Electrons In A Foursome Make A New State Of Matter

Electron quadruplets were observed in this iron-based superconductor material, Ba1−xKxFe2As2 seen mounted for experimental measurements in Professor Babaev's research. Image Credit:Vadim Grinenko, Federico Caglieris

Physicists have confirmed the first-ever observation of four electrons coupling. This quadruplet is a previously predicted state of matter, and it could provide insight into superconductivity, the peculiar state where materials can transmit electricity with no resistance.

The findings are published in the journal Nature Physics.

In the state of superconductivity, electrons couple in pairs, leading to unusual macroscopic properties. Electrons should just repel each other but in particular conditions – low temperatures and within crystals – they can bond. And once they do, they can flow without scattering from defects and obstacles. The material is now a superconductor and it has no electrical resistance.

Some scientists have wondered if it was possible to create quadruplets too. That idea goes against expectations from the Bardeen–Cooper–Schrieffer (BSC) theory, which underpins the microscopic understanding of superconductivity. For that to be possible something needs to stop electron pairs from forming and then allow for the formation of the quadruplet.

Back in 2018, the team found some peculiar behavior in a material that seemed to hint at such a rare state. Follow-up research at multiple institutions and labs has built the case for it and was published this week.

The material in question is a mixture of barium, potassium, iron, and arsenic. Its chemical formula is Ba1−xKxFe2As2 and if you are familiar with chemical formulae you might be surprised to find a letter x in there. That simply indicates a variable – scientists can toy about with how much barium and potassium they are putting in.

And that’s the key to this discovery. The material is usually a superconductor (below a certain temperature) but when x is 0.8 suddenly instead of seeing superconductivity, they see the opposite.

The crucial piece of evidence suggesting that this was likely a quadruple electron formation is the breaking of time-reversal symmetry. If time-reversal is not broken when you are observing a state it will look the same if time is flowing forward or backward. This is what happens in superconductors.

“However, in the case of a four-fermion condensate that we report, the time-reversal puts it in a different state,” senior author Professor Egor Babaev from KTH Royal Institute of Technology, said in a statement.

“It will probably take many years of research to fully understand this state," he said. "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.”

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