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.
Reporting today in Nature Physics, Professor Egor Babaev and collaborators
presented evidence of fermion quadrupling 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 a paper predicting that it could occur in the
material.
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.
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 didn’t allow for such behavior, so when
Babaev’s experimental collaborator at Technische Universtät Dresden, Vadim
Grinenko, found in 2018 the first signs of a fermion quadrupling condensate,
it challenged years of prevalent scientific agreement.
What followed was three years of experimentation and investigation at labs
at multiple institutions in order to validate the finding.
Babaev says that key among the observations made is that fermionic quadruple
condensates spontaneously break time-reversal symmetry. In physics
time-reversal symmetry is a mathematical operation of replacing 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 one inverts time direction, the fundamental laws of physics still hold.
That also holds for 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 report, the time
reversal puts it in a different state,” he says.
“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.”
Reference:
Vadim Grinenko, State with spontaneously broken time-reversal symmetry above
the superconducting phase transition, Nature Physics (2021).
DOI: 10.1038/s41567-021-01350-9
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Physics