‘Unparticles’ May Hold The Key To Superconductivity, Say Physicists

A mysterious form of matter may help to explain one of the most important phenomena in physics

Back in 2007, the Harvard physicist Howard Georgi put forward an exotic and revolutionary new theory of physics. Until then, all of physics could be described in terms of particles and the forces between them.

Georgi proposed something different — that there was an entire sector of physics involving energy, momentum and mass but with no particles. He coined this stuff “unparticles” and developed the mathematical framework that describes it and its properties.

Nobody has ever observed an unparticle but physicists have toyed with this new model of physics and proposed various ways in which it might be measured for the first time. Indeed, the most recent thinking is that conventional particles can behave like unparticles under certain conditions.

Today, James LeBlanc and Adolfo Grushin at the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany, say this effect may explain unconventional superconductivity, the ability for materials to conduct with zero resistance.

Conventional superconductivity was explained in the 1950s and thought to work only at very low temperature. But the discovery of superconductivity at relatively high temperatures in the 1980s took physicists entirely by surprise and they have struggled to understand this unconventional phenomenon ever since. Now LeBlanc and Grushin are suggesting that unparticles could be the key.

First, some background about unparticles. Massless particles such as photons have an interesting property — their energy and momentum can take any value ranging across many orders of magnitude. In the language of physics, photons are “scale invariant”, the physics that describes them is the same regardless of the scale at which they are considered.

By contrast, electron always have the same mass regardless of their energy or momentum. So massive particles like electrons are not scale invariant and consequently, the physics that describes them changes according to the scale which they are considered. For example, the physics that describes electrons on the quantum scale is very different to the physics that describes them at the scale of conventional transistors and diodes.

Georgi’s concept of unparticles comes about by conjecturing that some “stuff” may have mass, energy and momentum and yet also be scale invariant. The physics that describes this stuff would be the same regardless of the length scale or energy scale at which they are considered. Consequently, this stuff must be entirely unlike ordinary particulate matter, hence the name unparticles.

Although unparticles have never been observed, physicists have proposed that ordinary particles can behave in this scale invariant way under certain circumstances. In graphene, electrons are confined entirely to two dimensions and at temperatures close to absolute zero behave as if they have zero mass.

In the hinterland between this zero mass behaviour and ordinary behaviour, electrons can be described by a scaling law which is mathematically similar to the definition of an unparticle. So at these very low temperatures, electrons should behave like unparticles although the experiments to confirm this have yet to be done.

Nevertheless, the idea that ordinary particles can behave like unparticles under certain circumstances has driven LeBlanc and Grushin thinking. Perhaps unparticles might also emerge in complex three-dimensional systems at low temperatures, such as in superconducting materials, they suggest.

Physicists have long known that the behaviour of electrons in high-temperature superconductors is extremely complex. This behaviour can only be described in terms of the interaction between the various entities in the material, such as holes and electrons inside a complex crystal structure.

What LeBlanc and Grushin show is that under certain conditions the interaction between these entities can become scale invariant and is therefore described by the physics of unparticles. In very simple terms, when that happens, material properties such as resistance no longer depend on the length scales involved. So if electrons move without resistance on a tiny scale, they should also move without resistance on much larger scales too. Hence the phenomenon of superconductivity.

“We have described how it is possible for unparticles in strongly correlated matter to mediate superconductivity,” say LeBlanc and Grushin.

That is a fascinating idea that leads to many interesting additional questions. For example, physicists have measured other states of matter in which the mass of an electron takes on fractional values. One interesting question is how these so-called Hall states might be linked with the unparticle approach. Another is how unparticle physics might lead to superconductivity in 2 dimensional structures such as graphene.

There is plenty more gold to be mined in this area of theoretical physics, one that is clearly fizzing.

Ref: arxiv.org/abs/1407.8492 : Unparticle Mediated Superconductivity
