Tunneling and rattling of clathrate crystals Ce3Pd20Ge6 and La3Pd20Ge6

摘要

We have made ultrasonic measurements on clathrate crystals Ce3Pd20Ge6 and La3Pd20Ge6 with a cage filled by a rare-earth atom. The elastic constants C-44 of both compounds show a marked dependence in ultrasonic frequencies of 10-300 MHz around 10-30 K, which are caused by a thermally activated rattling of the off-center rare-earth ion in the cage over the potential hill. At low temperatures below 3 K down to 20 mK, C-44 of La3Pd20Ge6 without 4f-electron exhibits a softening proportional to 1/T, which indicates a tunneling of the off-center rare-earth ion in the cage through the potential hill. (c) 2005 Elsevier B.V. All rights reserved.