Researchers at the University of Oxford and University of Bath have identified an unusal slowing down of the mobilities of the negative charges inside thin flakes of iron-based superconductors. This new effect will help to shed light on the superconducting mechanism of two-dimensional materials and the different routes to enhance their superconducting transition temperatures above liquid nitrogen temperatures.
FeSe, grown in a single layer form on a suitable substrate, superconducts at world record temperatures for a two-dimensional system, above liquid nitrogen temperatures. The remarkable properties of this system and its related family could be implemented as components for tuneable superconducting devices and, with clever chemistry, one could conceive a topological superconductor that could become an important component for quantum computers. In order to design routinely high-temperature superconductors, one needs to assess what are the relevant electronic characteristics and how the charge carriers pair up to form the superconducting condensate.
Interestingly, FeSe in single crystal form superconducts at much lower temperature than the single layer form. In order to address these disparities in superconducting behaviour, scientists explore the transport properties in devices made of thin flakes of crystalline FeSe, by exfoliating single crystals in thinner and thinner layers. They found that the superconductivity gets weaker and weaker as the flakes are thinned down. Additionally, mobilities of the positive and negative charges have very different temperature dependencies between the thin flakes and bulk single crystal. This implies that the negatively charged carriers are severely disturbed on local scale by additional effects introduced by strong orbitally-dependent electronic interactions and scattering via spin fluctuations that could provide the pairing mechanism. As superconductivity is suppressed at the same time as the negative charges become more localized, it suggests that high-temperature superconductivity requires full participation of electron pockets in the superconducting pairing.
Professor Amalia Coldea and her group carried out the work in Oxford which was recently published in the paper Unconventional localization of electrons inside of a nematic electronic phase in PNAS. The thin flakes of FeSe exfoliated from single crystals were prepared in collaboration with Liam Farrar and Simon Bending at the University of Bath. Transport measurements were performed in the Oxford Centre for Applied Superconductivity up to 16T and quantum oscillations studies in the high-magnetic field facility in Nijmegen up to 38T.
Professor Amalia Coldea comments: "This work opens up a new exciting avenue to explore complex unconventional superconductors and quantum materials in thin flake devices in Oxford. The transition from single crystals to devices using the single crystalline flakes is the necessary step for designing the quantum devices of the future."