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

Superconductors are materials that can conduct electricity without any resistance when cooled below a critical temperature. These materials have revolutionary applications in a variety of fields, including electric motors, generators, high-speed maglev trains and magnetic resonance imaging. Among these materials, CuO₂ Superconductors like Bi2212 stand out for their high critical temperature, which exceeds the Bardeen-Cooper-Schriever limit (the theoretical maximum temperature limit for superconductivity). However, the origin of this superconductivity in high-temperature superconductors such as Bi2212 remains one of the most interesting mysteries in physics.

The key to this puzzle lies in two-dimensional CuO₂ The crystal planes of these materials have been studied extensively through various experiments. Optical reflectance measurement analyzes how light of different wavelengths reflects from crystal faces in different directions, and reveals that Bi2212 exhibits obvious optical anisotropy in “two aspects”ab” and”alternating current“Crystalline planes. Optical anisotropy describes how a material’s optical properties change depending on the direction of light traveling through the material. Now, while reflectance measurements provide valuable information, optical “transmittance” studies different wavelengths of light. Measurements of optical anisotropy across crystals of Bi2212 can provide a more direct understanding of bulk properties, but few such studies have been performed before.

To bridge this gap, a Japanese research team led by Professor Toru Asahi, Ph.D., researcher Kenta Nakagawa, Ph.D., and master’s student Keigo Tokita from Waseda University’s School of Science and Engineering, Comprehensive Research Institute, investigated the origin of strong electricity. Rate measurement of optical anisotropy in lead-doped Bi2212 single crystals. Professor Asahi further elaborated: “Achieving room temperature superconductivity has always been a dream, requiring an understanding of the superconducting mechanisms in high-temperature superconductors. Our unique method of using UV-visible light transmission measurements as a probe allowed us to elucidate these mechanisms in Bi2212, bringing us closer to this One goal is one step closer. This research also involved Professor Masaki Fujita from the Institute of Materials, Tohoku University, and was published in Scientific Reports on November 7, 2024.

In previous work, the researchers studied the wavelength dependence of the optical anisotropy of Bi2212 along its “c” crystallographic axis at room temperature using a universal high-precision universal polarimeter. This powerful instrument delivers simultaneous measurements of optical anisotropy markers—linear birefringence (LB) and linear dichroism (LD)—as well as optical activity (OA) and circular dichroism in the UV to visible region Sex (CD). Their early findings revealed significant peaks in the LB and LD spectra, which they hypothesized arise from incommensurate modulations of the Bi2212 crystal structure, characterized by periodic changes that are out of proportion to the usual pattern of its atomic arrangement.

To clarify whether this is indeed the case, the research team studied the optical anisotropy of lead-doped Bi2212 crystals in this study. “Previous studies have shown that partial replacement of Bi by Pb in Bi2212 crystals can suppress asymmetric modulation,” Mr. Tokita explained. To this end, the team used the floating zone method to create Bi2212 single cylindrical crystals with different lead contents. These crystals are then peeled off with water-soluble tape, yielding ultrathin slab samples that allow transmission of ultraviolet and visible light.

Experiments show that the large peaks in the LB and LD spectra decrease significantly with increasing lead content, which is consistent with the suppression of disproportional modulation. This reduction is crucial as it allows for more accurate measurements of OA and CD in future experiments.

Commenting on the findings, Professor Asahi said: “This discovery enables the study of the pseudogap and whether symmetry breaking exists in the superconducting phase, which is a key issue in understanding the mechanism of high-temperature superconductivity. It contributes to the development of new high-temperature superconductors. development.