New findings about why FeSe/SrTiO3 is superconducting at so high temperature
Researchers from Uppsala University, have in an international collaboration, made a substantial progress in understanding why the superconductivity is observed at the interface between iron selenide and strontium titanate at a significantly higher temperature than in a pure iron selenide.
The researches could, with the help of scanning transmission electron microscopy, identify particular vibrational modes within the interface between iron selenide (FeSe) and strontium titanate (SrTiO3). In this study, which was published in the scientific journal Nature, resarchers have analyzed the material at the atomic level and saw that these vibrational modes interact very strongly with electrons in the iron selenide layer within the material.
It was known earlier that iron-based superconductors can have relatively high superconducting transition temperature. Though, before 2012 there was no known iron-based superconductor that would have the superconducting transition temperature higher than 55 K. Then researchers have observed that iron selenide, with a thickness of a single unit cell grown on a strontium titanate, which by itself is not a superconductor, reached a transition temperature of over 80 K. This surprising discovery led to intense research efforts within the field.
A structure model of FeSe/SrTiO3-system and simulations of electron scattering. Figure: Jan Rusz.
Usually the superconductivity is related to atomic vibrations and how the electrons inside the material interact with the vibrational modes. Thanks to recent technical developments, it was possible to study the atomic vibrations with a help of the scanning transmission electron microscope in this new study.
In the measurements, the researchers have identified new vibrational modes at the interface between iron selenide and strontion titanate, which do not appear in the original materials. By simulations of vibrational spectra they have concluded that these modes are responsible for the higher superconducting transition temperature in the boundary layer.
"In the simulations we could observe that oxygen atoms at the interface in these new vibrational modes swing back and forth towards the iron selenide layer", says Jan Rusz, associate professor at the Department of Physics and Astronomy.
The Simulation method used in the study was developed by Paul Zeiger and Jan Rusz from Uppsala university.
"Our method made it possible to simulate how electrons scatter on vibrational modes, not only for iron selenide/strontium titanate, but also for other complicated nanostructured materials", says Paul Zeiger, researcher at the Department of Physics and Astronomy.
Measurement have also shown that these new vibrational modes are extremely sensitive to the details of the atomic structure at the interface. If there i.e. are irregularities in the growth of the substrate, the iron selenide can be slightly displaced further from strontium titanate. Even if this displacement is only as small as an extra layer of atoms, the superconducting transition temperature steeply decreases.
"The study increases our understanding of the unexpectedly high transition temperature in the iron selenide/strontium titanate. However, it also contributes to the development of methods for future studies of other superconducting materials, down to atomic scale", says Jan Rusz.
Camilla Thulin
About the study
The study has been conducted in a collaboration with University of California, Irvine (USA), Chinese Academy of Sciences, Beijing (China), Princeton University, Princeton (USA) and Uppsala University.
Article Reference
Yang, H., Zhou, Y., Miao, G. et al. Phonon modes and electron-phonon coupling at the FeSe/SrTiO3 interface. Nature 635, 332-336 (2024). https://doi.org/10.1038/s41586-024-08118-0
Contact
Paul Zeiger, researcher at the Department of Physics and Astronomy, Uppsala university, paul.zeiger@physics.uu.se
Jan Rusz, associate professor at the Department of Physics and Astronomy, Uppsala university, jan.rusz@physics.uu.se