This Letter discusses the possibility of realizing half-metallic antiferromagnets, i.e. , systems with 100% spin polarization of the conduction electrons without showing a net magnetization.The predictions are based on electronic structure calculations using the local density approximation.

Abstract. This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface.Measurements are presented of the circular-polarization-dependent photocurrent (the socalled helicity asymmetry) in thin-film tunnel junctions of Co/Al203/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/r-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of r-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

In order to understand the electronic structure of the mis6t-layer compound (SnS)&»NbS2 we carried out an ab initio band-structure calculation of the closely related commensurate compound (SnS) & ppNbS2.The band structure is compared with calculations for NbS2 and for hypothetical SnS with structure and interatomic distances as in (SnS) i 2pNbS&. The calculations show that the electronic structure is approximately a superposition of the electronic structures of the two components NbS2 and SnS, with a small charge transfer from the SnS layer to the NbS2 layer. The interlayer bonding between SnS and NbS2 is dominated by covalent interactions. X-ray and ultraviolet photoelectron spectra were obtained for the valence bands. The observed spectra are in good agreement with the band-structure calculations.

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Electronic structure calculations on Lao.#Zao.~Mn03 are reported. Calculations were performed with the Full Potential LAPW method within the Local Density Approximation (LDA) as well as the Generalised Gradient Approximation (GGA). The ferromagnetic phase of Lao.sCao.sMn03 is half-metallic, which is important in the relation to the colossal magnetoresistance properties of this compound.

Ab initio localized-spherical-wave calculations on crystalline Li 12 Si 7 are reported. The crystal consists of two one-dimensional units, ͑Li 6 Si 5 ͒ ϱ and ͑Li 12 Si 4 ͒ ϱ , that contain five-membered Si rings and four-membered Si stars, respectively. In the density of states the region below Ϫ5 eV is largely determined by the local ringlike and starlike arrangements of the Si atoms; above Ϫ5 eV there is a signature of delocalization of states across the subunits. Additional calculations on the separate units show that Li 12 Si 7 can be described as ͓Li 12 Si 4 ͔ 4ϩ ͓Li 6 Si 5 ͔ 2 2Ϫ where the charge transfer places the Fermi energy in the gaps of the aromatic subunits. A simple molecular orbital model is presented in order to rationalize the occurrence of these gaps. A short comparison to the theoretical density of states of liquid Li 12 Si 7 is also presented.

We present Si L2 3 emission-band spectra of a series of 3d and 4d transition-metal (TM) silicides, together wtih Si E emission-band spectra of four 3d TM disilicides. The data are compared with augmented-spherical-wave density-of-states (DOS) calculations, and good agreement is found. The trends we find are explained with a general scheme for chemical bonding in TM silicides. The differences between the experimental data and the calculated DOS curves are tentatively attributed to self-energy effects.