Electronic signatures of quasiparticles in high-T_c iron-chalcogenide superconductors under pressure

Iron-chalcogenide superconductors of the FeSe family provide an exciting platform to investigate the high-temperature superconductivity that often emerges from intertwined electronic nematic and spin-density wave orders. Tuning parameters, such as applied pressure and chemical pressure [1,2], can be used to explore the phase diagram and establish the characteristics of the normal quasiparticles. In this talk, I will present quantum oscillations studies under high applied hydrostatic pressure in single crystals of FeSe_1-xS_x using magnetotransport and tunnel diode oscillator experiments up to 45T [3,4,5]. I will discuss the evolution of the Fermi surface and the quasiparticle effective masses in the vicinity of the nematic end point as well as inside the high-pressure phase where superconductivity is enhanced. I will compare phase diagrams of different systems and discuss the role of Cu impurity scattering [6,7]. These studies emphasize the robustness of the high-T_c superconducting phase, the role of electronic correlations as well as the characteristics of the anomalous normal electronic phases.