Searching for sharp features in the pseudogap of icosahedral quasicrystals by NMR

Abstract

In a search for sharp features in the electronic density of states (DOS) of quasicrystals, a systematic comparative study of the DOS in the vicinity of Fermi energy ${E}_{F}$ was performed in icosahedral ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1},$ ${\mathrm{Al}}_{62}{\mathrm{Cu}}_{25.5}{\mathrm{Fe}}_{12.5},$ and ${\mathrm{Al}}_{70.5}{\mathrm{Pd}}_{21}{\mathrm{Re}}_{8.5}$ by the ${}^{27}\mathrm{Al}$ NMR spin-lattice relaxation. The investigated samples differ markedly in their electrical conductivity (and thus the DOS at ${E}_{F})$ and the content of paramagnetic centers. The paramagnetism of Mn and Fe d-electron moments in AlPdMn and AlCuFe introduces a strong relaxation mechanism of spin diffusion to paramagnetic centers that competes with the relaxation via conduction electrons and becomes dominant at low temperatures, typically below 100 K. The ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ and ${\mathrm{Al}}_{62}{\mathrm{Cu}}_{25.5}{\mathrm{Fe}}_{12.5}$ show a paraboliclike variation of the pseudogap in the DOS in the vicinity of ${E}_{F},$ but no sharp features were detected. In the ${\mathrm{AlPdRe}}_{8.5}$ the paramagnetic relaxation is absent except at temperatures below 7 K and a single sharp feature of the full width at half height 14 meV, displaced by 11 meV from ${E}_{F},$ was detected. The sharp feature is very deep, reducing the DOS at the minimum to a few percent of its value in the metallic aluminum. The effect of the sharp feature in the DOS on the NMR spin lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ is shown theoretically to introduce a minimum in the ${(T}_{1}{T)}^{\ensuremath{-}1}$ product at a temperature that is crucially related to the displacement of the sharp feature from ${E}_{F}.$