Iron-based superconductors: Superconductive FeSe multilayer films

08/31/2015

Potassium coating allows researchers to realize superconductive FeSe multilayer films for the first time

Depositing potassium (K) atoms (orange spheres) on top of a multilayer (in this case, bilayer) film of iron selenide (FeSe) on a strontium titanate (SrTiO3) substrate results in superconductivity due to doping of electrons (yellow spheres) in the film.
Depositing potassium (K) atoms (orange spheres) on top of a multilayer (in this case, bilayer) film of iron selenide (FeSe) on a strontium titanate (SrTiO3) substrate results in superconductivity due to doping of electrons (yellow spheres) in the film.

© 2015 Takashi Takahashi

A new way to explore the superconductivity of iron selenide (FeSe) thin films that involves coating them with a layer of potassium has been developed by AIMR researchers. The method has provided insights into what causes superconductivity in FeSe.

FeSe is an intriguing superconductor. In its bulk state, it superconducts at temperatures below 8 kelvin. This onset temperature for superconductivity — known as the critical temperature (Tc) — rockets up to about 65 kelvin when a single atomic layer of FeSe is placed on a substrate of strontium titanate (SrTiO3). However, when one or two layers of FeSe are added to the FeSe monolayer, all traces of superconductivity appear to vanish. Scientists are anxious to discover the cause of this variation because it may provide them with vital clues about how to realize room-temperature superconductors — the ultimate goal of researchers in this field.

Researchers have long had a hunch that charge-carrier doping of FeSe thin films plays a critical role in their superconductivity. But the only way they could test this was to dope a film via the substrate, which supplies only a limited number of carriers.

The team of Tohoku University researchers led by Takashi Takahashi of the AIMR struck on a new way to introduce carriers into FeSe films — depositing a potassium layer on top of the films (see image)1. Using this method, they were able to realize superconducting multilayer FeSe films for the first time. The result demonstrates that superconductivity had not been previously observed because insufficient carriers were doped from the substrate.

“Surprisingly, this simple method had not been tried previously,” explains Takahashi. “Consequently, prior studies had erroneously concluded that multilayer FeSe films are not superconducting.”

The method provides a powerful way for enhancing Tc in ultrathin films of iron-based superconductors. The finding also indicates that the origin of superconductivity in FeSe monolayers is probably solely electronic rather than due to interactions between electrons and vibrations of the crystal lattice of the SrTiO3 substrate, as had previously been suggested.

Furthermore, it is expected to lead to practical applications. “Demonstrating high-Tc superconductivity in atomically thin films represents an important step toward developing next-generation nanoscale superconducting devices,” says Takahashi.

The team plans to investigate the material further. “We suspect that the interface between the FeSe film and the substrate plays a critical role in generating superconductivity,” explains Takahashi, “and so we intend to fabricate FeSe thin films on various substrates and observe the change in electronic structure as well as Tc.”

References

  1. Miyata, Y., Nakayama, K., Sugawara, K., Sato, T. & Takahashi, T. High-temperature superconductivity in potassium-coated multilayer FeSe thin films. Nature Materials 14, 775–779 (2015). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.