Copper-oxide (CuO2) superconductors like Bi2Sr2CaCu2O8+δ (Bi2212) are known for their exceptionally high critical temperatures. While optical reflectivity measurements have highlighted the strong optical anisotropy of Bi2212, a deeper understanding of this phenomenon has been limited by a lack of optical transmittance studies. Now, researchers have made strides in uncovering the origins of this optical anisotropy by conducting ultraviolet and visible light transmittance measurements on lead-doped Bi2212 single crystals, offering new insights into its superconductivity mechanisms.

Superconductors, which conduct electricity without resistance at temperatures below a critical threshold, have vast potential applications in technology, including in electric motors, generators, and MRI machines. CuO2 superconductors, like Bi2212, are particularly notable for their high critical temperatures, which exceed the Bardeen-Cooper-Schrieffer limit—the theoretical maximum temperature for superconductivity. However, the exact cause of superconductivity in high-temperature materials such as Bi2212 remains one of the biggest unsolved questions in physics.

A key area of study is the two-dimensional CuO2 crystal plane, which has been extensively analyzed using various experimental methods. Optical reflectivity measurements reveal that Bi2212 exhibits significant optical anisotropy in both its "ab" and "ac" crystal planes. Optical anisotropy refers to how a material's optical properties vary depending on the direction of light passing through it. While reflectivity studies have provided valuable insights, optical transmittance measurements—how light passes through the crystal—can offer a more direct look at bulk properties. Despite their potential, transmittance studies have been rare until now.

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To address this gap, a research team from Waseda University in Japan, led by Professor Dr. Toru Asahi, Researcher Dr. Kenta Nakagawa, and master's student Keigo Tokita, has investigated the origin of the optical anisotropy in lead-doped Bi2212 single crystals using ultraviolet and visible light transmittance measurements. Asahi notes, "Achieving room-temperature superconductivity has long been a dream, requiring a deep understanding of superconducting mechanisms in high-temperature superconductors. Our unique approach using ultraviolet-visible light transmission measurements helps us elucidate these mechanisms in Bi2212, bringing us closer to this goal." The study, involving Prof. Dr. Masaki Fujita from the Institute for Materials Research at Tohoku University, was published in Scientific Reports on November 7, 2024.

In their earlier work, the researchers studied the wavelength dependence of Bi2212's optical anisotropy at room temperature along its "c" crystal axis using a high-accuracy universal polarimeter. This instrument allowed for the simultaneous measurement of linear birefringence (LB), linear dichroism (LD), optical activity (OA), and circular dichroism (CD) across the ultraviolet-to-visible light spectrum. Their findings revealed distinct peaks in the LB and LD spectra, which they attribute to incommensurate modulation in the crystal structure of Bi2212—periodic variations that do not align with the usual atomic arrangement patterns.