Magnetic-Field Effects on Quadrupolar Ordering in a<b>Γ</b>_<b>8</b>-Quartet System CeB_<b>6</b>

Abstract

A theory of the antiferro-quadrupolar ordering in CeB 6 is developed in the framework of the Γ 8 -quartet basis. Possible ordered phases under the external magnetic field are classified in terms of their symmetry properties, and the relations between quadrupolar order parameters and field-induced moments are established. Then we formulate a mean-field theory of Ohkawa's RKKY model based on the group-theoretical consideration. The stability of the possible phases and the resulting phase diagrams are analyzed in connection with the character of quadrupolar moments, the influence of induced moments and the form of Zeeman term. Considering available experimental results, we introduce an improved model with asymmetric interactions and discuss the phase diagram of CeB 6 . The connection between magnetostriction and uniform quadrupolar moments is discussed.