Juhani von Boehm

Fractal basin boundaries of an impacting particle

Abstract The basins and the basin boundaries of the dissipative sinusoidally driven particle moving and impacting in the harmonic potential are studied in the range where the winding number (the impact/period ratio) forms an incomplete Farey-organised devils staircase. The general shapes of the basins of the (periodic) devils staircase attractors are the same. The boundaries between the basins of the devils staircase attractors and the two other periodic attractors exhibit fractal structure. The fractal dimension, calculated from large regions of the phase space, does not depend on the subregion indicating that the boundaries are statistically self-similar and have a unique fractal dimension. The Farey-organisation affects the fractal dimension considerably. The calculated large fractal dimension, typically ≈ 1.7, indicates long transients and high sensitivity to noise although all the attractors are periodic in the devils staircase range.

Diffusion and clustering of substitutional Mn in (Ga,Mn)As

The Ga vacancy mediated microstructure evolution of (Ga,Mn)As during growth and postgrowth annealing is studied using a multiscale approach. The migration barriers for the Ga vacancies and substitutional Mn together with their interactions are calculated using first principles, and temporal evolution at temperatures 200–350°C is studied using lattice kinetic Monte Carlo simulations. We show that at the typical growth and annealing temperatures (i) Ga vacancies provide an efficient diffusion transport for Mn and (ii) in 10–20h the diffusion of Mn promotes the formation of clusters. Clustering reduces the Curie temperature, and explains its decrease during long-term annealing.

A multiscale study of ferromagnetism in clustered (Ga,Mn)N

Magnetic interactions in (Ga,Mn)N are studied on the microscopic intercluster and intra-cluster scales using first-principles calculations. Ferromagnetic transition temperatures are calculated using Monte Carlo methods. Randomness in Mn substitution is found to reduce Curie temperatures by about 10–20% fro mt hose of the corresponding regular (Ga,Mn)N lattice due to clustering. Nevertheless, high Curie temperatures reaching above room temperature are obtained even for completely random Mn distribution in (Ga,Mn)N for the Mn concentration of x 13.5%. Increasing clustering is always found to decrease the Curie temperature—especially when tetramer clusters are formed. The active search for diluted magnetic semiconductors (DMSs) with high Curie temperatures (TCs) exceeding room temperature started with the discovery of ferromagnetism in (In,Mn)As [1]. The calculations by Dietl et al predicted that (Ga,Mn)N should have the highest TC (� 400 K) among the prospective III–V DMS materials suggested fo rs pintronics applications [2]. However, the measured TC values for (Ga,Mn)N range from 10 to 940 K [3–6] and also paramagnetic behaviour is reported at lower Mn concentrations (typically x < 0.02) [7]. The reasons for the wide range of the observed TC values and especially for the high values are poorly understood. One of the suggestions for this anomalous behaviour of TC is that the increase is due to clustering (or precipitation) of Mn atoms and the formation of giant magnetic moments at the Mn clusters [6, 8–10]. However, Mn clustering is usually found to decrease calculated TC s[ 11–15 ]a lthough in some very special situations an increase may also be obtained [9–11]. In this paper we consider small clusters and define a Mnn cluster as a collection of n substitutional Mn atoms (n = 2, 3, and 4 for dimers, trimers, and tetramers, respectively) which have a common N neighbour. At high Mn concentrations x ac onsiderable amount of Mn clusters are present even in the case of a completely random distribution of substitutional

Self-consistent Ground State of Trigonal Tellurium

The purpose of the present paper is to report our non-relativistic Xα calculations of trigonal Te. In the self-consistent (SC) symmetrized OPW (SOPW) method used [1] both the valence states represented by SOPWs and the core states are included in the SC iteration. The only parameters entering into our calculations are the lattice constants a=0.44572 nm, u=0.11736 nm, c=0.5929 nm and the Xα-parameter α. Our SC α=1 band structure is quite similar to the SC pseudopotential one [2], Our bands are characteristically ∼0.2 eV broader and the gaps about the same amount narrower. Our rough α=2/3 density of states (DOS) histogram has two s, two p-bonding and one broader p-non-bonding peaks in agreement with earlier experience [2,4,5].