Transitions in vortex skyrmion structures in superfluid $^3$He-A driven by an analogue of the zero-charge effect

Abstract

In quantum electrodynamics, the zero-charge effect originates from the logarithmic dependence of the coupling constant in the action of the electromagnetic field on the ratio of the ultraviolet and infrared energy cutoffs. An analogue of this effect in Weyl superfluid $^3$He-A is the logarithmic divergence of the bending energy of the orbital anisotropy axis at low temperatures, where temperature plays the role of the infrared cutoff and the vector of the orbital anisotropy plays the role of the vector potential of the synthetic electromagnetic field for Weyl fermions. We calculate numerically the spatial distribution of the order parameter in rotating $^3$He-A as a function of temperature. At temperatures close to the superfluid transition, we observe formation of vortex skyrmions known as the double-quantum vortex and the vortex sheet. These structures include alternating circular and hyperbolic merons as a bound pair or a chain, respectively. As temperature lowers towards absolute zero, we find a continuous transition in the vortex structures towards a state where the vorticity is distributed in thin tubes around the circular merons. For the vortex sheet, we present a phase diagram of the transition in the temperature - angular velocity plane and calculations of the nuclear magnetic resonance response.