Physicists at the National University of Singapore (NUS) have made a significant breakthrough in the study of superconductivity by synthesizing extremely pure superconducting nickel-oxide materials. Their research has challenged previous assumptions about the role of hydrogen in nickel oxide superconductors, redefining its significance in these newly discovered materials. These findings, published in Nature Communications and Physical Review Letters, provide crucial insights into one of the most intriguing phenomena in modern physics.
Superconductivity, first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, is a remarkable state of matter where electrical resistance drops to zero below a certain critical temperature. This property has far-reaching implications for energy-efficient technologies, such as lossless power transmission, powerful superconducting magnets, and quantum computing. However, despite more than a century of research, the underlying mechanism behind superconductivity remains a profound mystery, especially in materials classified as “unconventional superconductors.”
The established Bardeen-Cooper-Schrieffer (BCS) theory, which won the Nobel Prize in 1972, explains superconductivity in many elemental metals and certain hydrogen-based compounds. In this theory, electrons in a material form pairs, known as Cooper pairs, which move through the lattice without scattering, thereby eliminating electrical resistance. While this theory successfully describes conventional superconductors, it fails to account for many complex materials, including copper-based high-temperature superconductors (cuprates) and, more recently, nickel oxides (nickelates).
The discovery of nickel oxide superconductors has sparked intense debate among physicists regarding the mechanisms responsible for their superconducting properties. Some researchers suggested that hydrogen plays a crucial role in the superconductivity of nickel oxides, a claim put forth in a recent study published in Nature. By using secondary ion mass spectrometry, a technique that detects elemental composition, researchers previously argued that hydrogen was present in nickel oxide superconductors and could be influencing their superconducting behavior through mechanisms similar to the BCS theory.
To test this hypothesis, a research team led by Professor Ariando from the Department of Physics at NUS, in collaboration with institutions such as A*STAR (Singapore), the National Institute of Standards and Technology (NIST), Harvard University, the University of Southern California, Arizona State University, and Cornell University (USA), set out to synthesize highly pure nickel oxide superconductors. Their goal was to rigorously examine the presence of hydrogen and its role in superconductivity.
Their findings contradicted the earlier hypothesis. The team demonstrated conclusively that hydrogen was not significantly present in their synthesized pure nickel oxide samples, and, more importantly, that superconductivity in these materials occurred independently of hydrogen. These results strongly challenge the previous study’s conclusions and redirect the scientific discussion towards alternative explanations for the superconducting mechanism in nickelates.
Mr. Lin Er Chow, a Ph.D. student and co-author of the study, emphasized the importance of their discovery, stating, “Surprisingly, hydrogen is not even abundantly present in pure superconducting nickel oxide. This observation suggests that hydrogen does not play an important role in the origin of superconductivity in these materials.”
Professor Ariando further elaborated on the implications of their findings: “These results help steer research efforts toward understanding the fundamental superconducting mechanism of high-temperature unconventional superconductors. By eliminating hydrogen as a key factor, we can now focus on other possible mechanisms that may be at play in these materials.”
The study is particularly significant because it refines the understanding of unconventional superconductors, a category that includes some of the most promising materials for future technological applications. Unlike conventional superconductors, which require extremely low temperatures near absolute zero, many unconventional superconductors operate at relatively higher temperatures, making them more practical for real-world applications.
Nickelates, in particular, are structurally similar to cuprates, the first class of high-temperature superconductors discovered in the 1980s. Cuprates revolutionized the field by demonstrating superconductivity at temperatures much higher than previously thought possible. Scientists hope that nickelates may offer similar breakthroughs, potentially leading to new materials that exhibit superconductivity at even higher temperatures or under more practical conditions.
By ruling out hydrogen’s influence, the NUS-led study refocuses attention on alternative explanations, such as electron correlations, magnetic interactions, or novel pairing mechanisms unique to nickelates. This work lays the foundation for future theoretical and experimental studies aimed at uncovering the true origin of superconductivity in these materials.
Furthermore, the research highlights the importance of material purity in scientific investigations. Even trace amounts of impurities can lead to misleading interpretations of experimental data, as demonstrated by the conflicting conclusions of previous studies. The NUS team’s meticulous approach to synthesizing pure samples ensures that their findings accurately reflect the intrinsic properties of nickel oxide superconductors.
The implications of this research extend beyond fundamental physics. Understanding the mechanism of unconventional superconductivity could pave the way for the development of new superconducting materials with applications in energy-efficient electronics, superconducting power grids, high-performance computing, and quantum technologies.
While the mystery of high-temperature superconductivity remains unsolved, the work of Professor Ariando and his colleagues represents a crucial step forward. By clarifying the role of hydrogen in nickelates, they have refined the scientific approach to studying these fascinating materials and brought researchers closer to unlocking the secrets of superconductivity.
Moving forward, the next steps in this research will involve further exploration of the electronic and magnetic properties of nickelates, as well as the development of new theoretical models that can explain their superconducting behavior. As scientists continue to investigate these questions, the potential for groundbreaking discoveries in superconductivity remains immense.
Ultimately, the quest to fully understand superconductivity is not just an academic endeavor—it has the potential to transform the future of technology and energy systems. By challenging previous assumptions and refining our understanding of these exotic materials, researchers like those at NUS are paving the way for a new era of superconducting science and innovation.
More information: Shengwei Zeng et al, Origin of a Topotactic Reduction Effect for Superconductivity in Infinite-Layer Nickelates, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.066503
Purnima P. Balakrishnan et al, Extensive hydrogen incorporation is not necessary for superconductivity in topotactically reduced nickelates, Nature Communications (2024). DOI: 10.1038/s41467-024-51479-3