Skyrmions, increasingly promising magnetic particles for future computers

Skyrmions, increasingly promising magnetic particles for future computers

During the second half of the nineteenth centuryH In the twentieth century, the famous British physicist Kelvin had a brilliant idea when he read the work of his German colleague Hermann Helmholtz. The latter showed that rings of rotating fluid were perfectly stable and would act on each other with forces reminiscent of those found in magnetic fields between wires carrying electric currents. Kelvin concluded that atoms were actually threads of liquid rotating around a central axis forming different nodes, one for each chemical element. The fluid carrying these threads has been interpreted as the ether, the material medium whose pressures and waves are supposed to be the origin of the electromagnetic field.

Although it is elegant and attractive, this… Unitary theory It was a dismal failure, as developments in the quantum theory of atoms have proven. However, physicists retained the idea that discrete stable structures, which can be interpreted as particles, could arise from nonlinear equations, such as the Navier-Stokes equations, which describe continuous fields.

Hence we know the existence of solitons, which are types of stable energy packets in media described by nonlinear partial differential equations. One of the most famous examples is found in hydrodynamics. This is a tidal wave, a single wave first observed by Scotsman John Scott Russell in the 19th century.H The horn that follows several kilometers is a wave heading upstream that does not seem to want to weaken.

Model of nucleons

Because of their stable nature, elementary particles have frequently been proposed as solitons. And also, nearly fifty years ago, before we discovered the theory of quantum dynamics, the great British theorist Tony Skyrmi He sought to better understand the nature of nucleons and the strong nuclear force. So he tried to play the same game that Kelvin played to explain the existence of nucleons and their properties. We already know that protons and neutrons are fermions with half-integer spin and that they exchange types of photons, the famous Yukawa boson with integer spin, the pion.

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At the same time, Heisenberg also sought to better understand nuclear forces, but he went further. He considered a nonlinear fundamental field equation based on a fermion field that must contain all matter and force particles known at the time. In this unified theory, photons and gravitons, for example, were viewed as ensembles of fermions. These, which have intrinsic angular momentum, spin, of values ​​1 and 2, can actually be states associated with an even number of spin ½ fermions.

Skyrme took a more modest approach (dealing only with baryons and nuclear forces) but it is very similar. He also considered a nonlinear equation but whose fundamental field is a zero-spin boson, the Yukawa pion.

At first glance, the idea seems ridiculous. How to get spin particles from composite states of zero spin particles?

This is where the non-linear nature of the equation comes into play. As in a fluid, also described by a nonlinear equation (the Navier-Stokes equation), stable vortices can be formed with angular momentum, we can consider protons and neutrons as types of vortices in a pion fluid. These configurations, reminiscent of those found in solitons, are today called skyrmions.

The discovery of quarks and the theory of quantum chromodynamics (QCD) eclipsed the Skirmey model of baryons (which, ironically, would later emerge as an approximation to the QCD equations). But after a few decades, we realized its importance in the field of condensed matter physics.

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