Critical velocities in flows of superfluid 4He

We review and discuss the underlying physics of critical velocities in flows of superfluid He-4 (for historical reasons known as He II). Critical velocities, which indicate changes in the character of quantum three-dimensional flows, are associated with intrinsic as well as extrinsic nucleation of quantized vortices or with generation of quasiparticles-phonons and rotons-and are affected by the length scale of a particular superflow. We consider both dc and ac flows of He II through channels and orifices, as well as flows due to moving or oscillating obstacles of various shapes and sizes in stationary He II. We also consider critical velocities associated with homogeneous and inhomogeneous cavitation, occurring in quantum as well as classical viscous flows of liquid He-4. We discuss relevant theoretical approaches leading to evaluations of critical velocities, focusing on the fundamental case of pure superflow of He II in the zero temperature limit and describe their experimental verification. Additionally, we point out the role of temperature in quantum flows of He II, describe various scenarios of transition to quantum turbulence, and discuss the crucial role of purity and history of the He II samples.