Evolution of the low-temperature Fermi surface of superconducting FeSe1−xSx across a nematic phase transition.

The existence of a nematic phase transition in iron-chalcogenide superconductors poses an intriguing question about its impact on superconductivity. To understand the nature of this unique quantum phase transition, it is essential to study how the electronic structure changes across this transition at low temperatures. Here, we investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition of FeSe_1−xS_x using Shubnikov-de Haas oscillations in high magnetic fields up to 45 T in the low temperature regime down to 0.4 K. Most of the Fermi surfaces of FeSe_1−xS_x monotonically increase in size except for a prominent low frequency oscillation associated with a small, but highly mobile band, which disappears at the nematic phase boundary near x ~ 0.17, indicative of a topological Lifshitz transition. The quasiparticle masses are larger inside the nematic phase, indicative of a strongly correlated state, but they become suppressed outside it. The experimentally observed changes in the Fermi surface topology, together with the varying degree of electronic correlations, will change the balance of electronic interactions in the multi-band system FeSe_1−xS_x and promote different k_z-dependent superconducting pairing channels inside and outside the nematic phase. Fe-based superconductors: Characterizing the nematic phase transition New measurements of quantum oscillations in FeSe1-xSx help understanding the nature of the nematic phase transition in Fe-based superconductors. Nematic order — an electronic order that breaks the rotational symmetry of the underlying substrate — is thought to play an important role in the appearance of superconductivity in Fe-based superconductors. A way to investigate the interplay between nematicity and superconductivity is to apply chemical pressure, in this case by substituting Se with S in FeSe samples. Amalia Coldea from the University of Oxford, UK, and colleagues used Shubnikov-de Haas oscillations to characterize the evolution of the Fermi surfaces across the nematic phase transition. They found a strongly correlated state inside the nematic phase and weakening electronic correlations outside it, and evidence for a Lifshitz transition that had not been detected before. [ABSTRACT FROM AUTHOR]