Date of Award
Doctor of Philosophy
Physics and Astronomy
[superscript]27Al and [superscript]63,65Cu NMR measurements are reported for powder samples of stable AlCuFe and AlCuRu icosahedral quasicrystals and their crystalline approximants, and for a AlPdMn single grain quasicrystal. Furthermore, [superscript]27Al NQR spectra at 4.2 K have been observed in the AlCuFe and AlCuRu samples. From the quadrupole perturbed NMR spectra at different magnetic fields, and from the zero field NQR spectra, a wide distribution of local electric field gradient (EFG) tensor components and principal axis system orientations was found at the Al site. A model EFG calculation based on a 1/1 AlCuFe approximant was successful in explaining the observed NQR spectra. It is concluded that the average local gradient is largely determined by the p-electron wave function at the Al site, while the width of the distribution is due to the lattice contribution to the EFG. Comparison of [superscript]63Cu NMR with [superscript]27Al NMR shows that the EFG distribution at the two sites is similar, but that the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more s-type wave function of the conduction electrons. The overall spread of EFG values is well reproduced by the calculation based on the approximant. However, the experimental spectra indicate a much larger number of non-equivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. It is argued that the short range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measurements of [superscript]27Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature yield results which indicate a reduction of the density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in the measured parameters were detected as a function of composition of the quasicrystalline alloys, arguing against a fine structure in the density of states at the Fermi level.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Shastri, Ananda, "NMR and NQR study of the thermodynamically stable quasicrystals " (1994). Retrospective Theses and Dissertations. 10645.