From: KTH Royal Institute of Technology (in Sweden)
October
18, 2021 -- 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.”
Contributing
to the research were scientists from the following institutions:
Institute for Solid State and Materials Physics, TU Dresden, Germany;
Leibniz Institute for Solid State and Materials Research, Dresden; Stockhom
University; Bergische Universtät at Wuppertal, Germany; Dresden High Magnetic
Field Laboratory (HLD-EMFL); Wurzburg-Dresden Cluster of Excellence ct.qmat,
Germany; Helmholtz-Zentrum, Germany; National Institute of Advanced Industrial
Science and Technology (AIST), Japan; Institut Denis Poisson, France.
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