Ferroquadrupole ordering and Gamma_5 rattling motion in clathrate compound Ce_3Pd_20Ge_6
Ferroquadrupole ordering and Gamma_5 rattling motion in clathrate compound Ce_3Pd_20Ge_6
复制标题
笼形化合物Ce_3Pd_20Ge_6中的铁四极有序化和Gamma_5震颤运动
DOI：
10.1103/physrevb.68.184109
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发表时间：
2003
影响因子：
3.7
通讯作者：
T. Komatsubara
中科院分区：
文献类型：
--
作者：
Y. Nemoto;Takashi Yamaguchi;T. Horino;M. Akatsu;T. Yanagisawa;T. Goto;O. Suzuki;A. Donni;T. Komatsubara
用超声和热膨胀方法研究了具有立方Cr23C6型结构的Ce_3Pd_20Ge_6包合物的晶格效应。在T_q1=1.25K以上，(C_11-C_12)/2和C_44的弹性软化与倒易温度1/T成正比，可以用基态四重态的四极磁化率很好地描述。在(C_11-C_12)/2中的50%的巨大软化和沿[001]方向的自发膨胀Dl/L=1.9×10^-4特别表明O_2^0在T_q_1以下具有铁四极有序。还发现了与T_n2=0.75K的反铁磁有序和无公度的反铁磁有序有关的弹性反常。C_(44)在10K附近的显著频率依赖性是由德拜型色散解释的，德拜型色散表明偏离中心的Ce离子沿[111]方向的伽马_5拍击运动，笼子中4a位周围有8个零星占据的位置。当尝试时间t_0=3.1×10~(-11)秒，激活能E=70K时，遵循弛豫时间t=t_0exp(E/k_BT)的热激活伽马_5拍击运动随温度降低而消失，此时4a位笼中的Ce离子将在低温下出现偏心隧穿状态。
Lattice effects in a cerium based clathrate compound Ce_3Pd_20Ge_6 with a cubic Cr_23C_6-type structure have been investigated by ultrasonic and thermal expansion measurements. Elastic softenings of (C_11-C_12)/2 and C_44 proportional to the reciprocal temperature 1/T above T_Q1 = 1.25 K are well described in terms of the quadrupole susceptibility for the ground state Gamma_8 quartet. A huge softening of 50 % in (C_11-C_12)/2 and a spontaneous expansion DL/L = 1.9x10^-4 along the [001] direction in particular indicate the ferroquadrupole ordering of O_2^0 below T_Q1. The elastic anomalies associated with the antiferromagnetic ordering at T_N2 = 0.75 K and the incommensurate antiferromagnetic ordering are also found. Notable frequency dependence of C_44 around 10 K is accounted for by the Debye-type dispersion indicating a Gamma_5 rattling motion of an off-center Ce ion along the [111] direction with eight fractionally occupied positions around the 4a site in a cage. The thermally activated Gamma_5 rattling motion obeying a relaxation time t = t_0exp(E/k_BT) with an attempt time t_0 = 3.1x10^-11 sec and an activation energy E = 70 K dies out with decreasing temperature, and then the off-center tunneling state of Ce ion in the 4a-site cage will appear at low temperatures.