Fermi surface and upper critical field of FeSe under applied pressure

Electronic nematic and spin-density wave phases of FeSe family of iron-chalcogenides superconductors can be intertwined and difficult to assess their relevance on superconductivity. A tuning parameter like applied pressure, which is a versatile clean tuning parameter, can be used to explore their relative importance. Here, we report a tunnel diode oscillator study of FeSe under pressure using both piston cell and diamond anvil cells up to 47 kbar. Using this contactless transport and thermodynamic measurement, we can detect quantum oscillations that change significantly their character outside the nematic phase and we determine the upper critical field behaviour as a function of applied pressure. The nature of this high-pressure electronic phase will be discussed.