Revisiting the physics of high-temperature superconductors

Changes in electronic structure induced by reduction annealing Top: Fermi surface (contour surface of electron energy) before reduction annealing (left) and band dispersion in direction of arrow (right). A band gap, that is, an energy region where electrons do not exist, opens due to antiferromagnetism. Bottom: Fermi surface of reduction-annealed sample (left) and band dispersion in direction of arrow (right). The influence of antiferromagnetism was eliminated and the band gap disappeared. © 2016 Masafumi Horio.

A research group led by the University of Tokyo, performing a process that removes oxygen impurities, has demonstrated that high-temperature superconductivity emerges in copper oxides in a broader electron density region and at higher temperatures than previously thought.

The phenomenon of superconductivity, by which electrons flow through a substance without electrical energy being lost as heat energy, finds applications in maglev trains, magnetic resonance imaging devices and the like. Among superconducting materials, a group of copper oxides in particular exhibit superconductivity at high temperatures. However, the nature of superconductivity that these substances demonstrate differs depending on how the superconducting state was generated. Specifically, a different superconducting state can be generated by adding either negatively-charged electrons (“electron doping”) or positively-charged electron holes to an insulating material completely impervious to passing electricity. In particular, oxygen impurities are easily incorporated into the crystal structure of the material when creating a superconducting state by electron doping, stabilizing what is termed an antiferromagnetic state, a kind of magnetism that was thought to prevent the emergence of superconductivity.

Professor Atsushi Fujimori and graduate student Mr. Masafumi Horio at the University of Tokyo, Graduate School of Science, Department of Physics, and their colleagues applied reduction-annealing, a type of heat treatment for removing oxygen impurities, to a high-temperature copper oxide superconductor created by electron doping. The group then directly observed the electron state of the superconductor, and discovered that sufficient reduction-annealing eliminates antiferromagnetism and enables the emergence of a high-temperature superconducting state that is stable over a much wider electron-concentration range and up to a higher temperature than previously reported.

“This finding concerning the influence of antiferromagnetism induced by impurities on the superconducting state challenges the conventional picture of the physics underlying superconductivity, and calls for an experimental and theoretical reexamination of the phenomenon,” says Professor Fujimori. He continues, “Thirty years have passed since the discovery of copper oxide high-temperature superconductors, yet the mechanism by which superconductivity emerges in these materials remains unknown. This outcome will bring about a new approach to the study of the mechanism of high-temperature superconductivity.”

This research was carried out in collaboration with Professor Tadashi Adachi at Sophia University, Professor Koike at Tohoku University, and the High-Energy Accelerator Research Organization (KEK).