Releasing half-quantum vortices via the coupling of spin polarization, charge- and spin-current

Motivated by recent experiments observing half-integer flux quanta in mesoscopic loops of superconducting Sr2RuO4, a theory is developed based on a "d-soliton", a topological defect of spin-triplet superconductors involving a change of the spin configuration. A phenomenological Ginzburg-Landau free energy is given which allows to describe such a soliton yielding a half-integer flux quantum inside a loop, assuming a chiral p-wave superconducting phase. The d-soliton couples to spin polarization perpendicular to the loop axis, which allows to reduce the energy of the d-soliton, fitting well to the experimental findings. The origin of this coupling lies in the combination of spin and charge current within the d-soliton generating a spin polarization analogous to a spin Hall effect. The effect of Fermi liquid corrections is discussed, in particular, in view of the spatial extension of the d-soliton and the energetics in comparison with a standard integer flux state.