Coexistence of single and double quantum vortex lines

We discuss the configurations in which singly and doubly quantized vortex l ines coexist in a rotating superfluid. General principles of energy minimiz ation lead to the conclusion that in equilibrium the two vortex species seg regate within a cylindrical vortex cluster in two coaxial domains where the singly quantized lines are in the outer annular region. This is confirmed with simulation calculations on discrete vortex lines. Experimentally the c oexistence can be studied in rotating superfluid He-3-A. With cw NMR techni ques Lye find the radial distribution of the two vortex species to depend o n how the cluster is prepared: (i) By cooling through T-c in rotation, coex istence in the minimum energy configuration is confirmed. (ii) A disordered agglomerate is formed if one starts with an equilibrium cluster of single quantum vortex lines and adds to it sequentially double quantum lines, by i ncreasing the rotation velocity in the superfluid state. This proves that t he energy barriers, which separate different cluster configurations, are to o high for metastabilities to anneal.