A new class of superconductors

A new theory that could explain how custom superconductivity arises in different combinations would never have happened if physicists Alchemists C and Emilian Nika had chosen a different name for their 2017 model of class-selective superconductivity.

In a study published this month NPJ quantum content, C of Rice University and Nika of Arizona State University argued that unconventional superconductivity in some iron-based and heavy-fermion materials arises from a common phenomenon called “multiorbital single pair”.

In superconductors, electrons form pairs and flow without resistance. Physicists cannot fully explain how pairs form in unconventional superconductors, where quantum forces stimulate strange behavior. Heavy fermions, another quantum material, feature electrons that appear thousands of times larger than ordinary electrons.

C and Nika proposed the idea of ​​creating a selective pair within the atomic orbit in 2017, to explain the unconventional superconductivity in alkaline iron selenides. The following year, they applied the orbital-selective model to heavier fermion material, first showing unconventional superconductivity in 1979.

After the famous related mathematical expression by quantum pioneer Wolfgang Pauli, they thought of naming this model, but chose to say D + D. The name refers to mathematical wave functions that describe quantum states.

Harry C. and Olga K. of Rice. “It’s like you have a pair of electrons that dance with each other,” said CA, a professor of physics and astronomy at Weiss. “You can characterize that dance by S-wave, P-wave and D-wave channels, and D + D refers to two different types of D-waves that merge together.”

In the years following the publication of the D + D model, CA gave several lectures on the work and found that audience members are often confused with the name “D + ID”, another pair of state names that physicists have debated for more than a quarter of a century. . .

“People would approach me after a lecture and say, ‘Your theory of D + ID is really interesting,’ and that meant compliments, but it happened so often that it was annoying,” said CA, director of the Rice Center. For Quantum Materials (RCQM).

In mid-2019, C and Nika met at noon while visiting the Los Alamos National Laboratory, and began sharing stories about the D + D versus D + ID confusion.

“This led to a discussion on whether D + D is meaningfully linked to D + ID, and we realized that was no joke.”

The alliance includes states that combine D + D and is famous for its Nobel Prize-winning discovery of helium-3 excess fluid.

“Liquid helium-3 has two types of hyperthermia, one called phase B and the other phase A,” Nika said. “From experience, phase B is very similar to our D + D, while phase A is almost like D + ID.”

The analogy became more interesting when they discussed mathematics. Physicists use matrix calculations to describe quantum pairing conditions in helium-3, and the same is true for the D + D model.

“You have different ways of organizing the matrix, and we realized that our D + D matrix for orbital space is a different form of the D + ID matrix that describes the helium-3 attached to the spin space.”

CA said the associations with the highly liquid helium-3 pair states have helped him and Nika advance the full description of the pairing states in both iron-based and heavy-fermion superconductors.

“As Emil and I talked further, we realized that the periodic table for creating superconducting pairs was incomplete,” CA said, using chart physicists to organize superconducting pair states.

“We use symmetry – such as mesh or spin adjustments, or whether the time to move backwards is exactly the same, which is symmetry as opposed to time – to adjust the possible connection phase,” he said. “Our revelation was that D + ID can be found in the existing list. You can use the periodic table to create it. But D + D, you can’t. It’s out of the periodic table, because the table doesn’t include orbitals.”

CA said iron rbitals are important for describing the behavior of materials such as iron-based superconductors and heavy fermions, where “very strong electron-electron correlation plays an important role.”

“Based on our work, it is necessary to expand the table to include orbital indicators,” CA said.