Stable singular fractional skyrmion spin texture from the quantum Kelvin-Helmholtz instability

TL;DR

This paper reports the experimental observation of a novel singular skyrmion in a ferromagnetic superfluid, generated via quantum Kelvin-Helmholtz instability, which challenges traditional topological classification and reveals new nonlinear dynamics.

Contribution

It introduces the first observation of eccentric fractional skyrmions beyond standard topology, stabilized by quantum Kelvin-Helmholtz instability in a superfluid system.

Findings

Observed stable singular skyrmions after 2 seconds.

Produced skyrmions via emission from magnetic domain walls.

Confirmed universality of classical and quantum Kelvin-Helmholtz instabilities.

Abstract

Topology profoundly influences diverse fields of science, providing a powerful framework for classifying phases of matter and predicting nontrivial excitations, such as solitons, vortices, and skyrmions. These topological defects are typically characterized by integer numbers, called topological charges, representing the winding number in their order parameter field. The classification and prediction of topological defects, however, become challenging when singularities are included within the integration domain for calculating the topological charge. While such exotic nonlinear excitations have been proposed in the superfluid $^3$He-A phase and spinor Bose-Einstein condensate of atomic gases, experimental observation of these structures and studies of their stability have long been elusive. Here we report the observation of a singular skyrmion that goes beyond the framework of topology in a ferromagnetic superfluid. The exotic skyrmions are sustained by undergoing anomalous symmetry breaking associated with the eccentric spin singularity and carry half of the elementary charge, distinctive from conventional skyrmions or merons. By successfully realizing the universal regime of the quantum Kelvin-Helmholtz instability, we identified the eccentric fractional skyrmions, produced by emission from a magnetic domain wall and a spontaneous splitting of an integer skyrmion with spin singularities. The singular skyrmions are stable and can be observed after 2~s of hold time. Our results confirm the universality between classical and quantum Kelvin-Helmholtz instabilities and broaden our understanding on complex nonlinear dynamics of nontrivial texture beyond skyrmion in topological quantum systems.