Formation of a New State of Matter by Joining up 4 Electrons

Nearly 20 years after researchers first predicted electron quadruplets and evidence of their existence are shown to do in experimental setups, representing a brand-new state of matter which opens-up an entire new field of possibilities in physics.

The central principle of superconductivity is that electrons form pairs. But can they also condense into foursomes? Recent findings have suggested they can, and a physicist at KTH Royal Institute of Technology today published the first experimental evidence of this quadrupling effect and the mechanism by which this state of matter occurs.

If you think about superconductivity, where electrical resistance is zero, you need pairs of electrons – known as Cooper pairs – to form and condense. Something very similar occurs with four electrons in this new state.

“It will probably take many years of research to fully understand this state,” says theoretical physicist Egor Babaev from the KTH Royal Institute of Technology in Sweden and the senior researcher for the new study.

Babaev made the original 2004 prediction about this state of matter.

The pairing of electrons enables the quantum state of superconductivity, a zero-resistance state of conductivity which is used in MRI scanners and quantum computing. It occurs within a material as a result of two electrons bonding rather than repelling each other, as they would in a vacuum. The phenomenon was first described in a theory by, Leon Cooper, John Bardeen and John Schrieffer, whose work was awarded the Nobel Prize in 1972.

So-called Cooper pairs are basically “opposites that attract”. Normally two electrons, which are negatively-charged subatomic particles, would strongly repel each other. But at low temperatures in a crystal they become loosely bound in pairs, giving rise to a robust long-range order. Currents of electron pairs no longer scatter from defects and obstacles and a conductor can lose all electrical resistance, becoming a new state of matter: a superconductor.

The iron-based superconductor material, Ba1−xKxFe2As2, is mounted for experimental measurements.

For electron quadrupling to occur, the particles need to be prevented from pairing up and flowing without resistance in normal superconductor conditions, something that scientists weren’t even sure was possible until recently.

Babaev and his colleagues looked at an iron-based superconductor material called Ba1−xKxFe2As2 (seen above) for their experiments, previously identified as potentially producing unusual effects. The material was tested for electrical resistance and other properties at a range of different temperatures.

The experiments showed evidence of the breaking of time-reversal symmetry, a concept in physics where turning time expressions negative in formulas can run the same event backward or turn motions in the opposite directions.

“However, in the case of a four-fermion condensate that we report, the time reversal puts it in a different state,” says Babaev.

Taken together, the measurements recorded from the experiments point to long-range order: not between pairs of electrons (as in superconductivity), but between pairs of pairs. That’s fermionic quadrupling and a new state of matter.

The state of superconductivity is used everywhere, from quantum computers to magnetic resonance imaging (MRI) scanners, but it remains to be seen what’s in store for the new state of matter made possible by fermionic quadrupling. It’s being called a BTRS (broken time-reversal symmetry) quartic metal phase.

“It will probably take many years of research to fully understand this state,” he says. “The experiments open up a number of new questions, revealing a number of other unusual properties associated with its reaction to thermal gradients, magnetic fields and ultrasound that still have to be better understood.”