NMR probe of pseudogap characteristics in Ca Al2-x Si2+x

We report the results of a Al27 nuclear magnetic resonance (NMR) study of Ca Al2-x Si2+x, near the stoichiometric composition with x=0. The low-temperature NMR relaxation rates for stoichiometric (x=0) and nonstoichiometric (x=-0.1 and 0.1) compounds follow a Korringa law, associated with a finite density of carriers at the Fermi level. High-temperature relaxation rates for x≥0 go over to a semiconductorlike activated form, providing information about the electronic structure near the Fermi energy. The results are consistent with pseudogap features identified by recent band-structure calculations. An analysis of the pseudogap change vs composition further points out that the band-filling picture is proper for the understanding of the NMR observations in Ca Al2-x Si2+x.