Iron and Superconductors

About 100 years ago, scientists discovered materials that could conduct electrons without losing energy to resistance, but, these "superconductors" had to be very cold. The electron-electron repulsion in these low-temperature superconductors was so weak that electrons could overcome it, pair up and move freely.

In 1986, scientists discovered new materials that became superconductors at temperatures above 100K. These high-temperature superconductors were made of layers of copper alloys sandwiched between layers of nonconducting material that were doped with trace amounts of material that could contribute a few extra electrons to the mix. If these materials were not doped with insulating material they did not conduct electricity as the electrons locked themselves at a distance from their neighbours. This locked pattern was named the "Mott localization".

In 2008 a second class of high-temperature superconductors was discovered. These pnictides are iron-based superconductors which are also layered and need to be doped. However, undoped pnictides are not Mott insulators.

Early in 2010, scientists replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This brought the iron atoms a little closer together and further away from the Mott tipping point.

Rice University researchers are now using iron oxychalcogenides which are layered materials like pnictides, but with greater distance between the iron atoms, and this greater distance is enough to push the system into a Mott insulating state.

A better understanding of the behaviour of high-temperature superconductors is essential to future improvements in electric generators, MRI scanners, high-speed trains and other devices.

Reference:
Jian-Xin Zhu, Rong Yu, Hangdong Wang, Liang L. Zhao, M. D. Jones, Jianhui Dai, Elihu Abrahams, E. Morosan, Minghu Fang, and Qimiao Si. Band Narrowing and Mott Localization in Iron Oxychalcogenides La₂O₂Fe₂O(Se,S)₂. Physical Review Letters, 2010; 104 (21): 216405 DOI: 10.1103/PhysRevLett.104.216405