L.T. Wille

Oxygen vacancy ordering in YBa2Cu3O7-y

Abstract First principles total energy calculations have been performed for YBa 2 Cu 3 O x with x varying from 6.0 to 7.5. The results of these calculations have been used to determine the effective pair interactions for the oxygen ordering in the basal plane. The phase diagram calculated with these “first principles” pair interactions is in very good agreement with experiment. Comparison of the total energies for the O 6 , O 6.5 and O 7 structures shows that the double-cell phase is thermodynamically stable.

Modeling daily realized futures volatility with singular spectrum analysis

Using singular spectrum analysis (SSA), we model the realized volatility and logarithmic standard deviations of two important futures return series. The realized volatility and logarithmic standard deviations are constructed following the methodology of Andersen et al. [J. Am. Stat. Ass. 96 (2001) 42–55] using intra-day transaction data. We find that SSA decomposes the volatility series quite well and effectively captures both the market trend (accounting for about 34–38% of the total variance in the series) and, more importantly, a number of underlying market periodicities. Reliable identification of any periodicities is extremely important for options pricing and risk management and we believe that SSA can be a useful addition to the financial practitioners’ toolbox.

Atomistic and continuum studies of carbon nanotubes under pressure

The results of finite element simulations of carbon nanotubes under pressure are presented and compared with atomistic studies. The dominant failure mode is shell buckling, characterized by diamond shaped bulges. Critical pressures are calculated for two types of boundary conditions.

Simulations of the elastic response of single-walled carbon nanotubes

Using the Tersoff-Brenner potential we have performed molecular dynamics simulations of nanotubes under axial strain, analyzing both compression and stretching forces. These large-scale simulations were carried out on a MasPar massively parallel computer. The elastic response is investigated and expressions for various elastic constants are derived from the simulations. Typical failure modes are also shown and discussed.

Growth processes of magnetic clusters studied by direct simulation Monte Carlo method

Film formation with deposited magnetic clusters has attracted strong attention as a new manufacturing technique to realize high-density magnetic recording media and to create materials with unique magnetic properties. Such clusters are typically obtained by adiabatic expansion of a metal vapor. It is therefore important to clarify the growth mechanism since this has a profound effect on the cluster magnetic moment. In this article a new simulation method based on a combination of Direct Simulation Monte Carlo (DSMC) and a cluster collision model is introduced to examine the effect of experimental conditions in cluster beam growth. We simulate the behavior of clusters and inert gas atoms in the flight path under different experimental conditions. In particular, we find a bimodal size distribution curve and a decreasing average moment as a function of flight path length.

Multiscale simulation of cluster growth and deposition processes by hybrid model based on direct simulation Monte Carlo method

A cluster growth and deposition model based on hybrid modeling is introduced to examine the experimental conditions of the cluster growth process in vacuum chamber and deposition process on substrate. This hybrid model is a simulation method including physical length and time scale characteristics of macro and microscale. We simulated the behavior of the cluster during the flight path by direct simulation Monte Carlo (DSMC) method and the deposition behavior on the substrate by a simple MC model. Several size distributions of the clusters and various morphologies of deposited film were obtained, and the relationship between macroscopic and microscopic physical phenomena during deposition process was examined.

MICROSCOPIC THEORY OF SURFACE SEGREGATION IN BINARY ALLOYS

Abstract A new method, involving a series of coupled electronic structure calculations and free energy minimizations, has been developed to determine surface compositions in binary alloys. Layer- and concentration-dependent point and pair interactions are calculated self-consistently by means of a configurational averaging procedure within the tight-binding recursion method and the Bragg-Williams approximation. Results for the segregation profile of TicRh1−c above the bulk order-disorder temperature are discussed and found to be in good agreement with experiment.

Parallelization of an ecological landscape model by functional decomposition

A functional scheme is described to parallelize computer simulations of grid-based ecological landscape models. The method is implemented using the Message Passing Interface protocol and is applied to the Everglades Landscape Vegetation Model. On a two-processor system, the speed-up is satisfactory and the overall performance of the program is competitive with traditional parallelization techniques such as geometrical decomposition. The method is discussed, timing information is provided for three different parallel machines, and some further developments are indicated.

Simulated annealing and the topology of the potential energy surface of Lennard-Jones clusters

Low-energy configurations of atomic clusters held together by Lennard-Jones (LJ) interactions are determined by simulated annealing. Using a combination of quenches and annealings at various cooling rates the topology of the potential energy surface is elucidated and the probability of convergence to the global minimum is analyzed. An understanding of this phenomenon is relevant to glass formation, protein folding, and other problems with competing minima.

Monte Carlo study of domain formation in YBa2(Cu1−xFex)3Oz

Abstract Upon progressive doping with Fe the YBa 2 (Cu 1−x Fe x ) 3 O z compound transforms from an orthorhombic to a tetragonal structure without an accompanying loss of superconductivity. This behavior has been interpreted by assuming that the tetragonal structure consists of many conflicting orthorhombic microdomains pinned by the Fe-atoms. Extending an Ising model previously used to model the undoped compound, we show by Monte Carlo simulation that such microdomains do indeed occur. The oxygen content is allowed to vary in the simulations, while the FeCu sublattice is handled in the canonical ensemble (fixed Fe-content). The Fe-atoms are mobile and found to cluster along the 〈11〉 direction in precise agreement with experimental observations.