Nag A, Nocera A, Agrestini S, Garcia-Fernandez M, Walters A C, Cheong Sang-Wook, Johnston S, Zhou Ke-Jin
Diamond Light Source, Harwell Campus, Didcot, OX11 0DE, United Kingdom.
SwissFEL, Paul Scherrer Institut, 5232, Villigen, Switzerland.
Nat Commun. 2022 Apr 28;13(1):2327. doi: 10.1038/s41467-022-30065-5.
The microscopic origins of emergent behaviours in condensed matter systems are encoded in their excitations. In ordinary magnetic materials, single spin-flips give rise to collective dipolar magnetic excitations called magnons. Likewise, multiple spin-flips can give rise to multipolar magnetic excitations in magnetic materials with spin S ≥ 1. Unfortunately, since most experimental probes are governed by dipolar selection rules, collective multipolar excitations have generally remained elusive. For instance, only dipolar magnetic excitations have been observed in isotropic S = 1 Haldane spin systems. Here, we unveil a hidden quadrupolar constituent of the spin dynamics in antiferromagnetic S = 1 Haldane chain material YBaNiO using Ni L-edge resonant inelastic x-ray scattering. Our results demonstrate that pure quadrupolar magnetic excitations can be probed without direct interactions with dipolar excitations or anisotropic perturbations. Originating from on-site double spin-flip processes, the quadrupolar magnetic excitations in YBaNiO show a remarkable dual nature of collective dispersion. While one component propagates as non-interacting entities, the other behaves as a bound quadrupolar magnetic wave. This result highlights the rich and largely unexplored physics of higher-order magnetic excitations.
凝聚态物质系统中涌现行为的微观起源编码于其激发态之中。在普通磁性材料中，单个自旋翻转会引发被称为磁振子的集体偶极磁激发。同样，在自旋S≥1的磁性材料中，多个自旋翻转能够引发多极磁激发。遗憾的是，由于大多数实验探测受偶极选择定则支配，集体多极激发通常仍难以捉摸。例如，在各向同性的S = 1海森堡自旋系统中仅观测到了偶极磁激发。在此，我们利用镍L边共振非弹性X射线散射揭示了反铁磁S = 1海森堡链材料YBaNiO中自旋动力学隐藏的四极成分。我们的结果表明，无需与偶极激发或各向异性微扰直接相互作用就能探测到纯四极磁激发。源自局域双自旋翻转过程，YBaNiO中的四极磁激发展现出集体色散的显著双重性质。其中一个分量作为非相互作用实体传播，而另一个则表现为束缚四极磁波。这一结果凸显了高阶磁激发丰富且在很大程度上未被探索的物理特性。
