Ultrasonic Dispersion in All Elastic Moduli and Softening at Low Temperatures in Filled Skutterudite LaFe4Sb12

Abstract

To clarify the origin of ultrasonic dispersion due to the so-called rattling motion in LaFe 4 Sb 12 , we measured the temperature dependences of elastic moduli and ultrasonic attenuation. We found the ultrasonic dispersion to be between 35 and 80 K in all elastic moduli, suggesting no-mode-selectivity of ultrasound. Experimental results and theoretically calculated density of states indicate that the d -electronic states of transition metal ions play an important role in ultrasonic dispersion. The theory based on the coupling between an acoustic phonon and some optical phonons interacting with electrons is compatible with no-mode-selective ultrasonic dispersion, and it suggests a stronger electron–phonon coupling in all elastic modes in LaFe 4 Sb 12 . Low-temperature elastic softening was observed in all elastic moduli, and it continues down to 0.4 K. There is a possibility that low-temperature softening originates from quantum tunneling motion or a novel quantum effect of rattling motion.