Low-temperature behavior of the elastic constant C₄₄ in Ce_xLa_1-xB₆

We have made the ultrasonic experiments to examine the low-temperature elastic properties in the Kondo compound Ce_xLa_1-xB₆ for x = 0.70, 0.65 and 0.60. The transverse C₄₄ mode in x = 0.70 and 0.65 shows steep decrease as much as 25% around the transition point into the ordered phase IV. The remarkable softening of C₄₄ down to 25 mK reveals that the phase IV remains to be stable even at absolute zero in x = 0.70 and 0.65. The C₄₄ of x = 0.60 shows gentle decrease about 9% below 1 K with a minimum around 0.4 K suggesting the absence of phase IV. A considerable discrepancy between the low-temperature limit of C₄₄ in x = 0.60, 0.50 and that in x = 0.70, 0.65 indicates that the ground state of x = 0.60 and 0.50 is the nonmagnetic state attributed to the Kondo screening effect.