Scientists at Brookhaven National Laboratory, a part of the U.S. Department of Energy, have made significant discoveries about the behavior of electrons in a group of superconducting materials based on nickel. Published in two papers in Physical Review X, the research highlights both similarities and differences between these nickel-based superconductors and their well-known counterparts made of copper. By comparing these “high-temperature” superconductors, scientists hope to uncover the essential features responsible for their remarkable ability to conduct electricity without any energy loss as heat.
According to Mark Dean, the leader of the research conducted at Brookhaven Lab's Condensed Matter Physics & Materials Science Department, understanding high-temperature superconductors has been a long-standing challenge. Ever since the discovery of copper-based superconductors, known as cuprates, in the 1980s, scientists have been striving to comprehend their underlying principles.
The appeal of these superconductors lies in their potential for energy-saving applications. Imagine power lines delivering electricity from distant wind and solar farms to homes without any energy loss, or computers and devices operating flawlessly without the need for costly and energy-intensive cooling.
However, a major hurdle with cuprate superconductors is their reliance on extremely low temperatures to function—well below zero degrees Fahrenheit. Unraveling the mechanisms that enable electrons in these materials to overcome their natural repulsion and flow without resistance could potentially pave the way for superconductors that operate under more practical conditions.
“These materials also serve as a testing ground for understanding other quantum materials where electrons interact very strongly,” noted Steven Johnston, a co-author of the paper and a theorist at the University of Tennessee. “One could argue that this is the most crucial unresolved problem in the field of materials physics.”
Nickel analogs
In their pursuit of unraveling the mysteries of cuprate superconductors, scientists have sought out comparable compounds to study and analyze for insights into enhancing their properties.
Yao Shen, a postdoctoral researcher at Brookhaven National Laboratory and the first author of the published papers, explained that by making slight adjustments to these compounds, it might be possible to achieve higher superconducting transition temperatures or develop materials that utilize more cost-effective elements for practical applications.
Nickel, being a neighboring element to copper on the periodic table, presented itself as a logical choice. The similarity between these transition metals implied that compounds composed of nickel might exhibit similar behaviors to cuprates, while also highlighting key distinctions that contribute to superconductivity.
However, even prior to the successful creation of a nickel-based superconductor by scientists at Stanford University in 2019, there were doubts about whether nickel compounds could be truly considered analogs to cuprates. Once the nickelates were synthesized, the investigation into their properties commenced, marking the beginning of the quest for answers.
‘Seeing' electronic behavior
The research conducted by the scientists involved the utilization of X-rays at the National Synchrotron Light Source II (NSLS-II), located at Brookhaven National Laboratory. This facility, supported by the DOE Office of Science, offers the capability to investigate the microscopic structure, chemistry, and various properties of diverse materials. Specifically, the team employed the Soft Inelastic X-Ray (SIX) beamline, overseen by Valentina Bisogni and Jonathan Pelliciari, who collaborated on the study. This beamline enabled the researchers to compare the electronic characteristics of a layered nickelate superconductor (La4Ni3O8) with those of a well-known cuprate (La2−xSrxCuO4).
The primary objective was to identify the contributing electrons from each element in both compounds that are responsible for superconductivity and other electronic properties, including the presence of a “charge-density wave.” This orderly arrangement of electrons may play a crucial role in generating the superconducting behavior of the material.
Michael Norman, a scientist from Argonne National Laboratory who collaborated on the study, explained that there is evidence suggesting that superconductivity in cuprates is associated with intense magnetic interactions among the copper ions. Hence, in addition to comparing the electrons involved in superconductivity in the two materials, the scientists also aimed to uncover indications of magnetic interactions among the nickel ions in the nickelates. Furthermore, they sought to understand which elements contribute electrons to the formation of both charge and magnetic density waves in these materials.
The SIX beamline, renowned for its exceptional energy resolution, enabled the scientists to observe these intricate details on a subatomic scale. By precisely tuning the X-ray energy to match the individual elements within the sample using resonant inelastic X-ray scattering (RIXS), they could discern the electronic properties of specific elements. Mark Dean emphasized the significance of this approach, stating that it provided a comprehensive understanding of the electronic workings of these materials, supported by theoretical calculations.
Key similarities and differences
The research revealed significant similarities between nickelate and cuprate superconductors, but also highlighted some distinctions. Both types of materials demonstrated the involvement of the transition metal (copper or nickel) and oxygen in their electronic properties. However, the magnetic interactions among nickel atoms, mediated by intervening oxygens, were found to be slightly weaker compared to the oxygen-mediated magnetic interactions among copper atoms in cuprates.
Yao Shen pointed out that cuprates exhibit a well-aligned energy relationship between copper and oxygen, which accounts for their strong magnetism. Similarly, the nickel compounds display a similar alignment, albeit to a somewhat lesser degree.
One key difference emerged in the electronic properties related to the development of charge order, specifically the charge density wave, in the two classes of superconductors. The charge density wave in nickelates was found to be considerably more complex than that observed in cuprates, originating from the combined interactions of all the different elements present in the material.
These findings indicate the potential of nickel compounds in advancing the understanding of cuprate behavior. They also suggest possible strategies for modifying nickel compounds to exhibit stronger magnetism or enhanced superconductivity, making them more akin to cuprates.
The utilization of X-rays, particularly the capabilities offered by NSLS-II, proved instrumental in investigating these complex phenomena. The advanced RIXS instruments available at NSLS-II facilitated a rapid exploration of the underlying physics—a feat that would have been challenging without these cutting-edge tools, as highlighted by collaborator Matteo Mitrano from Harvard University.
Future research endeavors will involve investigating the contributions of rare-earth elements, such as lanthanum and strontium, to the properties of these materials. The role of the rare-earth layer, which is considered electronically inactive in cuprates, remains an open question in nickel-based materials, as emphasized by Mark Dean.
Source: Brookhaven National Laboratory