A new theory that could explain how unconventional superconductivity arises in a diverse set of compounds would never have happened if physicists Kimiao Xi and Emilian Nika had chosen a different name for their 201
In a study published this month in npj quantum materials, Si of Rice University and Nika of Arizona State University argue that unconventional superconductivity in some iron- and heavy-fermion-based materials stems from a common phenomenon called “multiorbital singlet pairing.”
In superconductors, electrons form pairs and flow without resistance. Physicists cannot fully explain how pairs form in unconventional superconductors, where quantum forces give rise to strange behavior. Heavy fermions, another quantum material, have electrons that look thousands of times more massive than ordinary electrons.
Si and Nica proposed the idea of selective coupling in atomic orbitals in 2017 to explain the unconventional superconductivity in alkaline iron selenides. The following year, they applied an orbital-selective model to the heavy fermion material, in which unconventional superconductivity was first demonstrated in 1979.
They considered naming the model after a related mathematical expression known to quantum pioneer Wolfgang Pauli, but chose to call it d + d. The name refers to mathematical wave functions that describe quantum states.
“It’s like you have a pair of electrons dancing with each other,” said Si, Rice of Harry S. and Olga K. Wiess, a professor of physics and astronomy. “You can characterize this dance by s-wave, p-wave and d-wave channels, and d + d refers to two different types of d-waves that merge into one.”
In the year following the publication of the d + d model, Si gave many lectures on the work and found that audience members often confused the name with “d + id,” the name of another pairing state that physicists have debated for more than a quarter of a century.
“People would turn to me after a lecture and tell me ‘Your d + id theory is really interesting,’ and they took it as a compliment, but it happened so often that it was annoying,” said Si, who also leads Quantum Rice Center (RCQM).
In mid-2019, Si and Nika met at noon while visiting the National Laboratory in Los Alamos and began sharing stories about the confusion of d + d versus d + id.
“This led to a discussion about whether d + d could be linked to d + id in a meaningful way, and we realized it wasn’t a joke,” Nika said.
The relationship included d + d coupling states and those famous for the Nobel Prize for the discovery of helium-3 hyperfluidity.
“There are two types of superfluid states of liquid helium-3 pairing, one called phase B and the other phase A,” Nika said. “Empirically, phase B is similar to our d + d, while phase A is almost like d + id.”
The analogy became more intriguing when discussing mathematics. Physicists use matrix calculations to describe the quantum coupling states in helium-3, and this also applies to the d + d model.
“You have a number of different ways to organize this matrix, and we realized that our d + d matrix for orbital space is like a different form of the d + id matrix, which describes the pairing of helium-3 in the spin space,” Nika said.
He told himself that associations with superfluid helium-3 pairing states had helped him and Nika make a more complete description of pairing states in both iron-based and heavy fermionic superconductors.
“As we talked more with Emil, we realized that the periodic table for superconducting pairing is incomplete,” Xi said, referring to the diagram that physicists use to organize superconducting pairing states.
“We use symmetries – such as lattice or arrangements, or whether the time to move backwards is equivalent, which is symmetry to reverse time – to organize possible pairing states,” he said. “Our discovery was that d + id can be found in the existing list. You can use the periodic table to build it. But d + d, you can’t. This is outside the periodic table because the table does not include orbitals.”
Si said that orbitals are important for describing the behavior of materials such as iron-based superconductors and heavy fermions, where “very strong electron-electron correlations play a crucial role.”
“Based on our work, the table needs to be expanded to include orbital indices,” Xi said.
The orbital-selective pairing theory applied to the first heavy fermion superconductor
Emilian M. Nika et al., Multiorbital singlet pairing and d + d superconductivity, npj quantum materials (2021). DOI: 10.1038 / s41535-020-00304-3
Provided by Rice University
Quote: New class of superconductors (2021, January 25) retrieved on January 25, 2021 from https://phys.org/news/2021-01-class-superconductors.html
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