Searching for sharp features in the pseudogap of icosahedral quasicrystalsby NMR

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 o f Fermi energy E-F was performed in icosahedral Al72.4Pd20.5Mn7.1, Al62Cu25 .5Fe12.5, and Al70.5Pd21Re8.5 by the Al-27 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 paramagn etism of Mn and Fe d-electron moments in AlPdMn and AlCuFe introduces a str ong relaxation mechanism of spin diffusion to paramagnetic centers that com petes with the relaxation via conduction electrons and becomes dominant at low temperatures, typically below 100 K. The Al72.4Pd20.5Mn7.1 and Al62Cu25 .5Fe12.5 show a paraboliclike variation of the pseudogap in the DOS in the vicinity of E-F, bur no sharp features were detected. In the AlPdRe8.5 the paramagnetic relaxation is absent except at temperatures below 7 K and a si ngle 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 D OS at the minimum to a few percent of its value in the metallic aluminum. T he effect of the sharp feature in the DOS on the NMR spin lattice relaxatio n rate T-1(-1) shown theoretically to introduce a minimum in the (T1T)(-1) product at a temperature that is crucially related to the displacement of t he sharp feature from E-F.