Photoemission experiments at the Fudan University challenge the unconventional type of high-temperature superconductivity in certain bismuth oxides
Bismuth oxide is found naturally as the mineral bismite and sphaerobismoite. However, it is usually obtained as a by-product of the smelting of copper and lead ores. Bismuthates, a type of bismuth oxide is a very strong oxidizing agent. It reacts with hot water to make bismuth (III) oxide and oxygen. Sodium bismuthate is the most common bismuthate. Bismuthates exhibit high-temperature superconductivity. Such superconductivity in bismuthates is related by several researchers to unconventional superconductors such as cuprates and iron pnictides. Now, a research led by Donglai Feng at Fudan University in China have possibly uncovered the basics of superconductivity in bismuthates. The novel photoemission experiments suggest that bismuthates are not unconventional superconductors as previously thought. However, these bismuthates are conventional Bardeen-Cooper-Schrieffer superconductors that are characterized by superconductivity due to strong coupling between electrons and phonons.
The technique used for the study of superconductors is Angle-Resolved Photoemission Spectroscopy (ARPES). The technique offers a direct measurement of an electronic structure of material by mapping the momenta of electrons emitted by the material when illuminated by UV or x-ray light. Previous experiments of ARPES for bismuthates measurements were unfeasible as crystals with large, clean, flat surfaces were unavailable. The researchers overcame the hurdle by improving the synthesis process for bismuthate crystals. Moreover, the ARPES technique was upgraded to use a small-spot-size light beam that facilitated the researchers to probe crystal domains as small as 50 μm.
The findings revealed a stronger-than-expected electron-phonon coupling in bismuthates. The researchers compared the measured electronic bands with density-functional-theory calculations, which in turn explained that the strong coupling is due to long-range Coulomb interactions between electrons in the material. Such interactions were not considered in previous theoretical experiments. According to the researchers the long-range interactions could assist to predict other conventional superconductors with high critical temperatures. The research was published in the journal Physical Review Letters on September 13, 2018.