Hidden octupole order in URu2Si2

We propose that the hidden order in URu2Si2 is an incommensurate octupole order, which is derived from a spin-1 XXZ model with XX octupole and Z dipole interactions based on the singlet - doublet crystal-field level scheme. The octupole moments break time-reversal invariance and give rise to finite hyperfine fields on the nuclei of the ligand Si and Ru ions. In order for the hyperfine fields to be observed as a nuclear magnetic resonance ( NMR) linewidth, the order must be incommensurate, with the ordering vector determined so as to account for the Si-29 NMR linewidth data quantitatively.