tuned by applied hydrostatic pressure*

Bulk FeSe suffers a remarkable four-fold enhancement in its critical temperature under hydrostatic pressure. Furthermore, combined chemical and applied pressure in FeSe_1-xS_x provides important tuning parameters of the phase boundaries between the competing nematic and magnetic phases from which superconductivity emerges. While the isoelectronic substitution leads to the suppression of the nematic order in the vicinity of x~0.170(5), the applied hydrostatic pressure gives additional access to the tetragonal high-pressure phase. We present a quantum oscillations study in magnetic fields up to 45T under applied hydrostatic pressure up to 22kbar and determine the evolution of the Fermi surface and electronic correlations inside the high-Tc tetragonal phase. We detect a monotonous increase in the sizes of the different Fermi surface pockets inside the tetragonal phase, whereas the electronic correlations remain relatively unaffected.