C. P. Burmester

Simulation of multicomponent thin film deposition and growth

Results from a multicomponent Monte Carlo simulation of the deposition and growth of YBa{sub 2}Cu{sub 3}0{sub 7} are presented and discussed. In particular, a detailed examination of the growth modes active during different morphological growth conditions is performed. At higher deposition rates, both (001) and (100) epitaxial variants (`c` and `a` type growth, respectively) are observed to grow by modes attributed to the classic Volmer-Weber mechanism. At very low deposition rates, the film is observed to grow in a distinct, cyclic, multi-stage process. Small islands of (0011) epitaxy nucleate and grow to one unit cell height followed by primarily horizontal growth or ``ledge extension`` until one unit cell layer has formed. This process then repeats. Simulated RHEED amplitude data from this growth process compares favorably to experimentally obtained data.

Microscopic model and computer simulation of detwinning in YBa2Cu3Oz

Abstract The results of Monte Carlo simulations of the detwinning of the high-temperature superconductor YBa 2 Cu 3 O z are reported. To study the effects of detwinning and the concomitant domain growth in this system, a sample is first produced having a representative domain structure using a Monte Carlo simulation at fixed temperature and oxygen chemical potential based on an asymmetric two-dimensional Ising model. The resulting microstructures typically consist of finely textured orthogonal domains indicative of a twinned structure. Next, a symmetry-breaking detwinning potential is imposed and the time evolution of the system is followed. At low temperatures of detwinning potentials only partial detwinning is typically achieved as the system is often frozen in “glassy” domain structures, but a higher temperatures or detwinning potentials complete detwinning is attained. Domain kinetics is mapped out as a function of oxygen chemical potential and temperature and evidence is presented for an algebraic domain growth law of the form t n with n between 0.33 and 0.75 depending on the control parameters.

Monte Carlo simulation of oxygen ordering in YBa2Cu3Oz.

Abstract Oxygen ordering in the basal plane of YBa2Cu3Oz has been studied by Monte Carlo simulations of a two-dimensional anisotropic Ising model. The orthorhombic to tetragonal transition is found to be second order at high temperature in agreement with previous CVM calculations. At low temperatures, a cell-doubled orthorhombic phase appears near stoichometry z=6.5, intermediate between the single cell orthorhombic and tetragonal phases. At very low temperatures, long-lived transient O Cu O chain ordering is observed to lead to triple and fivefold unit cells. Where not obscured by transient ordering, the transition lines bordering the orthorhombic cell-doubled phase are found to be second order. The observed equilibrium and non-equilibrium structures correlate well with atomic resolution micrographs and diffraction data from the YBa2Cu3Oz system.

Oxygen ordering and chain formation in YBa2Cu3Oz

A distinctive aspect of the thermodynamic properties of YBa2Cu3Oz is the basal-plane oxygen ordering which leads to the formation of Cu-O chains and the development of orthorhombic phases. Theoretically, this phenomenon may be explained in terms of an Ising Hamiltonian with two types of second-neighbor interactions (ASYNNNI model). This symmetry breaking and ground-state degeneracy lead to kinetically arrested multi-domain structures. This paper reports new simulation results for these mesoscopic features, with specific emphasis on the effects of dopants, elastic deformation, and possible structural phase separation.

Strain and domain evolution in YBa2Cu3Oz

Abstract A Monte Carlo study of the time evolution of atomic positions and occupancies within the basal plane of YBa2Cu3Oz is performed. The simulations are based upon an anisotropic Ising model, and, for the first time, include elastic interactions associated with continuous displacements of atoms within the basal plane. Samples quenched to temperatures below the disorder-order transition curve from the disordered tetragonal state rapidly form orthorhombic domains separated by twin boundaries. The resulting microstructure is observed to coarsen and eventually to anneal into a single domain that exhibits the experimentally observed 21/2 a 0 × 21/2 a 0 superlattice reflections. Strain maps are used to elucidate and interpret these observations.

Theoretical Modeling and Experimental Characterization of Planar Defects in Y2Ba4Cu6+Xo14+X

Crystallographic defects and phase transformations in the system Y 2 Ba 4 Cu 6+x O 14+x (0≤≤4) are investigated by high resolution transmission electron microscopy (TEM) and static lattice, three dimensional Monte Carlo computer simulations. High resolution images of partially transformed (x=2 to x=l) material reveal a prevalence of CuO planar defects (stackingfaults) associated with the transformation and an absence of disturbance to the perovskite Ba-Y-Ba blocks. An atomic mechanism involving the intercalation and removal of extra CuO planes by partial dislocation climb, and requiring only a-b plane diffusion, is developed for the formation of such planar defects during changes in the layered YBaCuO crystal structure. Monte Carlo simulations based on the proposed transformation mechanism accurately reproduce the observed defects andknown equilibrium structures.