Landau critical velocity ($v_L={\inf }_{p>0}E(p)/p$)

The excitation-based kinematic velocity bound in the Landau criterion. For a heavy weak obstacle and an isotropic excitation spectrum, emitting momentum parallel to the obstacle's motion costs $E(p)-vp$; requiring this to remain positive for every excitation gives the displayed infimum. Vortex nucleation or other dynamical processes can make a measured critical velocity lower than this bound.