What is a soliton

Spintronics with Solitons

Spintronics promises to exceed the limits of conventional semiconductor technology by far. In the computer of the future, spin wave solitons could transfer data between individual components super-fast. It all began almost 200 years ago in a water canal in Scotland.

The Scottish explorer John Scott Russell actually wanted to improve the design of canal boats in August 1834. But while a boat was being pulled through a water canal by horses and stopped abruptly, Russell inadvertently created a very special type of water wave: a single large crest of waves detached from the bow of the boat and sped through the canal without losing height or losing its shape change. Russell followed the wave crest on horseback for kilometers until the phenomenon finally left him behind with the feeling of a great discovery. Russell had observed a soliton for the first time - a wave crest that is stable for a long time and does not dissipate, as happens with other types of waves.

Creation of a solitary wave

Waves are a fundamental phenomenon in nature. In everyday life, in addition to vivid water waves, there are, for example, sound waves in the form of noises, microwaves that heat food, and radio waves for television and radio. Individual wave crests, so-called "wave packets", are of particular interest for technical applications. For example, in modern communication technology, zeros and ones are sent in the form of short lightwave packets through optical fibers to transmit digital information.

However, a wave packet always consists of many waves with different frequencies. These individual components spread at different speeds, depending on the material through which the wave packet is moving. Physicists call this relationship between the speed of propagation and the wavelength "dispersion". As a result, the wave packet dissolves over time and a steep wave crest becomes a broad, flat something. In a fiber optic cable, that would mean the loss of information.

Creation of a soliton

The solitons discovered by Russell are nowadays of great importance in various areas of physics. In the case of a soliton, two opposing effects are exactly in balance and thus lead to a stable wave crest: On the one hand, the wave packet dissolves due to the dispersion. This is counteracted by the so-called non-linearity effect. Because with particularly strong waves, the high parts of the wave crest can move faster than the low parts. Similar to a breaking water wave, the crest of the wave overtakes the lower part of the wave crest. This will compress the shaft. If the non-linear compression and the dispersion-related dissipation are exactly in balance, the wave packet gets its shape and one speaks of a soliton. Although Russell's contemporaries dismissed the phenomenon as an unimportant fringe phenomenon, solitons are now being studied for various technical applications: for safety in shipping, for data transmission in fiber optic cables and in the form of spin wave solitons for the computer of the future.

Spintronics - the next generation of electronics

Conventional electronics are based on the movement of electrical charges, the electrons. However, these particles have a property that has not yet been used, namely the spin, which is responsible for the magnetic moment of the electron. By mastering the motion of the electron's spin, a completely new type of electronics could be realized, which is now called "spintronics". The electron spin creates a magnetic field similar to that of a tiny bar magnet. Neighboring electron spins influence each other like a chain of such bar magnets If you hit a spin, the excitation leads to a spin wave in which the spins rotate together in a wave-like manner.

Model of a spin wave

Particularly interesting are stable spin wave packets in which a deflection of the spins propagates through the material like a single wave crest. Such packages can be used, for example, to exchange information between different electronic components. The circuits could send each other spin packets without the need for electrical connections between the components, as was previously the case. This is because a layer of magnetic material would suffice for the spin packets to spread between the components. Since the packages can be sent from one component to the next through the magnetic layer, cables connecting two components would be superfluous. The speed of such a spintronic technique would far overshadow semiconductor technology as it is not subject to the fundamental limitations of conventional semiconductor electronics. For example, the fundamental limits of miniaturization could be overcome and significantly more circuits could be accommodated in the same area than before.

But as is so often the case, hard work comes before pleasure. As with all other wave packages, a spin package also dissipates over time and becomes a broad spin wave. However, stable wave packets are required for data transfer and so the solitons are also of great importance in spintronics.

On the trail of the spin soliton

Investigation of spin wave solitons

While Russell could still ride after his water soliton back then, scientists nowadays have to resort to more sophisticated methods to study the spin solitons. The long water channel is replaced by a crystal made of iron, yttrium and oxygen, in which spin waves can propagate. Since the material only weakens the spin wave a little, a spin pulse in the crystal can propagate over several centimeters. That doesn't sound like much at first. But if you consider that modern electronic components are only a few nanometers (billionths of a meter) in size, the distance seems huge.

The spin wave is triggered in the crystal with the aid of an antenna. Similar to a vibrating tuning fork that is placed on a table, the electromagnetic pulse from the antenna is transmitted to the crystal in the form of a spin wave packet. This spin soliton then moves over the surface of the crystal at around 10,000 kilometers per hour. In order not to be left behind by the soliton like Russell once did, researchers nowadays use laser light to study the wave. Because compared to the speed of light, spin wave solitons still propagate slowly in the crystal. This gives the researchers enough time to measure the shape of the spin wave on the surface of the crystal at various points in time.

Observation of a spin bullet

Findings: fit for the future

Since spin-wave solitons were discovered by Russian scientists in 1983, many experimental and theoretical studies have dealt with them. Thanks to modern experiments, it is now possible to observe not only broad spin waves, but even point-like spin solitons. These so-called spin wave bullets (engl. bullet = Floor) are particularly interesting in information technology. The short pulses would be perfect intermediaries between integrated circuits. And so, after 200 years, Russell's discovery could find its way into the computer of the future: as a spin wave soliton.