Pnictides can be Key to Questions on High-Temperature Superconductivity

A new iron-based material that provides insight into the microscopic origins of high-temperature superconductivity has been synthesized by a team of physicists from the Center for Quantum Materials (RCQM), Rice University.

Rice University graduate student Yu Song created a new material from iron, sodium, copper and arsenic that is offering physicists clues about the microscopic origins of high-temperature superconductivity. Credit: Photo by Jeff Fitlow.

Rice graduate student Yu Song synthesized the new material—a formulation comprising iron, arsenic, copper, and sodium—in the laboratory of physicist Pengcheng Dai, which has been reported in the Nature Communications journal this week.

According to Dai, Song’s technique of mixing the ingredients in an atmosphere of pure argon, sealing the mixture in niobium canisters and baking it at approximately 1000 °C yields a layered alloy with alternating stripes of iron and copper. RCQM Director Qimiao Si stated that this striping is highly crucial for applicability of the material in deciphering the origins of high-temperature superconductivity.

By forming this regular pattern, Yu Song has physically removed disorder from the system, and that is crucially important for being able to say something meaningful about what’s going on electronically.

Qimiao Si, RCQM Director

Si is a theoretical physicist who has dedicated his research in explaining the origins of high-temperature superconductivity and similar phenomena for almost 20 years.

It was in the year 1986 that high-temperature superconductivity was discovered. This phenomenon takes place when electrons get paired up and freely flow in layered alloys similar to the one synthesized by Song.

Of the dozens of high-temperature superconducting alloys synthesized to date, most are complex crystals comprising a transition metal (usually copper or iron) and various other elements. Usually, high-temperature superconductors are poor conductors at ambient temperature and only become superconductors upon being cooled to a critical temperature.

The central problem of high-temperature superconductivity is to understand the precise relationship between these two fundamental states of matter and the phase transition between them. The macroscopic change is evident, but the microscopic origins of the behavior are open to interpretation, largely because there are many variables in play, and the relationship between them is both synergistic and nonlinear.”

[Two schools of thought] developed from the very beginning of this field. One was the itinerant camp, which argues that both states ultimately arise from itinerant electrons. After all, these materials are metals, even if they may be poor metals.

Pengcheng Dai, professor of physics and astronomy at Rice University

The other is the localized camp proclaiming that fundamentally new physics surface (because of the electron-electron interactions) at the critical point at which transitioning of materials from one phase to another occurs.

According to Dai, the measurements on the new material synthesized by Song support the localized theory. Specifically, the new material is the first member of a class of iron-based superconductors known as pnictides, i.e. pronounced NIK-tides, which can be tuned between two competing phases, namely, the superconducting phase where there is no resistance to the flow of electrons, and a “Mott insulating” phase where electrons are locked in place and do not flow at all.

The discovery that Yu Song made is that this material is more correlated, which is evident because of the Mott insulating phase. This is the first time anyone has reported an iron-based superconductor that can be continuously tuned from the superconducting phase to the Mott insulating phase.

Pengcheng Dai, professor of physics and astronomy at Rice University

Samples were synthesized and a few tests were carried out at RCQM. Further tests were carried out at Chalk River Laboratories’ Canadian Neutron Beam Center in Ontario, the Brookhaven National Laboratory in New York, National Institute for Standards and Technology’s Center for Neutron Research in Maryland, Paul Scherrer Institute’s Advanced Resonant Spectroscopies beamline in Switzerland, and Oak Ridge National Laboratory’s High Flux Isotope Reactor in Tennessee.

“In the paper, we showed that if the interaction was weak, then even replacing 50 percent of the iron with copper would still not be sufficient to produce the insulating state,” explained Si. “The fact that our experimentalists have managed to turn the system to be Mott insulating therefore provides direct evidence for strong electron-electron interactions in iron pnictides. That is an important step forward because it suggests that superconductivity should be tied up with these strong electron correlations.”

Other co-authors of the study include Andriy Nevidomskyy, Emilia Morosan, Yu Li, Chenglin Zhang and Justin Chen, all from Rice University; Chongde Cao, formerly from Rice University and now belonging to China’s Northwestern Polytechnical University; Chalk River Laboratories’ Zahra Yamani; Qingzhen Huang of the NIST Center for Neutron Research; Hui Wu belonging to the NIST Center for Neutron Research as well as the University of Maryland; Jing Tao and Yimei Zhu, both from Brookhaven National Laboratory; Wei Tian, Songxue Chi and Huibo Cao from Oak Ridge National Laboratory; Marcus Dantz and Thorsten Schmitt from the Paul Scherrer Institute; Yao-Bo Huang belonging to the Paul Scherrer Institute as well as the Chinese Academy of Science’s Beijing National Laboratory for Condensed Matter Physics; and Rong Yu belonging to Renmin University of China as well as to Shanghai Jiao Tong University.

The study was supported by the Department of Energy, the Alexander von Humboldt Foundation, the Welch Foundation, the Swiss National Science Foundation, the National Science Foundation, and the National Science Foundation of China.

RCQM capitalizes on the strengths of over 20 Rice University research teams as well as global partnerships to answer questions pertaining to quantum materials. RCQM is supported by Rice’s offices of the Provost and the Vice Provost for Research, the Brown School of Engineering, the Smalley Institute for Nanoscale Science and Technology, the Wiess School of Natural Sciences, and the departments of Physics and Astronomy, Electrical and Computer Engineering, and Materials Science and NanoEngineering.

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