NMR evidence for triple-(q)over-right-arrow multipole structure in NpO2 -: art. no. 137209

In order to elucidate the nature of the exotic ordered phase of NpO2 below T-0=26 K, we have initiated the first O-17-NMR measurements on this system. From the O-17-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below T-0. It has also been shown that the characteristic features of the hyperfine interaction at the oxygen sites are well explained by invoking a hyperfine interaction with field-induced antiferromagnetic moments which appear as a result of the triple-q -> antiferroquadrupolar order. The NMR findings strongly support the occurrence of the longitudinal triple-q -> multipole structure in NpO2.