Zero-point entropy in 'spin ice'
Zero-point entropy in 'spin ice'
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DOI：
10.1038/20619
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发表时间：
1999-05-27
期刊：
影响因子：
64.8
通讯作者：
Shastry, BS
中科院分区：
文献类型：
--
作者：
Ramirez, AP;Hayashi, A;Shastry, BS
普通水冰(冰i-h)是一种不寻常的固体--氧原子形成周期性结构，而氢原子由于存在两个不相等的O-H键长而高度无序。鲍林指出，这两个键长的存在导致了可能的基态(2，3)的宏观简并，使得当温度趋于零时，系统具有有限的熵。然而，与这种简并相关的动力学在实验上是无法获得的，因为冰融化了，氢的动力学不能独立于氧的运动来研究。一个可以研究这种简并性的类似系统(5)是一个具有焦绿石结构的磁体--称为自旋冰--其中自旋取向起着类似于冰i-h中氢位置的作用。这里我们给出了一个这样的系统Dy2Ti2O7的比热数据，从中我们推导出总的自旋熵为0.67Rln2。这类似于为冰i-h确定的值0.71Rln2，因此确认了对应关系的有效性。我们还发现，通过施加磁场，在水冰中无法达到的行为--恢复了大部分基态熵，以及涉及横向自旋自由度的新转变。
Common water ice (ice I-h) is an unusual solid-the oxygen atoms form a periodic structure but the hydrogen atoms are highly disordered due to there being two inequivalent O-H bond lengths'. Pauling showed that the presence of these two bond lengths leads to a macroscopic degeneracy of possible ground states(2,3), such that the system has finite entropy as the temperature tends towards zero. The dynamics associated with this degeneracy are experimentally inaccessible, however, as ice melts and the hydrogen dynamics cannot be studied independently of oxygen motion(4). An analogous system(5) in which this degeneracy can be studied is a magnet with the pyrochlore structure-termed 'spin ice'-where spin orientation plays a similar role to that of the hydrogen position in ice I-h. Here we present specific-heat data for one such system, Dy2Ti2O7, from which we infer a total spin entropy of 0.67Rln2. This is similar to the value, 0.71Rln2, determined for ice I-h, SO confirming the validity of the correspondence. We also find, through application of a magnetic field, behaviour not accessible in water ice-restoration of much of the ground-state entropy and new transitions involving transverse spin degrees of freedom.