolation model relating the superconducting temperature to the oxygen ordering in the basal plane. The destruction of the plateau is related to changes in the structure of the orthorhombic phase. A small concentration of Al dopants is sufficient to cause a misorientation of the O - Cu - O chains and the disappearance of the phase ordering. Assuming that the coupling of spontaneous orthorhombic strain to the oxygen ordering is influenced by the substitution of rare earths for Y, we find that the phase region shrinks, and qualitatively describe the shift of the metal-to-insulator transition with the change of ionic radius of the substituent.

Exploring the Antipolar Nature of Methylammonium Lead Halides: A Monte Carlo and Pyrocurrent Study

The high power conversion efficiency of the hybrid CH3NH3PbX3 (where X = I, Br, Cl) solar cells is believed to be tightly related to the dynamics and arrangement of the methylammonium cations. In this Letter, we propose a statistical phase transition model which accurately describes the ordering of the CH3NH3+ cations and the whole phase transition sequence of the CH3NH3PbI3 perovskite. The model is based on the available structural information and involves the short-range strain-mediated and long-range dipolar interactions between the cations. It is solved using Monte Carlo simulations on a three-dimensional lattice allowing us to study the heat capacity and electric polarization of the CH3NH3+ cations. The temperature dependence of the polarization indicates the antiferroelectric nature of these perovskites. We support this result by performing pyrocurrent measurements of CH3NH3PbX3 (X = I, Br, Cl) single crystals. We also address the possible occurrence of the multidomain phase and the ordering entropy of our model.

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.

Holes amount and aging of quenched samples of YBa2Cu3O6 + x

Abstract The increase of the number of holes in the CuO 2 plane of YBa 2 Cu 3 O 6 + x during room temperature aging, Δp , is numerically calculated using the asymmetric next-nearest-neighbour interaction model. A maximum in the interval 0.35 x Δp ( x ) dependence. The position of this maximum depends on the annealing temperature. From a fit of the simulated data to experiment, a linear relation between T c and p is proposed. The number of holes is calculated using a simple chemical model for Al-doped YBa 2 Cu 3 O 6 + x compounds with the Al atoms ordered in the basal plane. It is demonstrated that the 3 × 3 and 4 × 4 Al ordering is favourable for superconductivity if the sample is free of Tb dopants.

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.