Revealing the Nature of Antiferroquadrupolar Ordering in Cerium Hexaboride: CeB6

The cerium hexaboride (CeB6) f-electron compound displays a rich array of low-temperature magnetic phenomena, including a "magnetically hidden" order, identified as multipolar in origin via advanced x-ray scattering. From first-principles electronic-structure results, we find that the antiferroquadrupolar (AFQ) ordering in CeB6 arises from crystal-field splitting and yields a band structure in agreement with experiments. With interactions of p electrons between Ce and B-6 being small, the electronic state of CeB6 is suitably described as Ce(4f(1))3(+)(e(-))(B-6)(2-). The AFQ state of orbital spins is caused by an exchange interaction induced through spin-orbit interaction, which also splits the J = 5/2 state into a Gamma(8) ground state and a Gamma(7) excited state. Within the smallest antiferromagnetic (AFM) (111) configuration, an orbital-ordered AFQ state appears during charge self-consistency, and it supports the appearance of a "hidden" order. Hydrostatic pressure (either applied or chemically induced) stabilizes the AFM (AFQ) states over a ferromagnetic one, as observed at low temperatures.