Particle physics experiments

Experiments reveal the formation of a new state o


image: The iron-based superconducting material, Ba1 − xKxFe2As2, is set up for experimental measurements.
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Credit: Vadim Grinenko, Federico Caglieris

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

Report today in Physics of nature, Prof. Egor Babaev and colleagues presented evidence for the quadrupling of fermions in a series of experimental measurements on the iron-based material, Ba1 − xKxFe2As2. The results follow nearly 20 years after Babaev first predicted this kind of phenomenon, and eight years after he published an article predicting it could occur in the material.

The pairing of electrons enables the quantum state of superconductivity, a state of zero resistance conductivity that is used in MRI scanners and quantum computing. It occurs in a material as a result of the bonding of two electrons instead of 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 received the Nobel Prize in 1972.

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

It is only in recent years that the theoretical idea of ​​four-fermion condensates has gained wide acceptance.

For a fermion-quadrupling state to occur, there must be something that prevents the condensation of the pairs and prevents their flow without resistance, while still allowing the condensation of the four-electron composites, Babaev says.

The Bardeen-Cooper-Schrieffer theory did not allow such behavior. So when Babaev’s experimental collaborator at the Technische Universtät Dresden, Vadim Grinenko, in 2018 found the first signs of a quadruplant fermion condensate, he questioned years of widespread scientific agreement.

What followed was three years of experimentation and investigation in the laboratories of several institutions in order to validate the results.

Babaev says that the key to the observations made is that the fermionic quadruple condensates spontaneously break the time inversion symmetry. In physics, time inversion symmetry is a mathematical operation of replacing the expression of time with its negative in formulas or equations so that they describe an event in which time goes back or all movements are reversed.

If we reverse the direction of time, the fundamental laws of physics still apply. This also applies to typical superconductors: if the arrow of time is reversed, a typical superconductor would still be the same superconducting state.

“However, in the case of a four-fermion condensate that we’re reporting, the time reversal puts it in a different state,” he says.

“It will probably take many years of research to fully understand this condition,” he says. “The experiments open 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 need to be better understood.

Scientists from the following institutions contributed to the research: Institute of Solids and Materials Physics, TU Dresden, Germany; Leibniz Institute for Solid State and Materials Research, Dresden; Stockholm University; Bergische Universtät in Wuppertal, Germany; Dresden High Magnetic Field Laboratory (HLD-EMFL); Pole of excellence Würzburg-Dresden ct.qmat, Germany; Helmholtz-Zentrum, Germany; National Institute of Advanced Industrial Science and Technology (AIST), Japan; Denis Poisson Institute, France.

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