Institut für Theoretische Physik, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 1, 60438, Frankfurt am Main, Germany.
Department of Physics and Astronomy, University of California, Irvine, CA, 92697, USA.
Nat Commun. 2017 Oct 27;8(1):1152. doi: 10.1038/s41467-017-01177-0.
The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons-quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital-coupled magnets, such as α-RuCl, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here, we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmonicity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of α-RuCl and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.
量子集体激发的描述是凝聚态物质量子理论的一个里程碑。一个突出的例子发生在传统磁体中，传统磁体支持玻色子磁振子——有序自旋的量子谐波涨落。相比之下，最近的发现令人震惊，即强自旋轨道耦合磁体（如 α-RuCl）可能表现出与传统磁振子不一致的宽激发连续体。由于对揭示这种连续体本质的潜在相互作用了解不完整，因此阐明其本质仍然具有挑战性。最常讨论的解释是指类似于著名的 Kitaev 自旋液体的分数激发相干连续体。在这里，我们提出了一个更普遍的情况。我们提出，观察到的连续体代表了源自强磁非谐性的非相干激发，这种非谐性在这类材料中自然发生。这种情况完全解释了 α-RuCl 的观察到的非弹性磁响应，并揭示了此类材料中存在的非平凡激发远远超出了 Kitaev 态。
