Copper-oxide (CuO2) superconductors, like Bi2Sr2CaCu2O8+δ (Bi2212), are known for their remarkably high critical temperatures. Research using optical reflectivity measurements has indicated that Bi2212 displays significant optical anisotropy. However, this characteristic has not been explored using optical transmittance measurements, which can provide more direct information about the material’s bulk properties. Recently, scientists have revealed the reasons behind this optical anisotropy by conducting ultraviolet and visible light transmittance measurements on lead-doped Bi2212 single crystals, facilitating a clearer exploration of its superconductivity mechanisms.
Superconductors are unique materials that allow electricity to flow without any resistance when cooled below a specific critical temperature. These materials are vital in numerous applications, including electric motors, generators, high-speed maglev trains, and magnetic resonance imaging. Among them, CuO2 superconductors such as Bi2212 are particularly noteworthy due to their high critical temperatures, which exceed the Bardeen-Cooper-Schrieffer limit—the theoretical ceiling for superconductivity. Nevertheless, the mechanism behind superconductivity in high-temperature materials like Bi2212 remains one of the great mysteries in physics.
A crucial aspect lies within the two-dimensional CuO2 crystal plane of these superconductors, which has been thoroughly examined through various experiments. Optical reflectivity measurements have demonstrated that Bi2212 shows significant optical anisotropy along its “ab” and “ac” crystal planes. Optical anisotropy refers to the differences in a material’s optical characteristics based on the direction of light passage. While reflectivity measurements have yielded valuable insights, investigating how light transmits through the crystal at various wavelengths via optical transmittance measurements will provide a more direct understanding of the material’s bulk properties. Nonetheless, such research has been infrequent until now.
To fill this gap, a Japanese research team, led by Professor Dr. Toru Asahi, Dr. Kenta Nakagawa, and master’s student Keigo Tokita from Waseda University’s Faculty of Science and Engineering, focused on understanding the source of the pronounced optical anisotropy in lead-doped Bi2212 single crystals through ultraviolet and visible light transmittance measurements. Prof. Dr. Asahi notes, “Achieving superconductivity at room temperature has long been a dream, requiring a deeper understanding of how superconductivity works in high-temperature superconductors. Our distinctive method of using UV-visible light transmission measurements as a probing technique allows us to elucidate these mechanisms in Bi2212, bringing us closer to this objective.” This study, which also included Prof. Dr. Masaki Fujita from Tohoku University’s Institute for Materials Research, was published in Scientific Reports on November 07, 2024.
In earlier research, the team examined the wavelength dependence of Bi2212’s optical anisotropy along its “c” crystal axis at room temperature, using a state-of-the-art universal polarimeter. This advanced instrument enables simultaneous measurements of various optical anisotropy indicators—linear birefringence (LB), linear dichroism (LD), along with optical activity (OA) and circular dichroism (CD) in the ultraviolet-to-visible light spectrum. Their findings indicated notable peaks in the LB and LD spectra, which they suggest originate from incommensurate modulation of Bi2212’s crystal structure, identified by periodic variations that do not align with its typical atomic arrangement.
To verify this theory, the research team delved into the optical anisotropy of lead-doped Bi2212 crystals in their current study. Mr. Tokita explains, “Previous research demonstrated that replacing some Bi with Pb in Bi2212 crystals curtails incommensurate modulation.” To this end, the team produced single cylindrical crystals of Bi2212 with varying lead concentrations utilizing the floating zone method, and subsequently obtained ultrathin plate specimens suitable for ultraviolet and visible light transmission via water-soluble tape exfoliation.
The experimental results showed that the prominent peaks in the LB and LD spectra diminished significantly as lead content increased, confirming the suppression of incommensurate modulation. This reduction is vital as it allows for more precise measurements of OA and CD in future investigations.
Regarding these findings, Prof. Dr. Asahi states, “This discovery paves the way for examining whether symmetry breaking occurs in the pseudo-gap and superconducting phases, a fundamental question in deciphering the mechanism behind high-temperature superconductivity. It also aids in the development of new high-temperature superconductors.”
