Quantum-spin-liquid states in the two-dimensional kagome antiferromagnets ZnxCu4-x(OD)6Cl2
Quantum-spin-liquid states in the two-dimensional kagome antiferromagnets ZnxCu4-x(OD)6Cl2
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DOI：
10.1038/nmat1986
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发表时间：
2007-11-01
期刊：
影响因子：
41.2
通讯作者：
Kiefer, K.
中科院分区：
文献类型：
--
作者：
Lee, S.-H.;Kikuchi, H.;Kiefer, K.
一个三维的相互作用的自旋系统在冷却时通常会发展出静态的长程有序。然而，如果自旋是量子的（S = 1/2），则可能出现新的量子顺磁态。其中最受关注的状态是共振价键态（1，2），其中每对相邻的量子自旋形成纠缠的自旋单线态（价键），这些单线态之间发生量子力学共振。在这里，我们提供了这样的量子顺磁态存在于阻挫反铁磁体，ZnxCu 4-x（OD）（6）Cl-2，其中S = 1/2的磁性Cu 2+的时刻形成一个二维kagome晶格层的实验指示。我们发现，在Cu-4（OD）（6）Cl-2，扭曲的kagome平面弱耦合，一个无色散的激发模式出现在磁激发谱低于类似20 K，其特性类似于量子自旋单线态在固态，称为价键固体，打破平移对称性。掺杂非磁性的Zn ~（2+）离子减少了kagome晶格的畸变，减弱了晶面间的耦合，但也稀释了kagome晶格的磁性占位。对于ZnCu_3（OD）（6）Cl-_2，当kagome面未发生畸变且90%被Cu ~（2+）离子占据时，低能自旋涨落变得无特征。
A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S = 1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating-valence-bond state(1,2), in which every pair of neighbouring quantum spins forms an entangled spin singlet ( valence bonds) and these singlets are quantum mechanically resonating among themselves. Here we provide an experimental indication for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4-x(OD)(6)Cl-2, where the S = 1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu-4(OD)(6)Cl-2, where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below similar to 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence-bond solid, that breaks translational symmetry. Doping with non-magnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The valence-bond-solid state is suppressed, and for ZnCu3(OD)(6)Cl-2, where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low-energy spin fluctuations become featureless.