Hopfion

A hopfion is a topological soliton.^[1][2][3][4] It is a stable three-dimensional localised configuration of a three-component field ${\displaystyle {\vec {n}}=(n_{x},n_{y},n_{z})}$ with a knotted topological structure. They are the three-dimensional counterparts of 2D skyrmions, which exhibit similar topological properties in 2D. Hopfions are widely studied in many physical systems over the last half century.^[5]

Model of magnetic hopfion in a solid. B_em is emergent magnetic field (orange arrows); in a hopfion, it does not align to the external magnetic field (black arrow).

The soliton is mobile and stable: i.e. it is protected from a decay by an energy barrier. It can be deformed but always conserves an integer Hopf topological invariant. It is named after the German mathematician, Heinz Hopf.

A model that supports hopfions was proposed as follows:^[1]

${\displaystyle H=(\partial {\bf {n}})^{2}+(\epsilon _{ijk}{\bf {n}}\cdot \partial _{i}{\bf {n}}\times \partial _{j}{\bf {n}})^{2}}$

The terms of higher-order derivatives are required to stabilize the hopfions.

Stable hopfions were predicted within various physical platforms, including Yang–Mills theory,^[6] superconductivity^[7][8] and magnetism.^{[9][10][11][4]}

Experimental observation

Hopfions have been observed experimentally in chiral colloidal magnetic materials,^[2] in chiral liquid crystals,^[12][13] in Ir/Co/Pt multilayers using X-ray magnetic circular dichroism^[14] and in the polarization of free-space monochromatic light.^[15][16]

In chiral magnets, a helical-background variant of the hopfion has been theoretically predicted to occur within the spiral magnetic phase, where it was called a "heliknoton".^[17] In recent years, the concept of a "fractional hopfion" has also emerged where not all preimages of magnetisation have a nonzero linking.^[18][19]

See also

• Skyrmion
• Hopf fibration