Topological Excitations in Spinor Bose-Einstein Condensates
  A rich variety of order parameter manifolds of multicomponent Bose-Einstein
condensates (BECs) admit various kinds of topological excitations, such as
fractional vortices, monopoles, skyrmions, and knots. In this paper, we discuss
two topological excitations in spinor BECs: non-Abelian vortices and knots.
Unlike conventional vortices, non-Abelian vortices neither reconnect themselves
nor pass through each other, but create a rung between them in a topologically
stable manner. We discuss the collision dynamics of non-Abelian vortices in the
cyclic phase of a spin-2 BEC. In the latter part, we show that a knot, which is
a unique topological object characterized by a linking number or a Hopf
invariant [$\pi_3 (S^2)=Z$], can be created using a conventional quadrupole
magnetic field in a cold atomic system.
