NMR probe of pseudogap characteristics in
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We report the results of a (Formula presented) nuclear magnetic resonance (NMR) study of (Formula presented) near the ordered composition with (Formula presented) at temperatures between 4 and 500 K. All compositions are found to be metallic or semimetallic, though electrical resistivity has indicated a metal-insulator transition at (Formula presented) The low-temperature NMR relaxation rates for stoichiometric (Formula presented) and nonstoichiometric (Formula presented) and (Formula presented) compounds follow a Korringa law, associated with a finite density of carriers at the Fermi level. High-temperature relaxation rates for (Formula presented) go over to a semiconductorlike activated form, providing information about the gap structure near the Fermi energy. The results are consistent with pseudogap features identified by recent band-structure calculations, although with smaller energy splittings. We analyze pseudogap changes vs composition, and compare the band-filling behavior to a recent coherent potential approximation calculation. 2000 The American Physical Society.
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