Pressure dependence of the Fermi surface of the nematic superconductor FeSe$_{1-x}$S$_x$*

Upon application of hydrostatic pressure the phase diagram of bulk FeSe evolves from a nematic phase with low T$_c \approx $ 11K towards a magnetic phase which harbours a high-T$_c$ superconductor with T$_c \approx$ 40K [1, 2]. This complex interplay between different competing orders suggests that superconductivity may be dominated by both nematic and spin-fluctuations that are tuned by applied pressure. Similar to hydrostatic pressure, chemical pressure by sulphur doping suppresses the nematic phase but no magnetic order has been detected yet [3]. Here, we will present quantum oscillation studies of FeSe$_{1-x}$S$_x$ up to 45T under applied hydrostatic pressure and we will follow the evolution of the Fermi surface from the nematic phase towards the high pressure high T$_c$ state. The temperature dependence of the quantum oscillations allows us to determine the quasiparticle masses and to follow the effect of electronic correlations as a function of applied pressure. \\ \, [1] Terashima \textit{et al.}, Phys. Rev. B \textbf{93}, 094505 (2016) \\ \, [2] Medvedev \textit{et al.}, Nat. Mater. \textbf{8}, 630 - 633 (2009) \\ \, [3] Watson \textit{et al.}, Phys. Rev. B \textbf{91}, 155106 (2015)