P. Bogusławski

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.

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.

Influence of the Mn interstitial on the magnetic and transport properties of (Ga,Mn)As

We report on measurements of the hole density, ferromagnetic transition temperature TC, and magnetization in a series of as-grown and annealed (Ga,Mn)As samples. Estimating the fraction of incorporated Mn occupying interstitial and substitutional sites allows a direct comparison of the predictions of mean field theory with experiment, and a determination of the magnetic moment per substitutional Mn. The saturation of TC at high Mn concentration is consistent with the mean field prediction. The estimated magnetic moment per Mn is close to the expected 5μB for all samples studied if an antiferromagnetic coupling between interstitial and substitutional Mn is assumed.

Electron-electron spin-flip scattering and spin relaxation in III–V and II–VI semiconductors

Abstract Spin relaxation time of conduction electrons resulting from electron-electron spin-flip collisions has been investigated in III–V and II–VI semiconductors with InSb-like energy band structure. Analytical expression has been obtained for non-degenerate electron gas. The proposed mechanism can be dominent in the experimental conditions of electron spin resonance in InSb at higher temperatures.

Theory of nitrogen doping of carbon nanoribbons: Edge effects

Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initio calculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is a deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.

Electron spin relaxation in InSb at high magnetic fields

Spin relaxation of conduction electrons in InSb at high magnetic fields is considered theoretically and compared with existing experiments. A three-level model of band structure is used, which takes into account the main peculiarities of InSb: narrow energy gap and strong spin-orbit interaction. Spin mixing in the electron wavefunctions allows for spin-flip transitions due to electric-type perturbations (Elliot mechanism, 1954). Times of spin relaxation between the two lowest spin sub-bands are calculated for scattering by ionised impurities, acoustic phonons, optic phonons and for scattering by localised magnetic moments. It is shown that the spin relaxation in existing experimental conditions is mainly due to the scattering by ionised impurities and acoustic phonons (deformation potential interaction). The theory well describes some experiments performed with a spin-flop Raman laser, while in others the observed spin relaxation times are shorter than the calculated ones. Possible sources of these discrepancies are considered.

Excess elastic energy and the instability of (GaAs)1(InAs)1(0 0 1), Ga3InAs4, GaIn3As4 and Ga1−xInxAs alloys

Abstract Self-consistent ab initio pseudopotential calculations indicate that the (GaAs) 1 (InAs) 1 (0 0 1) superlattice and the Ga 3 InAs 4 and GaIn 3 As 4 cubic crystals are unstable against phase segregation at zero temperature. Instability results from the excess elastic energy of stretched GaAs and compressed InAs bonds, which is not compensated by the energy released in the distortion of the anion sublattice. A simple statistical scheme, based on the above results predicts an even stronger instability of random Ga 1− x In x As alloys.

Ionicity-dependent structural properties of zinc-blende semiconductors

Abstract Structural properties of Si, AlP and MgS, obtained within the density-functional pseudopotential method, are analyzed as a function of crystal ionicity. For all considered crystals both kinetic and exchange-correlation energies are affected mostly by the pressure-induced density increase, whereas electron-core interaction and the Hartree energy are influenced mainly by the pressure-induced density metallisation. The role of the metallisation effect increases with ionicity, which leads to a qualitatively different pressure dependence of the electron gas energy for Si and MgS.

Optical-gap reduction in the ordered phases of GaInAs solid solution

Abstract The measured optical band gap of the (GaAs)1(InAs)1 (001) superlattice is smaller than that of the corresponding Ga0.5In0.5As random alloy. Ab-initio pseudopotential calculations show that the effect is caused by a repulsion between the lowest conduction bands, or, equivalently, by the onset of confinement of electrons to GaAs layers, and is accompanied by the decrease of the electron effective mass. Similar reductions are obtained for the indirect band gaps, and for cubic phases of GaInAs solid solution.

DFT calculations of magnetic anisotropy energy of Ge1−xMnxTe 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.