Title: Magnon hydrodynamics in an atomically-thin ferromagnet Show Chinese title

Title: 原子薄铁磁体中的磁振子水力学

Abstract: Strong interactions between particles can lead to emergent collective excitations. These phenomena have been extensively established in electronic systems, but are also expected to occur for gases of neutral particles like magnons, i.e. spin waves, in magnets. In a hydrodynamic regime where magnons are strongly interacting, they can form a slow collective density mode -- in analogy to sound waves in water -- with characteristic low-frequency signatures. While such a mode has been predicted in theory, its signatures have yet to be observed experimentally. In this work, we isolate exfoliated sheets of CrCl$_3$ where magnon interactions are strong, and develop a technique to measure its collective magnon dynamics via the quantum coherence of nearby Nitrogen-Vacancy (NV) centers in diamond. We find that the thermal magnetic fluctuations generated by monolayer CrCl$_3$ exhibit an anomalous temperature dependence, whereby fluctuations increase upon decreasing temperature. Our analysis suggests that this anomalous trend is a consequence of the damping rate of a low-energy magnon sound mode which sharpens as magnon interactions increase with increasing temperature. By measuring the magnetic fluctuations emitted by thin multilayer CrCl$_{3}$ in the presence of a variable-frequency drive field, we observe spectroscopic evidence for this two-dimensional magnon sound mode. Show Chinese abstract

Abstract: 强粒子相互作用可以导致新兴的集体激发。这些现象在电子系统中已经被广泛证实，但也预计会在像磁铁中的磁振子（即自旋波）这样的中性粒子气体中发生。在磁振子强烈相互作用的流体动力学状态下，它们可以形成一种缓慢的集体密度模式——类似于水中的声波——具有低频特征签名。虽然这种模式已在理论上被预测，但其特征尚未在实验中观察到。在这项工作中，我们分离出剥离的CrCl$_3$层，在其中磁振子相互作用很强，并开发了一种技术，通过金刚石中附近的氮-空位（NV）中心的量子相干来测量其集体磁振子动力学。我们发现单层CrCl$_3$产生的热磁波动表现出异常的温度依赖性，即随温度降低波动增加。我们的分析表明，这种异常趋势是由于低能磁振子声模式的阻尼率随着磁振子相互作用增强而增大所造成的。通过测量在可变频率驱动场存在下由薄多层CrCl$_{3}$发射的磁波动，我们观察到了这种二维磁振子声模式的光谱证据。