Quadrupole-Strain Interaction in Rare Earth Hexaborides

Abstract

Temperature dependence of the elastic constants in a series of rare earth hexaborides has been investigated by means of the ultrasonic method. The remarkable softening of the transverse C₄₄ mode in PrB₆, NdB₆, DyB₆ and HoB₆ has been discussed in connection with the cooperative Jahn-Teller effect due to the CEF ground state degenerated by the quadrupolar moment. The low temperature behavior of C₄₄ in Ce_xLa_1-xB₆ compounds is presented to show the Kondo effect on the quadrupolar moment of Γ₈ ground state. The anomaly of the bulk modulus C_B in SmB₆ is related to the valence fluctuation state in the system.