Quantum spin systems such as magnetic insulators usually show magnetic order, but such classical states can give way to quantum liquids with exotic entanglement through two known mechanisms of frustration: geometric frustration in lattices with triangle motifs, and spin-orbit-coupling frustration in the exactly solvable quantum liquid of Kitaev’s honeycomb lattice. Here we present the experimental observation of a new kind of frustrated quantum liquid arising in an unlikely place: the magnetic insulator Ba₄Ir₃O₁₀ where Ir₃O₁₂ trimers form an unfrustrated square lattice. The crystal structure shows no apparent spin chains. Experimentally we find a quantum liquid state persisting down to 0.2 K that is stabilized by strong antiferromagnetic interaction with Curie–Weiss temperature ranging from −766 to −169 K due to magnetic anisotropy. The anisotropy-averaged frustration parameter is 2000, seldom seen in iridates. Heat capacity and thermal conductivity are both linear at low temperatures, a familiar feature in metals but here in an insulator pointing to an exotic quantum liquid state; a mere 2% Sr substitution for Ba produces long-range order at 130 K and destroys the linear-T features. Although the Ir⁴⁺(5d⁵) ions in Ba₄Ir₃O₁₀ appear to form Ir₃O₁₂ trimers of face-sharing IrO₆ octahedra, we propose that intra-trimer exchange is reduced and the lattice recombines into an array of coupled 1D chains with additional spins. An extreme limit of decoupled 1D chains can explain most but not all of the striking experimental observations, indicating that the inter-chain coupling plays an important role in the frustration mechanism leading to this quantum liquid.

诸如磁性绝缘体之类的量子自旋系统通常表现出磁阶，但是这种经典状态可以通过两种已知的挫折机制让位于具有奇特纠缠的量子液体：由三角形图案组成的晶格中的几何挫折，以及正好在自旋轨道耦合中的挫折。 Kitaev蜂窝格的可解量子液体。在这里，我们介绍了一种新型的受挫量子液体在不大可能出现的地方的实验观察结果：磁绝缘体Ba ₄ Ir ₃ O ₁₀，其中Ir ₃ O ₁₂三聚体形成一个完整的方格。晶体结构未显示明显的自旋链。在实验中，我们发现，由于磁性各向异性，居里-魏斯温度介于-766至-169 K范围内的强反铁磁相互作用可稳定至0.2 K的量子液态。各向异性平均挫折参数是2000，在铱酸盐中很少见。低温下的热容量和热导率都是线性的，这在金属中是一个熟悉的特征，但在绝缘子中却指向奇特的量子液态。Ba仅2％Sr替代会在130 K产生远距离有序，并破坏线性T特性。尽管Ba ₄ Ir ₃ O _10中的Ir ⁴⁺（5d ⁵）离子似乎形成了面部共享的IrO ₆八面体的Ir ₃ O ₁₂三聚体，我们建议减少三聚体内部交换，使晶格重组为具有附加自旋的耦合1D链阵列。解耦的一维链的极端极限可以解释大多数但并非全部惊人的实验观察结果，这表明链间偶联在导致这种量子液体的无奈机制中起着重要作用。