Team makes progress on high-temperature superconductors

US researchers have confirmed some predictions about how iron-based high-temperature superconductors work in what is an important step for any future applications.

The team at Cornell University identified gaps in the energy levels of electrons thought to represent electrons that have paired up with twins from adjacent atoms to form so-called ‘Cooper pairs’ that move through the conductor without interference.

Superconductivity was first discovered in metals cooled to temperatures very near absolute zero. Recently discovered compounds of iron, arsenic and other elements become superconductors at much higher temperatures, offering a possible new route to room-temperature superconductivity.

The key lies in ‘spin’. Just as current rotating in a coil of wire generates a magnetic field, a spinning electron is thought to generate a tiny field, which physicists refer to as spin. It is believed that Cooper pairs form when two electrons with opposite spins join up, analogous to two bar magnets snapping together with their north and south poles meeting. The pair is magnetically neutral, so it can move without being impeded by the magnetic fields of other particles.

Theorists propose that, in iron-based superconductors, electrons pair up only with electrons in adjacent atoms that are at the same energy level, referring to how strongly the electron is bonded to the atom. If that is true, different pairing energy gaps should appear in association with each of the five active electronic energy levels in an iron atom.

Studying crystals of a compound of lithium, iron and arsenic — LiFeAs for short — that becomes a superconductor at 15K (Kelvins or °C above absolute zero), the Cornell researchers found three of the five possible electron bands.

‘There are two more pairing gaps that we should have been able to detect, and we don’t know yet why not,’ said Séamus Davis of Cornell University. ‘But finding these three along with the directionality is enough to strongly support the theory, and the measurements give the theorists numbers to plug in to refine and extend their predictions.’