Microscopic Investigation of Possible Multipole Ordering in SmRu₄P₁₂by³¹P-NMR

Abstract

To elucidate microscopically the metal–insulator transition ( T MI ∼16.5 K) and an additional transition ( T * ∼15 K) in Sm-based filled skutterudite compound SmRu 4 P 12 , we have carried out 31 P-NMR (nuclear magnetic resonance) at various fields from 0.8 to 11 T. The peaks in the spin lattice relaxation rate 1/ T 1 observed at T MI and T * under ∼ 7 T indicate that there are different magnetic fluctuation modes in SmRu 4 P 12 : one freezes below T MI and the other does below T * . The anomalous field and temperature dependences of the NMR spectrum are explained in terms of the occurrence of multipole ordering. We have examined the two possible scenarios to account for the experimental results, and both cases suggest that the M–I transition involves octapole ordering.