Calculated optical properties of Co in ZnO: internal and ionization transitions.

Previous luminescence and absorption experiments in Co-doped ZnO revealed two ionization and one intrashell transition of [Formula: see text] electrons. Those optical properties are analyzed within the generalized gradient approximation to the density functional theory. The two ionization channels involve electron excitations from the two [Formula: see text] gap states, the [Formula: see text] triplet and the [Formula: see text] doublet, to the conduction band. The third possible ionization channel, in which an electron is excited from the valence band to the [Formula: see text] level, requires energy in excess of 4 eV, and cannot lead to absorption below the ZnO band gap, contrary to earlier suggestions. We also consider two recombination channels, the direct recombination and a two-step process, in which a photoelectron is captured by [Formula: see text] and then recombines via the internal transition. Finally, the observed increase the band gap with the Co concentration is well reproduced by theory. The accurate description of ZnO:Co is achieved after including +U corrections to the relevant orbitals of Zn, O, and Co. The [Formula: see text] value was calculated by the linear response approach, and independently was obtained by fitting the calculated transition energies to the optical data. The respective values, 3.4 and 3.0 eV, agree well. Ionization of Co induces large energy shifts of the gap levels, driven by the varying Coulomb coupling between the [Formula: see text] electrons, and by large lattice relaxations around Co ions. In turn, over ∼1 eV changes of [Formula: see text] levels induced by the internal transition are mainly caused by the occupation-dependent [Formula: see text] corrections.

DFT calculations of magnetic anisotropy energy of Ge(1-x)Mn(x)Te ferromagnetic semiconductor.

Density functional theory (DFT) calculations of the energy of magnetic anisotropy for diluted ferromagnetic semiconductor Ge(1-x)Mn(x)Te were performed using OpenMX package with fully relativistic pseudopotentials. The influence of hole concentration and magnetic ion neighbourhood on magnetic anisotropy energy is presented. Analysis of microscopic mechanism of magnetic anisotropy is provided, in particular the role of spin-orbit coupling, spin polarization and spatial changes of electron density are discussed. The calculations are in accordance with the experimental observation of perpendicular magnetic anisotropy in rhombohedral Ge(1-x)Mn(x)Te (1 1 1) thin layers.

Point defects as a test ground for the local density approximation +U theory: Mn, Fe, and V(Ga) in GaN.

Electronic structure of the Mn and Fe ions and of the gallium vacancy V(Ga) in GaN was analysed within the GGA + U approach. First, the +U term was treated as a free parameter, and applied to p(N), d(Mn), and d(Fe). The band gap of GaN is reproduced for U(N) ≈ 4 eV. The electronic structure of defect states was found to be more sensitive to the value of U than that of the bulk states. Both the magnitude and the sign of the U-induced energy shifts of levels depend on occupancies, and thus on the defect charge state. The energy shifts also depend on the hybridization between defect and host states, and thus are different for different level symmetries. In the case of V(Ga), these effects lead to stabilization of spin polarization and the "negative-U(eff)" behavior. The values of Us were also calculated using the linear response approach, which gives U(Fe) ≈ U(Mn) ≈ 4 eV. This reproduces well the results of previous hybrid functionals calculations. However, the best agreement with the experimental data is obtained for vanishing or even negative U(Fe) and U(Mn).

Magnetism of solids resulting from spin polarization of p orbitals.

Magnetism in systems that do not contain transition metal or rare earth ions was recently observed or predicted to exist in a wide variety of systems. We summarize both experimental and theoretical results obtained for ideal bulk II-V and II-IV compounds, molecular crystals containing O(2) or N(2) molecules as structural units, as well as for carbon-based materials such as graphite and graphene nanoribbons. Magnetism can be an intrinsic property of a perfect crystal, or it can be induced by non-magnetic dopants or defects. In the case of vacancies, spin polarization is local and results in their high spin states. The non-vanishing spin polarization is shown to originate in the strong spin polarization of the 2p shell of light atoms from the second row of the periodic table.

Interfacial segregation and electro-diffusion of dopants in superlattices.

A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.

Mn interstitial diffusion in (ga,mn)as.

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.

Surface segregation of Ge at SiGe(001) by concerted exchange pathways.

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.