Bose-Einstein condensation of magnons is one of few macroscopic quantum phenomena observed at room temperature. Since its discovery, it became an object of intense research, which led to the observation of many exciting phenomena such as quantized vortices, second sound, and Bogolyubov waves. However, it remained unclear what physical mechanisms can be responsible for the spatial stability of the magnon condensate. Indeed, since magnons are believed to exhibit attractive interaction, it is generally expected that the condensate is unstable with respect to the real-space collapse, contrarily to experimental findings. Here, we provide direct experimental evidence that magnons in a condensate exhibit repulsive interaction resulting in the condensate stabilization and propose a mechanism, which is responsible for this interaction. Our experimental conclusions are additionally supported by the theoretical model based on the Gross-Pitaevskii equation. Our findings solve a long-standing problem, providing a new insight into the physics of magnon Bose-Einstein condensates.

磁振子的玻色-爱因斯坦凝聚是在室温下观察到的少数宏观量子现象之一。自从它被发现以来，它就成为了人们深入研究的对象，从而观察到了许多令人兴奋的现象，例如量子涡旋、第二声和博戈柳波夫波。然而，目前尚不清楚什么物理机制可以导致磁振子凝聚体的空间稳定性。事实上，由于磁振子被认为表现出吸引相互作用，因此通常预计凝聚态相对于真实空间塌缩是不稳定的，这与实验结果相反。在这里，我们提供了直接的实验证据，证明凝聚态中的磁振子表现出排斥相互作用，从而导致凝聚态稳定，并提出了一种负责这种相互作用的机制。我们的实验结论还得到基于 Gross-Pitaevskii 方程的理论模型的支持。我们的发现解决了一个长期存在的问题，为磁振子玻色-爱因斯坦凝聚体的物理学提供了新的见解。