a team of scientists from around the world, including the university of leipzig (germany), university of florida, university of copenhagen (denmark), cornell university, and the u.s. department of energy brookhaven national laboratory (upton, n.y.), has discovered that an orbital-selective pairing could take place and be the reason that certain materials develop the ability to carry electric current with no resistance at high temperatures.

iron-based superconductivity occurs in materials such as iron selenide (fese) that contain crystal
planes made up of a square array of iron (fe) atoms, depicted here. (brookhaven national laboratory)

the research studied insulation copper-based compounds and metallic iron-based compounds to examine why these very different materials share high-temperature superconductivity.

according to a report on the brookhaven lab website, copper oxide compounds act as electrical insulators because electrons in adjacent copper atoms interact strongly but without moving from one place. the article calls it a “quantum mechanical traffic jam” that blocks electrical current. only when some of the electrons are removed to provide holes for movement (after being cooled) then the material acts as a superconductor.

the nucleus of iron atoms exerts less pull on its electrons and therefore electrons in multiple orbits are unpaired and active. the article added, “the alignment of unpaired electrons in multiple orbitals gives simple iron its strong magnetic and metal properties, so it’s easy to see why iron compounds would be good conductors.” without the insulating state that copper contains, researchers were not sure why iron became a superconductor at high temperatures.

using a scanning tunneling microscope at brookhaven lab, researchers were able to measure energy and electron momentum in iron-selenide samples that were cooled to nearly absolute zero. “comparing the measurements with the predicted electronic signatures allowed the scientists to identify which electrons were associated with each orbital,” the article said.

the researchers discovered that almost all of the electrons in cooper pairs in iron selenide came from a lower energy orbital and that the electrons in the outermost orbital showed many of the insulating properties as electrons in copper oxide.

“with this outer-orbital insulating state, the iron compound has all the same requirements for superconductivity that the copper oxides do—a strong magnetic interaction (up/down pairing) of the almost localized electrons, and a metallic state that allows those pairs to move,” the article said. “the big difference is that in iron selenide, these contributions come from different electrons in three separate active orbitals, instead of the single electron in one active orbital in copper.”

the researchers believe that this demonstrates that magnetic, metallic materials with properties like iron but with an orbitally selective arrangement have potential to be high-temperature superconductors.

the research was recently published in science. the abstract stated:

“the superconductor iron selenide (fese) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. but its cooper pairing mechanism has not been determined.

“we used bogoliubov quasiparticle interference imaging to determine the fermi surface geometry of the electronic bands surrounding the γ = (0, 0) and x = (π/a_fe, 0) points of fese and to measure the corresponding superconducting energy gaps.

“we show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other.

“this complex gap configuration reveals the existence of orbital-selective cooper pairing that, in fese, is based preferentially on electrons from the d_yz orbitals of the iron atoms.”