Studies Show a New Design Principle for Tuning Quantum Materials to Achieve Unconventional Superconductivity

According to two studies performed by Rice University physicists and collaborators, certain iron-based superconductors may benefit from a tuneup.

“Our work demonstrates a new design principle for tuning quantum materials to achieve unconventional superconductivity at higher temperatures,” stated Rice University’s Qimiao Si, the lead theoretical physicist on the studies, which examine extraordinary patterns of superconductivity that have been formerly reported in iron selenide.

We show how nematicity, an unusual electronic order, can boost the chances that superconductivity will arise from electron-pairing in specific orbitals. Tuning materials to enhance this effect could foster superconductivity at higher temperatures.

Qimiao Si, Harry C. and Olga K. Wiess Professor of Physics and Astronomy, Director, Rice Center for Quantum Materials

When countless numbers of electrons jostle together, they tend to lose energy whenever they bump something; this causes the electrical current to heat up the wiring. Every year, approximately 6% of electricity on U.S. power grids is lost to this electrical resistance, or heating. On the other hand, the electrons in superconductors create pairs that flow easily, without heat or resistance.

Harnessing superconductivity for power grids, energy-efficient computing, etc. has long been a dream of engineers; however, electrons are known to be notorious loners, the most-extensively researched member of a quantum family referred to as fermions. Fermions do not share space with one another, to such an extent that they would rather momentarily wink out of existence. Due to their peculiar quantum nature, persuading electrons to form pairs usually needs extreme conditions, like strong temperatures or pressure colder than deep space.

Unusual superconductivity—the kind that takes place in iron selenide and similar materials—is completely different. For reasons cannot be explained by physicists, electrons in unusual superconductors are able to form pairs at extremely high temperatures. The behavior has been observed in lots of materials over the past four decades. And while the actual mechanism continues to be confounding, physicists like Si have learned to forecast the way unusual superconductors will act in certain circumstances.

In the latest studies, a hypothetical model of “orbital-selective pairing” was used by Si, collaborators, and Rice graduate student Haoyu Hu to elucidate past experimental results from iron selenide and also to predict how iron selenide and other materials will behave under other situations. The research team included Rong Yu of Renmin University of China, Haoyu Hu, graduate student at Rice University, Jian-Xin Zhu of Los Alamos National Laboratory, and Emilian Nica of Arizona State University. In its model, electrons in certain atomic shells are more inclined to form pairs when compared to other electrons. One way to envisage this is to think of atomic orbitals like lanes on a freeway, informed Si.

Cars travel at different speeds in different lanes. We expect those in the left lane to move fastest, but that’s not always the case. When many cars are on the highway, other lanes may move faster. The electrons in unconventional superconductors are like the cars on a crowded freeway. They must avoid one another and may end up being stuck in one lane. Tuning for electronic order is a way to coax electrons into specific orbitals, much like the highway cones and barriers that direct cars into specific lanes.

Qimiao Si, Harry C. and Olga K. Wiess Professor of Physics and Astronomy, Director, Rice Center for Quantum Materials

The year 2008 saw the discovery of iron-based high-temperature superconductors, and Si and collaborators proposed one of the first concepts to elucidate them: cooling these superconductors to the vicinity of a quantum critical point results in marked correlated-electron effects—behaviors that emerge from and can only be inferred by visualizing electrons as an overall system instead of the several separate objects.

The latest papers, which were reported in Physical Review Letters (PRL) and Physical Review B (PRB), are based on studies Si performed with Nica and Yu at the time of their graduate and postdoctoral studies at Rice University. In 2013, Yu and Si demonstrated that orbital-selective behavior may cause alkaline iron selenides to concurrently show the contradictory properties of both insulators and metals. In 2017, Nica, Si, and coworkers demonstrated that iron selenides can possibly have a superconducting state, wherein electron pairs related to a single orbital of a subshell were relatively different from the electron pairs of a closely associated orbital within the same subshell.

“In the present work, we showed that a nematic order drastically enhances orbital selectivity in the normal state at temperatures above the superconducting transition temperature,” stated Yu, lead author of the PRL paper.

In the case of nematic systems, a higher degree of order is observed in one direction than another. For instance, in a box of uncooked spaghetti, the noodles are aligned in longwise but disordered fashion if visualized in the perpendicular direction.

In order to examine the nature of superconductivity in the presence of the nematic electronic order, Si, Yu, and coworkers examined the “superconducting gap”—a measure comparing the energy costs related to breaking apart electron pairs in the perpendicular direction and the nematic direction. The team’s calculations showed a huge difference.

“Our results provide a natural understanding of very striking results that were recently reported based on painstaking measurements of the superconducting gap in iron selenide with scanning tunneling microscopy,” stated Hu, the lead author of the PRB paper.

Si stated that the work “sheds light on the interplay between orbital-selective pairing and electronic orders, which appear to be important ingredients for unconventional superconductivity in both iron-based superconductors and other strongly correlated quantum materials.”

The study was supported by the Basic Energy Sciences program of the Department of Energy’s Office of Science, DOE’s Center for Integrated Nanotechnologies, an Ulam Scholarship from the Center for Nonlinear Studies at Los Alamos National Laboratory, the Robert A. Welch Foundation, the Gordon and Betty Moore Foundation, the National Science Foundation of China, and the Ministry of Science and Technology of China.