High pressure studies of iron-based superconductors

Unconventional superconductors are promising systems that can be used in practical applications, due to their large critical temperature, critical current densities and upper critical fields. This thesis investigates the electronic and superconducting behaviour of a new family of unconventional iron-chalcogenide superconductors tuned by applied hydrostatic pressure. I will present transport measurements, at low temperatures and high magnetic fields of different single crystals of the FeSe family, to understand their complex electronic phase diagrams under pressure. The parent compound shows an unexpected fourfold increase in superconductivity under pressure, whereas as a monolayer on a substrate it displays superconductivity above the liquid nitrogen temperature. Furthermore, bulk FeSe has an unusual electronic nematic phase, but applied pressure stabilises an additional magnetic order which manifests as an upturn in resistivity.
Firstly, I report the effect of impurity scattering on the electronic behaviour induced by copper substitution in the conducting iron planes. This substitution is strongly disruptive to the nematic and superconducting phases which are quickly suppressed. The enhanced impurity scattering leads to a marked increase in resistivity and strongly reduced charge carrier mobilities, as determined from magnetotransport measurements. As the amount of impurities increases, both the suppression of critical temperatures and the temperature dependence of the upper critical field are consistent with a superconductor having a sign changing order parameter. In a subsequent study of Fe1−xCuxSe under pressure I find that, even in the presence of a small amount of Cu impurity, the high-Tc high pressure superconducting phase is robust. Surprisingly, no signatures of the pressure stabilised magnetic order are found in zero field, only in strong magnetic fields.
In the next chapters, I investigate the phase diagrams of systems with isoelectronic substitution of sulphur for selenium outside of the iron planes. The first system, FeSe0.82S0.18, explores the electronic structure in the absence of nematicity, which displays a threefold enhancement in the superconducting transition temperature in the high pressure phase. Quantum oscillations in high magnetic fields and simulations probe the expansion and topology of the large Fermi surface under pressure. Despite the enhanced critical temperature under pressure, the quasiparticle effective masses are almost pressure independent. Notably, the quantum oscillations of a different system, FeSe0.96Se0.04, only display small frequencies in the high pressure phase. I find its phase diagram shows no signature of magnetic order outside of the nematic phase in zero field, but upturns in resistivity are observed inside the nematic phase in zero field and at high pressures in strong magnetic fields. Overall, the high pressure phase of all systems investigated display an enhanced superconducting phase, and highlight the sensitivity of the magnetic phase to chemical substitution and magnetic fields.

Keywords: iron-based
transport
quantum materials
FeSe
FeSe-based
superconductors
pressure
Advisor: Coldea, A