J. von Boehm

Devil's attractors and chaos of a driven impact oscillator

Abstract The motion of a driven elastic impact oscillator: x + 0.4 x + x = cos (ωt) , x > 0 and x (t + ) = - x (t − ) at x = 0, is studied for ω ≈ 2−4. The oscillator exhibits Feigenbaums bifurcations (computed δ ≈ 4.70), the Feigenbaum and intermittent transitions to chaos, crises in chaos and a strong hysteresis region for ω ≈ 3.18–3.20 where the impact/period ratios of a group of attractors show the Devils staircase behaviour with locking values between 3 5 and 3 4 .

Absence of inversion-symmetric limit cycles of even periods and the chaotic motion of Duffing's oscillator

Abstract It is shown that anharmonic oscillators x + 2ζ x − F(x) = A cos (ωt), F(-x) = -F(x) , cannot have inversion-symmetric attractors of even periods. The computed motion related to chaos for Duffings oscillator is consistent with this rule as well as with the universal features of noninvertible one-dimensional maps.

Band structure of group IVA transition-metal dichalcogenides

A systematic study of the electronic properties of the layered group IVA transition-metal dichalcogenides TiS2,, TiSe2, ZrS2 ZrSe2 has been performed using the ab initio fully self-consistent symmetrised OPW method according to the local density formalism (Xalpha exchange correlation). Core states, energy bands and valence densities of states are presented. The valence eigenstates are analysed in terms of Slater-Koster LCAO interpolation models. A general discussion of properties and trends in the family of group IVA transition-metal dichalcogenides also includes HfS2 and HfSe2.

Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As

Donostia International Physics Centre (DIPC), POB 1072, 20018 San Sebastian, Spain(Dated: February 2, 2008)The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are investigated by all-electron total energy calculations using the projector augmented wave method within the density-functional formalism. It is found that the energetically most favorable clusters consist of Mn atomssurrounding one center As atom. As the Mn cluster grows the hole band at the Fermi level splitsincreasingly and the hole distribution gets increasingly localized at the center As atom. The holedistribution at large distances from the cluster does not depend significantly on the cluster size.As a consequence, the spin-flip energy differences of distant clusters are essentially independent ofthe cluster size. The Curie temperature T

High Curie temperatures in "Ga,Mn…N from Mn clustering

The effect of microscopic Mn cluster distribution on the Curie temperature (TC) of (Ga,Mn)N is studied using density-functional calculations together with the mean field approximation. We find that the calculated TC depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental TC values from a few K to well above room temperature. The partially dimerized Mn2-Mn1 distribution is found to give the highest TC>500K, and in general, the presence of the Mn2 dimer has a tendency to enhance TC. The lowest TC values close to zero are obtained for the Mn4-Mn1 and Mn4-Mn3 distributions.

Optical Absorption of Tellurium

The salient features of the imaginary part of the frequency-dependent permittivity 2 of trigonal Te are interpreted in terms of the transitions between the band triplets. The interpretation is based on the self-consistent Hartree-Fock-Slater calculation where the optical matrix elements are treated rigorously. The agreement with the experimental 2 is close. The role of the transitions from the s-bonding triplet in the higher energy region of 2 is clarified. The uppermost p-lone-pair triplet plays the dominant role in the formation of the 2 spectra, especially for the perpendicular component. The contribution from the p-bonding triplet falls into the energy region 4-10 eV and it is mainly responsible for the anisotropy of 2 here.

Relativistic p-d gaps of 1T TiSe2, TiS2, ZrSe2 and ZrS2

The relativistic corrections (RC) (spin-orbit, Darwin and mass-velocity) are calculated to the p-d gaps of 1T TiSe2, TiS2, ZrSe2 and ZrS2 using the self-consistent non-muffin-tin OPW band-structure results. RC are found to enlarge the p-d gaps. The corrected p-d gap of 1T TiSe2 is estimated to be small and positive. RC to the p-d gap of the semiconductor ZrSe2 improve agreement with the experimental gap. RC are found to be of secondary importance to the p-d gaps of the semiconductors TiS2 and ZrS2.

The conduction band structure of ZrS2 and ZrSe2

Abstract The conduction band structures of ZrS 2 and ZrSe 2 are studied with the Hartree-Fock-Slater theory using the self-consistent symmetrised orthogonalised-plane-wave (SCSOPW) method with 240 basis functions/ k . The SCSOPW d t 2g band group (without the d z 2 splitting off) is separated from the p bands by an indirect Γ 2− → L 1+ gap and from the d e g bands by an indirect gap (≈0.5 eV). The d e g bands overlap with the Zr 5s and 5p bands by ≈1.5 eV. The SCSOPW conduction bands resemble most the bands for HfS 2 by Mattheiss and those for ZrS 2 and ZrSe 2 by Bullett (except the e g -5s, p overlap).

Interstitial oxygen loss and the formation of thermal double donors in Si

The combination of first-principles total energy calculations and a general kinetic model, which takes into account all processes of association, dissociation, and restructuring, is used to study the kinetics of thermal double donors (TDDs) in silicon over the temperature range of 300–650 °C. A strong correlation is found between the formation rate of TDDs and the loss rate of interstitial oxygen atoms. Also, a close agreement with experiments is obtained. It is found that TDDs grow via consecutive reactions where fast diffusing oxygen dimers and all TDDs capture interstitial oxygen atoms.

Variational procedure for symmetry-adapted Wannier functions

An alternative derivation of the basic variational principle for Wannier functions is presented and combined with the symmetry properties so as to obtain a generalisation of the equations given by Koster (1953). An LCAO procedure for the solution of these equations is discussed and the outcome of such a procedure is compared with the Bloch functions obtained from a conventional LCAO calculation of the band structure.