Usually the Wrong Name Leads to Discovery

Levitation of a magnet on top of a superconductor. Credits: Jubobroff, Fbouquet, LPS CC BY-SA 3.0

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

In a study published last month in npj Quantum materialsBoth Rice University and Arizona State University Nice argue that unconventional superconductivity in some iron-based materials and heavy fermions arises from a general phenomenon called “single multiethletic coupling.”

In superconductors, electrons form pairs and flow without resistance. Physicists cannot fully explain how pairs are formed in unconventional superconductors, where quantum forces bring about strange behaviors. Heavy fermions, another quantum material, represent electrons that appear to be thousands of times more massive than ordinary electrons.

As well as Nica proposed the idea of ​​selective coupling within 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.

Qimiao Si

Qimiao Si is professor of physics and astronomy Harry C. and Olga K. Wiess at Rice University and director of the Rice Center for Quantum Materials. Credit: Jeff Fitlow / Rice University

They considered the model name after a related mathematical expression made famous by quantum pioneer Wolfgang Pauli, but decided to call it d + d. The name refers to the functions of mathematical waves that describe quantum states.

“It’s like having a pair of electrons jumping with each other,” said Si, a professor of physics and astronomy, Harry Rice and Olga K. Wiess. “You can characterize that dance from the channels of wave s, wave p and wave d, and d + di refers to two different types of waves d joining together into one.”

Emilian Nica

Emilian Nica is a postdoctoral researcher in physics at Arizona State University. Credit: E. Nica

In the year following the publication of the d + d model, Si gave many work-related lectures and found that audience members often confused the name with “d + id”, the name of another pairing state that physicists have discussed for more than one a quarter of a century.

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

In mid-2019, Si and Nica met over lunch while visiting the Los Alamos National Laboratory and began sharing stories about d + d versus d + id confusion.

“This led to a discussion about whether d + d could be related to d + id in a meaningful way, and we realized it was not a joke,” Nica said.

The link included the d + d coupling states and those made famous by the Nobel Prize-winning discovery of helium-3 superfluidity.

“There are two types of liquid helium-3 superfluid coupling states, 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 coupling states in helium-3, and this is also the case for the d + d model.

“You have a number of different ways to organize that matrix, and we realized that our d + d matrix for orbital space was like another form of the d + id matrix that describes the coupling of helium-3 in space of rotation,” he said. Nica.

As he said, associations with helium-3 superfluid coupling states have helped him and Nica advance a more complete description of coupling states in both iron-based and heavy-duty superconductors.

“As Emil and I talked more, we realized that the periodic table for superconducting mating was incomplete,” Si said, referring to the table that physicists use to organize superconducting mating states.

“We use symmetry – like lattice or rotation arrangements, or if the time moving forward and backward is equivalent, which is the symmetry of time reversal – to organize possible mating 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 not. “Beyond it is beyond the periodic table, because the table does not include orbitals.”

As the mentioned orbitals are important to describe the behavior of materials like 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.

Reference: “Single multiorbital coupling and d + d superconductivity” by Emilian M. Nica and Qimiao Si, January 5, 2021, npj Quantum materials.
DOI: 10.1038 / s41535-020-00304-3

The research was supported by an initial grant from Arizona State University, Department of Energy (DE-SC0018197), Welch Foundation (C-1411) and National Science Foundation (PHY-1607611).

RCQM is a multidisciplinary research effort that utilizes the strengths and global partnerships of more than 20 Rice research groups.

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