Electrons have time for foursomes in iron-based superconductor

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

In a paper in Nature Physics, Egor Babaev and his collaborators reported evidence of fermion quadrupling in a series of experimental measurements on the iron-based material, Ba_1−xK_xFe₂As₂. These results come nearly 20 years after Babaev first predicted this kind of phenomenon, and eight years after he published a paper predicting that it would occur in the material.

The pairing of electrons is behind the quantum state of superconductivity, a zero-resistance state of conductivity that 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 won 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 as pairs, giving rise to a robust long-range order. Currents of these electron pairs are no longer scattered by defects and obstacles, causing a conductor to lose all electrical resistance and become a new state of matter: a superconductor.

Only in recent years has the theoretical idea of four-fermion condensates become broadly accepted. For a fermion quadrupling state to occur there has to be something that prevents condensation of pairs and prevents their flow without resistance, while allowing condensation of four-electron composites, Babaev says.

The Bardeen-Cooper-Schrieffer theory doesn’t allow for such behavior, so when Vadim Grinenko, Babaev’s experimental collaborator at the Technische Universtät Dresden in Germany, detected the first signs of a fermion quadrupling condensate in 2018, it challenged years of prevalent scientific understanding. What followed was three years of experimentation and investigation at labs in multiple institutions to validate the finding.

According to Babaev, the key observation was that fermionic quadruple condensates spontaneously break time-reversal symmetry. In physics, time-reversal symmetry is a mathematical operation that replaces the expression for time with its negative in formulas or equations so that they describe an event in which time runs backward or all the motions are reversed.

If the direction of time is inverted, the fundamental laws of physics still apply. That also holds for typical superconductors: if the arrow of time is reversed, a typical superconductor would still be in the same superconducting state.

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

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

This story is adapted from material from KTH Royal Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.