Microscopic theory of multipole ordering in NpO2
Abstract
In order to examine the mysterious ordered phase of NpO2 from a microscopic viewpoint, we investigate an f-electron model on an fcc lattice constructed based on a j-j coupling scheme. First, an effective model with multipole interactions is derived in the strong-coupling limit. Numerical analysis of the model clearly indicates that the interactions for 4u and 5u moments are relevant to the ground state. Then, by applying mean-field theory to the simplified model including only such interactions, we conclude that longitudinal triple-q 5u octupole order is realized in NpO2 through the combined effects of multipole interactions and anisotropy of the 5u moment.
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