Collapse of metallicity and high-Tc superconductivity in the high-pressure phase of FeSe0.89S0.11

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using
transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed
pressure-temperature phase diagrams that indicate that the superconducting critical temperature is
strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal
signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a
putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With
further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature
dependence and the superconducting transition broadens significantly. Eventually, the system fails to
reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly
current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is
very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness.
This high-pressure region could be understood assuming a real-space phase separation caused by
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nearly concomitant electronic and structural instabilities.