Characterizing a Top Superconductor
The highest-temperature superconductor at ambient pressure is the cuprate Hg1223, which superconducts at 134 K. This material is not as well characterized as the cuprate Bi2223, which cleaves more readily into uniform, easily measurable samples. Last year researchers demonstrated a way to grow high-quality crystals whose composition and structure come close to matching those of Hg1223 [1]. Now Masafumi Horio at the University of Tokyo and his colleagues have analyzed those crystals and found that studies of Bi2223 have missed a detail that contributes to Hg1223’s record superconducting properties [2].
Hg1223 and Bi2223 are both trilayer cuprates, meaning their unit cells include three layers of copper oxide (CuO2) interspersed with, among other elements, mercury (in Hg1223) and bismuth (in Bi2223). The newly synthesized material, (Hg,Re)1223, has the same structure as Hg1223, but some mercury atoms are replaced by rhenium, which helps to stabilize the compound.
A popular technique for investigating superconductors is angle-resolved photoemission spectroscopy (ARPES). This technique can quantify a material’s superconducting gap, which is the binding energy of the electron pairs that carry the supercurrent: In general, the wider the gap, the higher the superconducting temperature. In Bi2223 the inner CuO2 layer has a much wider superconducting gap than the outer layers, suggesting that the inner layer’s properties are key to the material’s high critical temperature of 110 K.
Applying ARPES to (Hg,Re)1223, Horio and colleagues found a crucial difference. The superconducting gap of (Hg,Re)1223’s inner CuO2 layer was similar to that of Bi2223. But the outer layers of (Hg,Re)1223 exhibited a significantly larger gap than those of Bi2223. “It is now clear from our study that superconductivity in the outer plane is not a mere supporter of the inner plane but is itself a significant factor governing superconducting temperature,” Horio says.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- Y. Mino et al., “Single-crystal growth and characterization of cuprate superconductor (Hg,Re)Ba2Ca2Cu3O,” J. Phys. Soc. Jpn. 93, 044707 (2024).
- M. Horio et al., “Enhanced superconducting gap in the outer CuO2 plane of the trilayer cuprate (Hg,Re)Ba2Ca2Cu3O,” Phys. Rev. Lett. 135, 046501 (2025).