Unconventional superconductivity in iron arsenide

Not lattice vibrations but magnetic factors cause superconductivity

Duck Young Chung examines superconducting crystals © Argonne National Laboratory
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Iron arsenide is a superconductor: electrons flow through it without resistance at certain voltages. But why? Scientists have now shown for the first time that it is not lattice vibrations as in conventional superconductors that are responsible, but magnetic factors, as they report in "Nature".

Normally, electrons repel because of their same charge. But in the phenomenon of superconductivity they interact with the vibrations in the crystal lattice of the conductor and thus overcome the repulsion. They form pairs that continue to flow without resistance even when the voltage is switched off. With such materials, there exists a so-called energy gap between the superconducting and the normal electronic state, which is constant. It is always at the same level of supplied energy. But there are also exceptions, the so-called unconventional superconductors. In these, the energy gap varies with the direction in which the electrons move in the material. In some directions, it can even be zero.

Riddle of the "fixed" energy gap

One of the unconventional superconductors is iron arsenide. From this, researchers have previously assumed that the vibrations of the grid are not sufficient for normal superconductivity, so it must be an unconventional leader. It may be that magnetic factors are more likely than the vibrations that allow the flow of electrons without resistance.

On the other hand, in iron arsenide, the energy gap does not seem to be variable.

Scientists from the Argonne National Laboratory in the US, together with colleagues from other countries, have now studied the strange behavior of iron arsenide using a special method. "The techniques we use to uncover conventional unconventional superconductivity do not work with these compounds, " explains physicist Ray Osborn. "Inelastic Neutron Scattering is the only method that can be used to date." Display

Neutron experiment shows sign reversal

While previous techniques could not prove whether the hole in the arsenide may not change in size, but that it reverses in its sign, this is possible with the neutron method. The researchers discovered that iron arsenide in the superconducting state has a magnetic excitation. However, this can only occur if the energy gaps actually change their sign from one electron orbital to the next.

"Our results indicate that the mechanism that turns electrons into pairs is generated by iron arsenide through antiferromagnetic fluctuations rather than lattice vibrations, " says Rosenkranz. "This gives us proof that this superconductivity is indeed unconventional."

(Argonne National Laboratory, 14.01.2009 - NPO)