Author: Saulius Tumėnas
Dissertation title: Optical response of ZnMgRE quasicrystals
Fields of science: Physical sciences, Physics -02 P, Semiconductor physics P265
Scientific supervisor: Dr. Vytautas Karpus
Defence of the dissertation: 2013-09-25
The main goal of the dissertation was to reveal the electronic structure of ZnMgRE (RE = Y, Ho, Er) quasicrystals by investigations of their optical response. The dissertation comprises experimental X-ray diffraction (XRD) and optical spectroscopy studies of the quasicrystals, a construction of their electron subsystem model, and a theoretical description of their optical response.
The XRD studies were aimed at a determination of the reciprocal quasicrystalline- lattice vectors, which define the atomic field acting on an electron subsystem.
The optical spectroscopy studies were carried out by the combined spectroscopic ellipsometry and reflectance spectroscopy technique, based on a suggested anchor- window method. High-accuracy ZnMgRE optical conductivity σ(ω) spectra were recorded in the wide, 0.01-6 eV, spectral range.
The model of ZnMgRE electron energy spectrum, previously suggested for an interpretation of experimental ZnMgRE photoemission spectra, was developed. The nearly-free-electron gas model of independent intersections was formulated in the extended zone presentation. A scheme of the theoretical σ(ω) calculations was extended to account for various positions of the Fermi level with respect to a pseudogap.
The experimental ZnMgRE optical conductivity spectra was reproduced in detail by theoretical calculations performed within the framework of the suggested electron energy spectrum model. The set of electron energy spectrum parameters determined from an analysis of the optical data predicts actually the same structure of the Fermi level-vicinity electron energy spectrum, as was previously predicted from an analysis of photoemission data.
The electron subsystem in ZnMgRE quasicrystals maintains the nearly free electron gas character. The energy spectrum of electrons in a vicinity of the Fermi level is determined by the Fermi surface intersections with (222100) and (311111) families of Bragg planes.
The optical response of ZnMgRE quasicrystals, as of other metallic compounds, is determined by the intraband Drude-type and interband optical transitions. The intraband transitions contribute to the total optical conductivity with the relative spectral weight of about 10 %. The Drude relaxation times are of about 0.14-0.4 10-14 s. The intersections of the Fermi surface with Bragg planes lead to an essential increase of the optical mass in quasicrystals, as compared to the usual crystalline metals. The ZnMgRE optical mass is of the order of 10 m0.
The interband ZnMgRE optical conductivity is predominantly due to the optical transitions across 222100 and 311111 pseudogaps. Their relative spectral weight is of about 80 %. An influence of the low structure-factor Sg pseudopotentials on the optical response of ZnMgRE quasicrystals was revealed. The relative spectral weight of the low- Sg contribution is of about 10 %.