Spin-orbit phase behavior of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>Na</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Co</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>TeO</mml:mi></mml:mrow><mml:mn>6</mml:mn></mml:msub></mml:math> at low temperatures
Wenjie Chen, Xintong Li, Zhenhai Hu, Ze Hu, Yue Li, Ronny Sutarto, Feizhou He, Kazuki Iida, Kazuya Kamazawa, Weiqiang Yu, Xi Lin, Yuan Li
Abstract
We present a comprehensive study of single crystals of ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$, a putative Kitaev honeycomb magnet, focusing on its low-temperature phase behaviors. A new thermal phase transition is identified at 31.0 K, below which the system develops a two-dimensional (2D) long-range magnetic order. This order precedes the well-known three-dimensional (3D) order below 26.7 K, and is likely driven by strongly anisotropic interactions. Surprisingly, excitations from the 3D order do not support the order's commonly accepted ``zigzag'' nature, and are instead consistent with a ``triple-$\mathbf{q}$'' description, which has remained hitherto unexplored in the study of Kitaev honeycomb magnets. The 3D order exerts a fundamental feedback on high-energy excitations that likely involve orbital degrees of freedom, and it features strongly scattered spin waves until at much lower temperatures, a sign for ground state near degeneracy and frustration. These findings constrain microscopic models and render ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$ a spin-orbit entangled frustrated magnet that hosts very rich physics.