Mirebeau I, Goncharenko I N, Cadavez-Peres P, Bramwell S T, Gingras M J P, Gardner J S
Laboratoire Léon Brillouin, CEA-CNRS, CE Saclay, 91191 Gif sur Yvette, France.
Nature. 2002 Nov 7;420(6911):54-7. doi: 10.1038/nature01157.
Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie-Weiss temperature theta(CW) (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below theta(CW) (refs 5-7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb(2)Ti(2)O(7), under conditions of high pressure (approximately 9 GPa) and low temperature (approximately 1 K). This compound is a three-dimensional magnet with theta(CW) = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb(2)Ti(2)O(7) remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.
液体在低温下有望结晶。唯一的例外是氦气，由于量子涨落，它在0 K时仍可保持液态。同样，磁体中的原子磁矩（自旋）预计会在由居里 - 外斯温度θ(CW)（参考文献3）设定的温度尺度下有序排列。几何阻挫磁体则是个例外。在这些体系中，由于晶格对称性，成对的自旋相互作用无法同时达到最小化。这能够在远低于θ(CW)（参考文献5 - 7）的温度下稳定一种短程有序的波动磁矩的类液态状态。在此，我们利用中子散射在高压（约9 GPa）和低温（约1 K）条件下观测几何阻挫体系Tb(2)Ti(2)O(7)中的自旋液态。该化合物是一种三维磁体，其θ(CW) = -19 K，负值表明存在反铁磁相互作用。在常压下，Tb(2)Ti(2)O(7)至少在70 mK以下仍保持自旋液态（参考文献8）。但我们发现，在高压下，自旋在2.1 K以下开始有序排列或“结晶”，反铁磁序与类液态涨落共存。这些结果表明，在几何阻挫体系中，压力可诱导出自旋液态/固态混合物。
