Yu. A. Uspenskii

Ab initio study of the magnetism in GaAs, GaN, ZnO, and ZnTe-based diluted magnetic semiconductors

Abstract Electronic structure and magnetic properties of Ga 1− x Mn x As, Ga 1− x Mn x N, Zn 1− x M x O, and Zn 1− x M x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64 atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMS. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMS strongly correlates with the occupation and energy position of 3D-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM–AFM competition is discussed.

Efficient method for the determination of extreme-ultraviolet optical constants in reactive materials: application to scandium and titanium

The chemical reaction of a sample with atmospheric gases causes a significant error in the determinantion of the complex refractive index n = 1 - delta + ibeta in the extreme-ultraviolet region. The protection of samples removes this effect but hampers the interpretation of measurements. To overcome this difficulty, we derive the exact dependences on film thickness of the reflectivity and transmissivity of a protected film. These dependences greatly simplify the determination of delta and beta when the spectra of several films with different thickness and identical protection are measured. They also allow the verification of the delta(omega) obtained from the Kramers-Kronig relation and even make the Kramers-Kronig method unnecessary in many cases. As a practical application, the optical constants of Sc and Ti are determined at h omega = 18-70 eV and 18-99 eV, respectively. The essential feature of our experimental technique is deposition of a film sample directly on a silicon photodiode that allows easy operation with both thin (approximately 10-nm) and thick (approximately 100-nm) films. The comparison of calculated reflectivities of Si-Sc multilayers with the measured values shows the high accuracy of the determined delta(omega) and beta(omega).

Determination of XUV optical constants by reflectometry using a high-repetition rate 46.9-nm laser

We report the measurement of the optical constants of Si, GaP, InP, GaAs, GaAsP, and Ir at a wavelength of 46.9 nm (26.5 eV). The optical constants were obtained from the measurement of the variation of the reflectivity as a function of angle utilizing, as an illumination source, a discharge pumped 46.9-nm table-top laser operated at a repetition rate of 1 Hz. These measurements constitute the first application of an ultrashort wavelength laser to materials research.

Effect of polymer coating on leakage losses in Bragg fibers

It is found that the reflection of leaky radiation from the interface between the outer silica cladding and the coating polymer greatly modifies the loss spectrum of Bragg fibers. A simple model that describes this effect is proposed and confirmed by measurement and computation.

Sc}Si normal incidence mirrors for a VUV interval of 35}50 nm

The Sc/Si multilayers are suggested as high-re#ectivity coatings for a VUV interval of 35}50 nm. Fabricated mirrors show the normal incidence re#ectivity of 30}50% which is high enough to e!ectively manipulate the beams of synchrotron radiation and compact discharge and laser-driven X-ray lasers. The obtained values are not, however, limiting for the Sc/Si coatings. Theoretical estimations as well as electron microscopy studies of Sc}Si interfaces indicate a large potential for further raising the re#ectivity. ( 2000 Elsevier Science B.V. All rights reserved.

Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga1−xMnxAs, Ga1−xMnxN, Zn1−xMxO, and Zn1−xMxTe (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.

SKYLAB 3600 GROOVE/MM REPLICA GRATING WITH A SCANDIUM-SILICON MULTILAYER COATING AND HIGH NORMAL-INCIDENCE EFFICIENCY AT 38 NM WAVELENGTH

A Sc-Si multilayer coating was applied to a replica of the 3600 groove/mm grating, developed for the SO82A spectroheliograph that flew on the Skylab mission, for the purpose of enhancing the normal-incidence efficiency in the extreme-ultraviolet region. The efficiency, measured at an angle of incidence of 6 degrees with synchrotron radiation, had a maximum value of 7.2% at a wavelength of 38 nm and was a factor of 3 higher than the efficiency of the gold-coated Skylab grating. The measured efficiency of the Sc-Si grating was in good agreement with the efficiency calculated by use of the modified integral method.

Extreme UV optical constants of rare-earth metals free from effects of air contamination

The determination of EUV optical constants in rare-earth metals is much hampered by the high reactivity and easy air contamination of these materials. The most difficult regions are a long wavelengths part of the EUV interval and the vicinity of absorption edges. In this study the optical constants of La and Tb are determined in a wide energy interval 14-400 eV. The study is performed by our recent method, which is suitable for reactive materials and for intervals around the absorption edges of elements. The samples are identically protected films with thickness ~ 10 nm or ~ 100 nm, which are deposited on silicon photodiodes. Mathematical treatment separates the contributions of a capping layer and a rare-earth metal and provides data which are free from the effects of air contamination. The precision and behavior of optical constants, as well as the parameters of the O2,3 and N4,5 edges in La and Tb are analyzed.

Terbium-based extreme ultraviolet multilayers

We have fabricated periodic multilayers that comprise either Si/Tb or SiC/Tb bilayers, designed to operate as narrowband reflective coatings near 60 nm wavelength in the extreme ultraviolet (EUV). We find peak reflectance values in excess of 20% near normal incidence. The spectral bandpass of the best Si/Tb multilayer was measured to be 6.5 nm full width at half-maximum (FWHM), while SiC/Tb multilayers have a more broad response, of order 9.4 nm FWHM. Transmission electron microscopy analysis of Si/Tb multilayers reveals polycrystalline Tb layers, amorphous Si layers, and relatively large asymmetric amorphous interlayers. Thermal annealing experiments indicate excellent stability to 100 degrees C (1 h) for Si/Tb. These new multilayer coatings have the potential for use in normal incidence instrumentation in a region of the EUV where efficient narrowband multilayers have not been available until now. In particular, reflective Si/Tb multilayers can be used for solar physics applications where the coatings can be tuned to important emission lines such as O V near 63.0 nm and Mg X near 61.0 nm.

Spin, orbital moments and magneto-optical properties of MPt3 (M = Cr, Mn, Fe, Co) compounds

Abstract We have studied trends in the ab initio electronic structure, spin moments, orbital moments and magneto-optical (MO) properties of MPt 3 compounds. The calculated quantities agree well with the experimental data. It was found that values of the local orbital moments are closely connected with the predominant spin character of electronic states near the Fermi energy ( E F ). Features of the MO spectra of compounds are primarily defined by the absorptive off-diagonal conductivity σ 2, xy ( ω ). The spin decomposition of this quantity showed that at the MO resonance energy σ 2, xy ( ω ) is formed mainly by spin-up electronic transitions to an unoccupied part of the hybridized {M3d ↑-Pt5d ↑} band. These transitions are intense for CrPt 3 and MnPt 3 , having high MO activity. A reduction of the unoccupied part of {M3d ↑-Pt5d ↑} band in FePt 3 and CoPt 3 results in the weak electronic transitions and in small calculated Kerr rotation values for these compounds.