A new class of superconductors

A new theory that could explain how unconventional superconductivity appears in a diverse set of compounds may never have happened if physicists Qimiao Si and Emilian Nica had chosen a different name for their 2017 model of orbital selective superconductivity.

In a study published this month in Quantum materials npj, Si from Rice University and Nica from Arizona State University argue that unconventional superconductivity in some iron-based, heavy-fermion materials results from a general phenomenon called “multibital 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 appear to be thousands of times more massive than ordinary electrons.

Si and Nica proposed the idea of ​​selective pairing in atomic orbitals in 2017 to explain unconventional superconductivity in alkaline iron selenides. The following year, they applied the selective orbital 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 made famous by quantum pioneer Wolfgang Pauli, but chose to call it d + d. The name refers to the mathematical wave functions that describe quantum states.

“It’s like you have a pair of electrons dancing with each other,” said Si, physics and astronomy professor Harry C. and Olga K. Wiess of Rice. “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 release of the d + d model, Si lectured extensively on the job and found that members of the public often had the name confused with “d + id”, another’s name. pairing state that physicists have been discussing for over a quarter of a century. .

“People would approach me after a conference and say, ‘Your d + id theory is really interesting,’ and they took it as a compliment, but it happened so often that it got boring,” said Si, who directs also the Rice Center for Quantum Materials (RCQM).

In mid-2019, Si and Nica met over lunch during a visit to the Los Alamos National Laboratory, and began sharing stories about the confusion between d + d and d + id.

“This led to a discussion of whether d + d could be connected with d + id in any meaningful way, and we realized it wasn’t a joke,” Nica said.

The connection involved d + d pairing states and those made famous by the discovery of the superfluidity of Nobel Prize winning helium-3.

“There are two types of superfluid pairing states of liquid helium-3, one called phase B and the other called phase A,” Nica said. “Empirically, phase B is similar to our d + d, while phase A is almost like a d + id.”

The analogy became more intriguing when they discussed mathematics. Physicists use matrix calculations to describe quantum pairing states in helium-3, and this is also the case for 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 was like a different shape from the d + id matrix that describes the helium-3 pairing in spin space, ”Nica said.

Si said that associations with superfluid helium-3 pairing states have helped him and Nica advance a more complete description of pairing states in iron-based and heavy-fermion superconductors.

“As Emil and I spoke more, we realized that the periodic table of superconducting pairing was incomplete,” Si said, referring to the diagram physicists use to organize superconducting pairing states. .

“We use symmetries – like lattice or spin arrangements, or if the time ahead of the back is equivalent, which is time reversal symmetry – to organize the possible pairing states. “, did he declare. “Our revelation 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. It’s beyond the periodic table, because the table does not include orbitals. “

While so-called orbitals are important in 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,” Si said.