Department of Physics, University of Tokyo, Bunkyo-ku, Hongo 7-3-1, Tokyo 113-0033, Japan.
Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany.
Nature. 2018 Feb 14;554(7692):341-345. doi: 10.1038/nature25482.
The honeycomb lattice is one of the simplest lattice structures. Electrons and spins on this simple lattice, however, often form exotic phases with non-trivial excitations. Massless Dirac fermions can emerge out of itinerant electrons, as demonstrated experimentally in graphene, and a topological quantum spin liquid with exotic quasiparticles can be realized in spin-1/2 magnets, as proposed theoretically in the Kitaev model. The quantum spin liquid is a long-sought exotic state of matter, in which interacting spins remain quantum-disordered without spontaneous symmetry breaking. The Kitaev model describes one example of a quantum spin liquid, and can be solved exactly by introducing two types of Majorana fermion. Realizing a Kitaev model in the laboratory, however, remains a challenge in materials science. Mott insulators with a honeycomb lattice of spin-orbital-entangled pseudospin-1/2 moments have been proposed, including the 5d-electron systems α-NaIrO (ref. 5) and α-LiIrO (ref. 6) and the 4d-electron system α-RuCl (ref. 7). However, these candidates were found to magnetically order rather than form a liquid at sufficiently low temperatures, owing to non-Kitaev interactions. Here we report a quantum-liquid state of pseudospin-1/2 moments in the 5d-electron honeycomb compound HLiIrO. This iridate does not display magnetic ordering down to 0.05 kelvin, despite an interaction energy of about 100 kelvin. We observe signatures of low-energy fermionic excitations that originate from a small number of spin defects in the nuclear-magnetic-resonance relaxation and the specific heat. We therefore conclude that HLiIrO is a quantum spin liquid. This result opens the door to finding exotic quasiparticles in a strongly spin-orbit-coupled 5d-electron transition-metal oxide.
蜂窝晶格是最简单的晶格结构之一。然而，在这个简单的晶格上，电子和自旋通常会形成具有非平凡激发的奇特相。无质量的狄拉克费米子可以从巡游电子中出现，正如在石墨烯中实验所证明的那样，并且在自旋-1/2 磁体中可以实现具有奇特准粒子的拓扑量子自旋液体，正如 Kitaev 模型理论所提出的那样。量子自旋液体是一种长期以来被追求的物质奇异态，其中相互作用的自旋保持量子无序，没有自发对称性破缺。Kitaev 模型描述了量子自旋液体的一个例子，可以通过引入两种类型的马约拉纳费米子来精确求解。然而，在实验室中实现 Kitaev 模型仍然是材料科学中的一个挑战。已经提出了具有自旋轨道纠缠的赝自旋-1/2 矩的蜂窝晶格的莫特绝缘体，包括 5d 电子系统α-NaIrO（参考文献 5）和α-LiIrO（参考文献 6）以及 4d 电子系统α-RuCl（参考文献 7）。然而，由于非 Kitaev 相互作用，这些候选物在足够低的温度下被发现是磁有序的，而不是形成液体。在这里，我们报告了 5d 电子蜂窝化合物 HLiIrO 中赝自旋-1/2 矩的量子液体态。尽管相互作用能约为 100 开尔文，但该碘化物在 0.05 开尔文以下没有显示出磁有序。我们观察到来自核磁共振弛豫和比热中少数自旋缺陷的低能费米子激发的特征。因此，我们得出结论，HLiIrO 是一种量子自旋液体。这一结果为在强自旋轨道耦合的 5d 电子过渡金属氧化物中发现奇特的准粒子打开了大门。
