Towards room-temperature superconductivity: Insights into optical properties of bi-based copper-oxide superconductors

Superconductors are materials which conduct electricity without any resistance when cooled down below a critical temperature. These materials have transformative applications in various fields, including electric motors, generators, high-speed maglev trains, and magnetic resonance imaging. Among these materials, CuO₂ superconductors like Bi2212 stand out due to their high critical temperatures that surpass the Bardeen–Cooper–Schrieffer limit, a theoretical maximum temperature limit for superconductivity. However, the origin of this superconductivity in high-temperature superconductors, such as Bi2212, remains one of physics’ most intriguing mysteries.

A key piece of the puzzle lies in the two-dimensional CuO₂ crystal plane of these materials, which has been extensively studied using various experiments. Measurements of optical reflectivity, which analyze how light of varying wavelengths reflects off the crystal plane from different directions and reveal that Bi2212 displays pronounced optical anisotropy in both its "ab" and "ac" crystal planes. Optical anisotropy describes the variation in a material's optical properties based on the direction in which light travels through it. Now, while reflectivity measurements have provided valuable information, studying how light passes through a crystal at different wavelengths via optical “transmittance” measurements of the optical anisotropy of Bi2212 can offer more direct insights into bulk properties. However, such studies have been rarely conducted before.

To bridge this gap, a Japanese research team, led by Professor Dr. Toru Asahi, Researcher Dr. Kenta Nakagawa, and master’s student Keigo Tokita from the Faculty of Science and Engineering, Comprehensive Research Organization at Waseda University, investigated the origin of the strong optical anisotropy of lead-doped Bi2212 single crystals using ultraviolet and visible light transmittance measurements. Elaborating further, Prof. Dr. Asahi shares that, “Achieving room-temperature superconductivity has long been a dream, requiring an understanding of superconducting mechanisms in high-temperature superconductors. Our unique approach of using ultraviolet-visible light transmission measurements as a probe enables us to elucidate these mechanisms in Bi2212, taking us one step closer to this goal.” The study, also involving Prof. Dr. Masaki Fujita from the Institute for Materials Research at Tohoku University, was published in Scientific Reports on November 07, 2024.

In their previous work, the researchers studied the wavelength dependence of Bi2212’s optical anisotropy at room temperature along its “c” crystal axis, using a generalized high-accuracy universal polarimeter. This powerful instrument allows simultaneous transmission measurements of optical anisotropy markers—linear birefringence (LB) and linear dichroism (LD)—along with optical activity (OA) and circular dichroism (CD) in the ultraviolet-to-visible light region. Their earlier findings revealed significant peaks in the LB and LD spectra, which they hypothesize to be coming from incommensurate modulation of Bi2212’s crystal structure, characterized by periodic variations that are not commensurate with the usual pattern of its atomic arrangements.

To clarify whether this is indeed the case, the team investigated the optical anisotropy of lead-doped Bi2212 crystals in this study. “Previous studies have shown that the partial substitution of Bi by Pb in Bi2212 crystals suppresses incommensurate modulation,” explains Mr. Tokita. To this end, the team fabricated single cylindrical crystals of Bi2212 with varying lead content using the floating zone method. Ultrathin plate specimens, which allow the transmission of ultraviolet and visible light, were then obtained from these crystals by exfoliation with water-soluble tape.

The experiments revealed that the large peaks in the LB and LD spectra reduced considerably with increasing lead content, consistent with the suppression of incommensurate modulation. This reduction is crucial as it allows for more accurate measurement of OA and CD in future experiments.

Commenting on these findings, Prof. Dr. Asahi remarks, “This finding enables investigation into the presence or absence of symmetry breaking in the pseudo-gap and superconducting phases, a critical issue in understanding the mechanism of high-temperature superconductivity. It contributes to the development of new high-temperature superconductors.”

This study marks a crucial step in the quest for room-temperature superconductivity, a breakthrough that could revolutionize technologies ranging from energy transmission to medical imaging and transportation.

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Reference

DOI: 10.1038/s41598-024-78208-6

Authors: Keigo Tokita¹, Kenta Nakagawa², Kun Zhang¹, Komei Okano¹, Masataka Matsumoto³, Takuya Nakanishi², Masaki Fujita⁴, and Toru Asahi¹

Affiliations                         

¹Faculty of Science and Engineering, Waseda University, Japan

²Comprehensive Research Organization, Waseda University, Japan

³Wilczek Quantum Center, School of Physics and Astronomy, Shanghai Jiao Tong University, China

⁴Institute for Materials Research, Tohoku University, Japan

About Waseda University
Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015. 

To learn more about Waseda University, visit https://www.waseda.jp/top/en

About Professor Toru Asahi
Toru Asahi is currently a Professor at the Faculty of Science and Engineering at Waseda University. He graduated from the Department of Applied Physics, School of Science and Engineering, Waseda University in 1986, obtaining his Ph.D. (Doctor of Science) in 1992, and MBA in 2007. He is also the Dean of the School (Graduate) school of Advanced Science and Engineering at Waseda University, Director of the Global Consolidated Research Institute for Science Wisdom at Waseda University and the Vice-Chairperson of the Research Organization for Nano & Life Innovation. He has published over 280 articles that have received over 4,500 citations. His research interests include chiral science, biophysical sciences, crystal optics, functional thin films, broken symmetry, and cyclical food production systems.

About Assistant Professor Kenta Nakagawa
Kenta Nakagawa is currently an Assistant Professor and a researcher at the Global Consolidated Research Institute for Science Wisdom, Comprehensive Research Organization at Waseda University. He is also the group leader at the ERATO Yamauchi Material Space Tectonics Project. He is the recipient of the 19^th Osawa Prize of The Society of Fullerene, Nanotubes, and Graphene. He has published over 25 articles that have received over 120 citations. His research interests include electric and electronic materials, analytical chemistry, and fundamental physical chemistry, among others.

About Keigo Tokita
Keigo Tokita is currently a master’s student at the Faculty of Science and Engineering at Waseda University. He is affiliated with the Asahi Lab, led by Professor Toru Asahi.