Ghosh S, Rosenbaum T F, Aeppli G, Coppersmith S N
James Franck Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
Nature. 2003 Sep 4;425(6953):48-51. doi: 10.1038/nature01888.
Free magnetic moments usually manifest themselves in Curie laws, where weak external magnetic fields produce magnetizations that vary as the reciprocal of the temperature (1/T). For a variety of materials that do not display static magnetism, including doped semiconductors and certain rare-earth intermetallics, the 1/T law is replaced by a power law T(-alpha) with alpha < 1. Here we show that a much simpler material system-namely, the insulating magnetic salt LiHo(x)Y(1-x)F(4)-can also display such a power law. Moreover, by comparing the results of numerical simulations of this system with susceptibility and specific-heat data, we show that both energy-level splitting and quantum entanglement are crucial to describing its behaviour. The second of these quantum mechanical effects-entanglement, where the wavefunction of a system with several degrees of freedom cannot be written as a product of wavefunctions for each degree of freedom-becomes visible for remarkably small tunnelling terms, and is activated well before tunnelling has visible effects on the spectrum. This finding is significant because it shows that entanglement, rather than energy-level redistribution, can underlie the magnetic behaviour of a simple insulating quantum spin system.
自由磁矩通常在居里定律中表现出来，即弱外部磁场产生的磁化强度随温度的倒数（1/T）变化。对于各种不显示静态磁性的材料，包括掺杂半导体和某些稀土金属间化合物，1/T定律被α<1的幂律T^(-α)所取代。在这里，我们表明一个简单得多的材料系统——即绝缘磁性盐LiHo(x)Y(1 - x)F(4)——也能表现出这样的幂律。此外，通过将该系统的数值模拟结果与磁化率和比热数据进行比较，我们表明能级分裂和量子纠缠对于描述其行为都至关重要。这些量子力学效应中的第二个——纠缠，即具有多个自由度的系统的波函数不能写成每个自由度的波函数的乘积——对于非常小的隧穿项就变得明显，并且在隧穿对光谱产生可见影响之前就被激活。这一发现具有重要意义，因为它表明纠缠而非能级重新分布可以成为简单绝缘量子自旋系统磁行为的基础。
