Half-solitons in superfluid³He-A: Novel π/2-quanta of phase slippage

Abstract

Core structures of nontopological solitons between inequivalent vacua in superfluid³He-A are considered. We analyze the symmetries of these A-A interfaces, and compute their hard-core structures in the Ginzburg-Landau regime. We discuss both domain walls where the orbital anisotropy l-vector is flipped (l→−l), and those with the same l(x=−∞) and l(x=+∞) asymptotics. In particular, we find new classes of A-A boundaries: these novel π/2-solitons, which can occur in the absence of a change in the asymptotic l-vector field, constitute the elementary quanta of phase slippage in superfluid³He-A. We ascribe these “half-solitons” to a new topological scenario for the flaring-out of vorticity in the extended (k, r)-space. Edges of such walls serve to provide vortices with 1/4 quantum of circulation in³He-A. In analogy with the B-B domain walls in superfluid³He-B, solitons of pure phase slippage by π—with a normal core—prove unstable in³He-A; they either fission into a pair of ordinary l-solitons—domain walls with superfluid cores both flipping the orbital anisotropy axis (i.e., l→−l→l), or form a bound pair of walls, each of which constitutes an l-soliton with a phase shift of π/2. Our investigation of the superfluid A-A vacuum interfaces may prove useful in a broader context since the A-A boundaries exemplify the possible domain walls relevant for the Higgs-field solitons (“cosmic domain walls”) within the Weinberg-Salam model.