Density functional calculations of optical spectra and narrow band systems

Abstract: A calculational method, based on density functionaltheory, has been used to calculate optica spectra for a wide range of systems,as well as ground-state properties of some narrow-band systems.A systematic study of the trends in the optical properties of the 4Brefractory metal compounds has been performed. These materials are of technological interest as, e.g., energy efficient window coatings. Thecalculated optical spectra are in very good agree- ment with experimentalspectra. Furthermore, the spectra of all nine 4B refractory metal compounds are related, and it is shown that the differences can be qualitatively understoodin terms of band filling, nuclear charge, and band width. The opticalproperties of CeN, are also well described using our calculational method. It is shown that an itinerant description of the 4f electrons produces an optical spectrum in close agreement with the experimental results, whereas alocalized description fails to reproduce any of the low-energy features in the spectrum. Also calculated are the optical spectra of the narrow-gap semiconductors PbS, PbSe, and PbTe, and the wide-gap insulators SnI2and NaNO2. For PbSe and PbTe, there exist high-quality ellipsometry measurements, with which the calculations show very good agreement. Magneto-optical properties, including the polar magneto-optical Kerr effect (MOKE), of cubic and tetragonal Fe, Co, and Ni, as well as of MnAs, MnSb, andMnBi have been investigated. The calculations show good agreement with previous calculations and with experimental spectra. It is found that a tetragonal distortion of Fe, Co, and Ni results in a fairly minor effect on their magneto-optical spectra.The valence configuration energy balance in lanthanide systems,including the lanthanide elements, the Sm and Tm chalcogenides, and several Ce-systems has been studied using a combination of density-functional calculations and atomic data. With the generalized gradient approximation (GGA) to the exchange-correlation potential, our method succeeds in describing thedi- trivalent energy balance to within 0.15 eV.Two different approximations to the exchange-correlation potential, the local density approximation (LDA) and GGA, were compared using the ground-state properties of the lanthanide elements as testing ground. GGA was seen to correct most of the overbonding tendency of LDA. This study also revealed thatthe standard model of the lanthanides breaks down for the early lanthanide elements.