Tunneling and rattling in clathrate crystal
Abstract
We present tunneling and rattling motions of an off-center guest atom in cage referring to a clathrate crystal La3Pd20Ge6. The elastic constant C44 of La3Pd20Ge6 shows a Debye-type dispersion around 20 K obeying a relaxation time tau = tau0exp(E/kBT) with an attempt time tau0 = 2.0*10-12 sec and an activation energy E = 197 K. At low temperatures below 3 K down to 20 mK, the C44 shows a softening of C44 = C440(T-TC0)/(T-Theta) with TC0 = -337.970 mK and Theta = -338.044 mK. These facts are attributed to two different types of the off-center motions with Gamma5 symmetry in 4a-site cage of La3Pd20Ge6, a thermally activated rattling motion over the potential hill and a tunneling motion through the potential hill at low temperatures.
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