How Demon Particle could shed new light on understanding of superconductors

A new physics sensation, the so-called Pines’ Demon, or the Demon Particle, is not actually a demon. Nor is it a particle; and it’s not new, since its existence has been suspected for over 65 years. This is an amazing story of accidental discovery and it could potentially lead to far-reaching advances in understanding superconductors.

A team of researchers from the University of Illinois Urbana-Champaign and Kyoto University found a weird effect while studying a compound called strontium ruthenate (Sr2RuO4), a crystalline combination of two metals, strontium and ruthenium.

Researchers were shooting electrons at strontium ruthenate, and measuring the energies bouncing off it. The results were unexpected. Repeated experiments confirmed this was not an experimental error. The only way to explain the oddity was by reference to a possibility suggested by physicist David Pines (1924-2018) in 1956. As the team’s paper explains in Nature, they started to look at the electronic structure and what they found fitted Pines’ calculations.

All materials have some resistance to electrical current. Metals have less resistance, wood has more. Energy is required to generate a current to overcome resistance. According to Ohm’s law, which is taught in school, the strength of an electrical current (I) is equal to the voltage (V) divided by the resistance (R) of the medium, or I=V/R in physics notation. Voltage can be defined as the energy required to push a charge through a resistance, and 1 volt is defined as equal to 1 joule (unit of energy) divided by 1 coulomb (unit of charge).

At very low temperatures (below minus 130 Celsius), some materials see resistance drop to zero. A current can pass uninterrupted for years, at zero voltage, without any energy used. Ohm’s law turns into nonsense. Resistance becomes zero, and you cannot divide by zero.

These materials are known as superconductors. The world record for running an uninterrupted zero-energy current through a superconductor is over 28 years (this is an ongoing experiment). One of the holy grails of physics is finding or making materials that are superconducting at normal temperature and pressure. There was much excitement recently when a Korean team claimed to have found such a material (LK-99) but the claims turned out to be false.

Superconductors have some other properties. They levitate when put in a magnetic field — this is called the Meissner effect. They can be moved with zero friction. Under very high pressures, superconductivity is known to occur at normal temperatures.

Superconductors are used to run maglev trains, in MRI (magnetic resonance imaging) machines, in cyclotrons where particles are smashed together, and, of course, in many electrical circuits. The challenges involved in using superconductors include maintaining very low temperatures, or generating very high pressures. Obviously, utility would improve massively if the phenomenon can be replicated in normal room conditions.

Electrical currents are caused because electrons, which are negatively charged, move around. An electron is famously both a wave and a particle. When waves interact, constructive or destructive resonance can occur. In constructive resonance, waves are in phase, and the peaks get higher and the troughs go lower. In destructive resonance, waves cancel because they are out of phase — when one wave is peaking, the other is at a trough. Noise cancelling earphones use destructive resonance to cancel sound waves.

Pines speculated electrons in superconductors could undergo resonance and collectively behave like a “quasiparticle”. After all, if a particle can be a wave, a wave can also be a particle. If you don’t understand this, join the club of geniuses like Einstein and Schrodinger — two quantum pioneers who said they didn’t understand quantum effects!

Nobody would believe in quantum mechanics except for one rather important detail — it offers very exact predictions which fit very exactly with experimental results. Pines offered exact predictions for this “plasmon” — or plasma wave, if at all it existed. Importantly, Pines predicted such a “demon” (named in honour of Maxwell’s Demon, a famous thought experiment) could exist at room temperature. Pines’ Demon would be transparent, it would have no charge, and it would have no mass (because charge and mass would be cancelled out). So how do you find this thing, if it exists?

Other materials, which are not superconductors, have some properties similar to superconductors. Strontium ruthenate is one of those materials. When the team looked at the electronic structure of the crystal, they found such a plasma wave. This consisted of two electron bands oscillating out-of-phase with nearly equal magnitudes, which behaved like a particle just like Pines theorised. If this team or some other researchers can make sense of the results and manipulate these effects, they may achieve a better understanding of how superconductors work and maybe, just maybe, figure out how to make superconductors at closer to normal pressure-temperature gradients.

Maxwell’s Demon

James Clerk Maxwell (1831-1879) conceptualised a “demon which could play games of skill with molecules”. The demon opens or shut a door, which connects two chambers that are both filled with gas. The demon allows only fast-moving molecules to pass through to one chamber and slow-moving molecules into the other chamber. Fast-moving molecules have higher temperatures, while slow molecules have lower temperatures. So the chambers would have different temperatures. This has important implications and it could violate the second law of thermodynamics if such a demon (think of it as an AI if you like) could exist. The second law is the reason why a glass dropped on the floor shatters into many pieces (increase of entropy) but the glass pieces thrown together don’t turn into a glass (decrease of entropy).

Nobody has managed to make Maxwell’s demon yet but the term passed into physics vocabulary. Pines’ Demon does exist and it has been retconned to “Distinct Electron Motion” to describe how this model works.