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.
