Understanding the origin of superconductivity in high-temperature copper oxide superconductors

Superconductors are materials that can conduct electricity with zero resistance when cooled to a certain temperature, called the critical temperature. They have applications in many fields, including power grids, maglev trains, and medical imaging. High-temperature superconductors, which have critical temperatures higher than normal superconductors have significant potential for advancing these technologies. However, the mechanisms behind their superconductivity remain unclear.

Copper oxides or cuprates, a class of high-temperature superconductors, exhibit superconductivity when electrons and holes (vacant spaces left behind by electrons) are introduced into their crystal structure through a process called doping. Interestingly, in the low-doped state, with less-than-optimal electrons required for superconductivity, a pseudogap ­­–a partial gap in the electronic structure– opens. This pseudogap is considered a potential factor in the origin of superconductivity in these materials.

Additionally, previous studies have revealed a long-range charge density wave (CDW) order, in the low-doped regime of cuprates that breaks the crystal symmetry of the copper oxide (CuO₂) plane. CDW is a repeating wave-like pattern of electrons that affects the material’s conductivity. This symmetry breaking is significant as superconductivity has been known to arise inside or near symmetry-broken states. Moreover, in the bismuth-based cuprate superconductor, Bi₂Sr_2-xLa_xCuO_6+δ  (Bi2201), it has been shown that strong magnetic fields can induce a long-range symmetry-breaking CDW order. Despite extensive research, the exact role of these phenomena in the occurrence of superconductivity in cuprates is still not known.

In a new study, a team of researchers led by Associate Professor Shinji Kawasaki from the Department of Physics at Okayama University, Japan investigated the origin of high-temperature superconductivity in the pseudogap state of cuprates using a novel approach. Prof. Kawasaki explains, “In this study, we have discovered the existence of a long-range CDW order in the optimally doped Bi2201, induced by tensile-compressive strain applied by a novel piezo-driven uniaxial strain cell, which deliberately breaks crystal symmetry of the CuO₂ plane.” Their findings were published in the journal Nature Communications on June 14, 2024. The team included Ms. Nao Tsukuda and Professor Guo-qing Zheng, also from Okayama University, and Dr. Chengtian Lin from Max-Planck-Institut fur Festkorperforschung, Germany.

The researchers used nuclear magnetic resonance (NMR) technique to observe the changes in the electronic structure of the optimally doped Bi2201 superconductor as uniaxial compressive and tensile strains were applied to the material. The results revealed that when the strain exceeded 0.15%, the material underwent a significant transformation, with the short-range CDW order transitioning into a long-range CDW order. Furthermore, increasing strain suppressed superconductivity while enhancing CDW order, indicating that both superconductivity and long-range CDW can coexist. These results suggest that a hidden long-range CDW order, not limited to the low-doped regime, exists in the pseudogap state of cuprates, which becomes apparent under strain.

“This finding challenges the conventional belief that magnetism is the primary driver in copper oxides and provides valuable insights for constructing theoretical models of superconductivity, “remarks Prof. Kawasaki. Highlighting the potential applications of this study, he adds, “The findings of this study hold immense promise for elucidating the underlying mechanisms of high-temperature superconductivity, paving the way for the development of more practical superconducting materials. High temperature superconductors hold great potential for lossless power transmission and storage, contributing significantly to energy conservation and the pursuit of carbon neutrality. Furthermore, the application of superconductors in MRI technology has the potential to reduce costs and make advanced medical imaging more accessible.”

Overall, this study marks a significant step towards understanding the origin of high-temperature superconductivity, highlighting the importance of uniaxial strain as a valuable tool for understanding superconductivity in other similar superconductors.

About Okayama University, Japan

As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.

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About Associate Professor Shinji Kawasaki from Okayama University, Japan

Shinji Kawasaki is currently an Associate Professor at the Faculty of Science, Department of Physics at Okayama University. He obtained his M.S. and Ph.D. degrees from Osaka University in 2001 and 2004, respectively. He has over 100 publications with over 2000 citations. Before joining Okayama University in 2006, he served as a research fellow of the Japan Society for the Promotion of Science of Japan Science (DC2, PD) and as a specially appointed assistant at the Center of Excellence of Osaka University. His research interests include magnetism, superconductivity, and strongly correlated electron systems.