Metastability in decelerating rotation of superfluid He-3-B

The maximum and the equilibrium numbers of vortex lines are measured in a r otating cylindrical container as a function of its rotation velocity Omega. We find that a slow cool-down through the superfluid transition at constan t rotation produces a state with the equilibrium number of lines, N-eq(Omeg a). In decelerating rotation, when vortex lines annihilate, the state is ob tained which includes the maximum possible number of lines, N-max(Omega). A t low velocities N-max(Omega) exceeds N-max(Omega) and the surplus N-max - N-eq is stabilized by an annihilation energy barrier, which during further deceleration ultimately vanishes in an instability. At high velocities extr insic effects, such as a misalignment between the container and rotation ax es, remove the annihilation barrier and cause N-max to equal N-eq. Our anal ysis concludes that the lowest annihilation barrier is provided by the bend ing of the peripheral vortex lines at one end of the container and not by c ollective instability in the peripheral shape of the vortex array. (C) 1998 Elsevier Science B.V. All rights reserved.