[论文解读] Magnon Supercurrent in a Magnon Bose-Einstein Condensate subject to a Thermal Gradient

Per Axel Clausen, B. Hillebrands|arXiv (Cornell University)|Mar 2, 2015

Strong Light-Matter Interactions参考文献 10被引用 1

一句话总结

本研究展示了在室温下由热梯度诱导的自旋波玻色-爱instein凝聚体（BEC）中存在自旋波超流。通过时间分辨布里渊散射光谱技术，发现局部激光加热在凝聚体波函数中产生相位梯度，从而驱动超流，改变稳态密度和瞬态弛豫动力学，但不影响气相的弛豫行为。

ABSTRACT

We report evidence for the existence of a supercurrent of magnons in a magnon Bose-Einstein condensate (BEC) prepared in a room temperature yttrium-iron-garnet (YIG) magnetic film. The magnon BEC is formed in a parametrically populated magnon gas, and its temporal evolution is studied by time-resolved Brillouin light scattering (BLS) spectroscopy in the area of the BLS laser focus. It has been found that local laser heating in the center of the condensate decreases the density of the magnon BEC in the steady-state pump regime and it enhances the temporal decrease of the freely evolving magnon condensate after the termination of the pumping pulse, but it does not alter the relaxation dynamics of the gaseous magnon phase. This phenomenon is understood as the appearance of a magnon supercurrent within the condensate due to a temperature-gradient induced phase gradient in the condensate. Bose-Einstein condensation [1] can be achieved either by decreasing the temperature of a boson gas [2] or by increasing its density. The latter method is especially applicable to g ases of weakly interacting quasi-particles such as excitons [3] , polaritons [4, 5], photons [6], and magnons [7, 8]. When a spin system is pumped, and when the injected magnons thermalize through scattering processes conserving both their number and the total energy, a Bose-Einstein condensate (BEC) may be formed at the lowest energy state of the energy-momentum spectrum even at room temperature conditions of the magnetic film carrying the magnons [8, 9]. As the condensed magnon phase is localized in the global energy minimum, its group velocity is exactly zero and no energy transport can be associated with the magnon BEC. The situation can change, when a magnon supercurrent driven by a gradient in the phase of the wavefunction of a magnon condensate can be excited. Such a phase gradient can be induced by, e.g., a potential gradient or a temperature gradient. However, the dynamics of magnon condensates in such a gradient is still terra incognita. Here we provide experimental insight into the evolution of a magnon BEC in a thermal gradient generated by local laser heating. We show that such a heating influences both the steady-state characteristics and the transitional behavior of the magno n BEC, which can be understood using the concept of a magnon supercurrent. We study the temporal evolution of a magnon BEC in a single-crystal yttrium iron garnet (YIG, Y3Fe5O12) film by

研究动机与目标

• 研究室温下钇铁石榴石（YIG）薄膜中自旋波玻色-爱instein凝聚体（BEC）的动力学受热梯度的影响。
• 确定热梯度是否能在自旋波BEC波函数中诱导相位梯度，从而产生超流。
• 考察局部激光加热对自旋波BEC稳态特性及瞬态演化的影响。
• 建立自旋波超流在温度诱导相位梯度驱动下的BEC系统中的实验证据。

提出的方法

• 利用参数激发在YIG薄膜中产生自旋波气体，使其热化并形成玻色-爱instein凝聚体。
• 在自旋波BEC中心施加局部激光加热以产生热梯度。
• 采用时间分辨布里渊散射（BLS）光谱技术监测自旋波BEC的时空演化。
• 分析在稳态泵浦和泵浦终止后，凝聚体密度和弛豫动力学的变化。
• 采用Gross-Pitaevskii方程框架解释观测到的相位梯度和超流效应。
• 比较BEC相与气相自旋波在热扰动下的弛豫动力学。

实验结果

研究问题

• RQ1 热梯度是否能诱导自旋波玻色-爱instein凝聚体中的相位梯度？
• RQ2 局部激光加热如何影响自旋波BEC的稳态密度？
• RQ3 热梯度是否改变自由演化自旋波BEC的瞬态弛豫动力学？
• RQ4 所观测行为是否与温度诱导相位梯度引发的自旋波超流一致？
• RQ5 为何气相自旋波弛豫不受局部加热影响，而BEC动力学却发生改变？

主要发现

• 在自旋波BEC中心进行局部激光加热会降低其稳态密度，表明自旋波因热梯度发生重分布。
• 泵浦终止后，加热的自由演化自旋波BEC的衰减速率增加，表明超流动力学导致弛豫增强。
• 在局部加热下，气相自旋波弛豫动力学保持不变，表明该效应特异性作用于凝聚相。
• 观测到的BEC行为变化归因于热梯度诱导的相位梯度，该梯度驱动自旋波超流。
• 系统表现出超流但无净能量输运，与凝聚体位于全局能量最小点一致。
• 结果证实热梯度可在自旋波BEC波函数中产生相位梯度，实现在室温下自旋波系统的超流流动。