Mark van Schilfgaarde

Spin‐polarized band structure of magnetically coupled multilayers

The magnetic properties of idealized bcc Fe[001]/Cu[001] and Fe[001]/Cr[001] superlattices have been investigated by carrying out self‐consistent spin‐polarized linearized muffin‐tin orbital/atomic sphere approximation and linearized augmented spherical wave methods/atomic sphere approximation calculations. Structures composed of 2–4 Fe, 2–11 Cu, and 2–6 Cr monolayers were considered. In Fe/Cu, the exchange coupling between successive Fe slabs does not exhibit the FM to AFM crossover observed experimentally at about 9 Cu monolayers. In Fe/Cr superlattices having atomically abrupt interfaces, the exchange coupling exhibits short‐range oscillations. When interfacial roughness is taken into account, these oscillations are smoothed. However, the averaged exchange coupling does not switch from FM to AFM at about 4–5 Cr monolayers as observed. Since experimental samples differ markedly from the idealized models employed, it is likely that structural and chemical imperfections will have to be taken into account ...

First-principles study of phase stability of Ti--Al intermetallic compounds

Thermodynamic and structural properties of fcc- and hcp-based Ti--Al alloys are calculated from first-principles and are used to perform an [ital ab] [ital initio] study of phase stability for the intermetallic compounds in this system. The full potential linear muffin tin orbital method is used to determine heats of formation and other zero-temperature properties of 9 fcc- and 7 hcp-based intermetallic compounds, as well as of elemental fcc and hcp Ti and Al. From the results of these calculations, sets of effective cluster interactions are derived and are used in a cluster variation method calculation of the solid-state portion of the composition-temperature phase diagram for fcc- and hcp-based alloy phases. The results of our calculations are compared with those of experimental studies of stable and metastable phases in the Ti--Al system.

InTlSb: An infrared detector material?

In1−xTlxSb is proposed as promising infrared material. A number of optical and structural properties are studied within local density‐functional theory. The alloy at x=0.09 is estimated to have a gap of 0.1 eV. Although TlSb is found to favor the CsCl structure, the zinc blende alloy is stable for low x values. A phase diagram is calculated to estimate the regions of stable phases and explore the conditions for growing narrow‐gap In1−xTlxSb alloys.

Bandstructure effect on high-field transport in GaN and GaAlN

The velocity-field characteristics in zinc-blende GaN are calculated from the Boltzmann equation, using realistic energy bands taken from ab initio theory. The drift velocity and the high-field negative differential resistance are shown to be largely determined by the inflection point in the bands centered around the Γ valley, instead of the usual intervalley scattering. We analyze the relative importance of these competing mechanisms for GaN and Al0.5Ga0.5N. The importance of this anomaly to device properties is also discussed.

Correlations in pseudobinary alloys

Correlations between atoms of Cd and Zn in Cd1−xZnxTe and Hg and Zn in Hg1−xZnxTe are calculated within a generalized quasichemical approximation. A tetrahedron of alloy atoms is used for the microcluster unit. The statistical mechanics of the correlations is outlined for the present problem, an approximate method of solution is indicated. Microcluster energies are obtained from an approximation of a Keating model due to Martin, plus a correction due to Harrison to account for the coupling between the tetrahedral bonds. It is found that the correlations are significant at typical growth temperatures (300–600 K). Hg1−xZnxTe is predicted to undergo spinodal decomposition between 300 and 600 K.

A first-principles study of the phase stability of fcc-based Ti-Al alloys

In this paper we present results of a first-principles phase stability study of fcc-based Ti-Al alloys. In particular the full-potential linear muffin tin orbital method has been used to determine heats of format on and other zero-temperature properties of 9 fcc ordered superstructures as well as fcc and hcp Ti, and fcc Al. From these results a set of effective cluster interactions are determined which are used in a cluster variation method calculation of the thermodynamic properties and the composition-temperature phase diagram of fcc-based alloys.

Electronic structure of ideal and nonideal metal/GaAs contacts

On the basis of explicit first‐principles electronic structure calculations of a number of ideal and nonideal metal/GaAs interfaces, we show that nonidealities and imperfections play a major role in determining the Schottky barrier height. For the ideal interfaces, the calculated pinning positions ranged from 0 to 1 eV above the valence‐band maximum, in contradiction to experiment. As one example of a defect, we incorporate an As antisite defect near the interface, and show that it significantly alters the electronic structure and Schottky barrier height.

Tight‐binding theory of force constant models

A theory of force constant models is derived for tetrahedral semiconductors from Harrison’s [Phys. Rev. B 27, 3592 (1983)] tight‐binding theory. A relation between the three independent elastic constants is derived that is independent of the detailed form of the matrix elements. This relation provides an approximate, but ab initio, theory on which two‐parameter force constant models can be based. The universal matrix elements of Harrison are also found to describe remarkably well the magnitude of the elastic constants and their scaling from one material to another. Analytic forms for c11 and c44 are given in terms of the matrix elements.

Scattering from ionized dopants in Schottky barriers

We calculate the true electrostatic potential that forms a Schottky barrier, arising from a random distribution of dopants in the depletion region. It is customary to model the electrostatic potential in a Schottky barrier by a one‐dimensional quadratic potential, as would obtain from a jellium of charge in the depletion region. The difference between the true potential and the ‘‘ideal’’ one acts as a perturbation to electrons traversing the depletion region. This perturbation is found to be small, and effectively one dimensional. Its effect on the transmission probability of an electron tunneling through the depletion region is shown to be negligible. It is shown, however, that because of the large quantum dipoles near the metal–semiconductor interface, the image‐force lowering correction does not in general vary as N1/4d, as is widely thought, at least for heavily doped materials. Local‐density calculations of the Schottky barrier height were made for several metal/GaAs interfaces, to obtain the magnitu...

DYNAMICAL SPIN SUSCEPTIBILITY IN THE TD-LDA AND QSGW APPROXIMATIONS

Abstract. This project was aimed at building the transverse dynamical spin susceptibility with the TD-LDA and the recently-developed Quasparticle Self-Consisent Approximations, which determines an optimum quasiparticle picture in a self-consistent manner within the GW approximation. Our main results were published into two papers, (J. Phys. Cond. Matt. 20, 95214 (2008), and Phys. Rev. B83, 060404(R) (2011). In the first paper we present spin wave dispersions for MnO, NiO, and #11;-MnAs based on quasiparticle self-consistent GW approximation (QSGW). For MnO and NiO, QSGW results are in rather good agreement with experiments, in contrast to the LDA and LDA+U descriptions. For #11;-MnAs, we find a collinear ferromagnetic ground state in QSGW, while this phase is unstable in the LDA. In the second, we apply TD-LDA to the CaFeAs2 A¢Âx80Âx94 the first attempt the first ab initio calculation of dynamical susceptibililty in a system with complex electronic structure Magnetic excitations in the striped phase of CaFe2As2 are studied as a function of local moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the NAx82´eel temperature, originating largely from a narrow band of Fe d states near the Fermi level, and coexisting with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model.