- 1. 山西大学理论物理研究所
- 2. 山西大学物理电子工程学院
- 3. 山西大学
摘要: 针对偶极相互作用的玻色-爱因斯坦凝聚体, 解析计算了点状杂质沿平行极化轴和垂直极化轴运动的能量耗散率, 证明了在超流临界速度更大的方向上耗散率也更高. 该结论为最近在162Dy 原子气体中观测到的实验现象提供了理论支持. 对于一般的运动方向, 给出了耗散率在高速极限下以及临界速度附近的渐近形式. 结合数值计算的结果, 论证了耗散率随方向角的变化总是表现出与临界速度一致的各向异性.
Anisotropic Dissipation in a Dipolar Bose-Einstein Condensate
- Received Date:
06 January 2020
Abstract: The ability to support frictionless motion is one of the manifestations of superfluidity. An impurity immersed in a superfluid can move without dissipation below the critical velocity, which, according to the Landau criterion, is related to the elementary excitation spectrum of the system. In quantum gases of the ultracold atoms, the critical velocity can be measured by stirring a laser beam through the atomic cloud, and the emergence of dissipation can be observed via the heating effect above the threshold stirring speed. Recently, such technique is exploited to study the
superfluidity of the Bose-Einstein condensate (BEC) of 162Dy atoms with dipole-dipole interactions. It is shown that both the critical velocity and the heating rate reflect the anisotropy of the underlying dipolar excitation spectrum.
In this work, we theoretically investigate the anisotropic dissipation of a point-like impurity moving through a dipolar BEC. For the motion along the principal axes, the dissipation rate above the critical velocity is analytically derived according to the linear response theory. At a given reduced velocity, we find the dissipation rate is of a higher value in the direction parallel to the dipole moment, which qualitatively explains the recent experimental observation in dysprosium atoms. Moreover, in the moving direction away from the principal axes, asymptotic s for the dissipation rate are obtained in the high-speed limit, as well as the regime close to the dissipation threshold. By combining these analytical results with numerical simulations, we conclude that, in a dipolar BEC, the angular dependence of the dissipation rate always shows the same anisotropy as the critical velocity. Our predictions can be examined in the current experiments with cold atoms, and the results presented here may be also helpful to understand the anisotropic superfluidity in other systems.