Sibille Romain, Gauthier Nicolas, Lhotel Elsa, Porée Victor, Pomjakushin Vladimir, Ewings Russell A, Perring Toby G, Ollivier Jacques, Wildes Andrew, Ritter Clemens, Hansen Thomas C, Keen David A, Nilsen Gøran J, Keller Lukas, Petit Sylvain, Fennell Tom
Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, California 94025, USA.
Nat Phys. 2020 May;16(5):546-552. doi: 10.1038/s41567-020-0827-7. Epub 2020 Mar 23.
Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles. Theories define such states using gauge fields and deconfined quasiparticle excitations that emerge from a local constraint governing the ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores. However, -electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders. Here we show that the correlated ground state of a Ce-based pyrochlore, CeSnO, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with a fluid-like state where degrees of freedom have a more complex magnetization density than that of magnetic dipoles. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies CeSnO as a unique example of frustrated multipoles forming a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.
自旋液体是由磁偶极子纠缠形成的高度关联但无序的状态。理论通过规范场和从控制受挫磁体基态的局部约束中出现的解禁准粒子激发来定义此类状态。例如，四面体上偶极矩的“2进2出”冰规则可导致稀土烧绿石中的量子自旋冰。然而，电子离子通常携带比偶极子更高阶的多极自由度，从而导致有趣的行为和“隐藏”序。在此，我们表明基于铈的烧绿石CeSnO的关联基态是磁八极子的量子液体。我们的中子散射结果与一种类似流体的状态一致，其中自由度具有比磁偶极子更复杂的磁化密度。八极子 - 八极子耦合的性质和强度，以及归因于自旋子的连续激发的存在，为受“2加2减”规则支配的八极子量子冰提供了进一步证据。我们的工作将CeSnO确定为形成“隐藏”拓扑序的受挫多极子的独特示例，从而将对量子自旋液体的观察推广到可以支持新型涌现场和激发的多极相。
