| Čoahkkáigeassu: | <p>This thesis presents the results of resistivity, torque magnetometry and magnetisation ex- periments on two different unconventional high-temperature iron-based superconductors in high magnetic fields. The superconducting phases and normal electronic states are ex- plored. These results provide details about superconductivity in these compounds, and link changes in the superconducting state to those of the electronic state above the supercon- ducting transition.</p>
<p>The first material that will be presented is the high-temperature, stoichiometric, clean and optimally doped iron-based superconductor CaKFe<sub>4</sub>As<sub>4</sub>. Magnetisation experiments on CaKFe<sub>4</sub>As<sub>4</sub> map the vortex phase diagram and reveal exceptional high critical current densities, the largest in the family of iron-based superconductors. The upper critical fields of CaKFe<sub>4</sub>As<sub>4</sub> require a two-band description, and become almost isotropic at low tem- peratures. The large upper critical fields, high critical current densities and strong vortex pinning force in CaKFe4As4, combined with its high transition temperature, metallic prop- erties and stoichiometry, make it an excellent candidate for practical applications.</p>
<p>The second superconductor that will be considered is FeSe, particularly with isoelec- tronic sulphur substitution in FeSe<sub>1−x</sub>S<sub>x</sub>. This system has been of much interest recently. The sulphur substitution suppresses the nematic transition temperature of FeSe, and super- conductivity persists across the entire phase diagram, although the transition temperature is suppressed where nematicity vanishes. These materials offer the opportunity to study the interplay between nematicity and superconductivity where no long-ranged magnetic order is present. This thesis experimentally explores the normal electronic state and supercon- ducting phase of FeSe<sub>1−x</sub>S<sub>x</sub> both inside the nematic state and in the tetragonal phase. The results show significant differences between the nematic and tetragonal phases for both the normal state and superconductivity, which correspond to changes in the Fermi surface. Near the nematic end point an extended region of ρ ∝ T<sup>1.5</sup> resistivity is uncovered. New characteristic signatures of nematicity have been found in FeSe<sub>1−x</sub>S<sub>x</sub> for x ≲ 0.18, with a resistivity power law in magnetic field of ρ ∝ H<sup>1.55</sup>, which returns to the usual H<sup>2</sup> power law in the tetragonal phase. The high-field transverse magnetoresistivity changes from metallic for low x to insulating-like at larger x inside the nematic state, which may arise from the emergence of an extra hole pocket. At low temperatures Fermi liquid behaviour emerges in clean samples, depending on the sulphur composition. The upper critical fields are suppressed across the nematic end point and reveal a two-band nature of supercon- ductivity in FeSe<sub>1−x</sub>S<sub>x</sub>. Inside the nematic state the upper critical fields are dominated by inter-band Cooper pairing, whereas in the tetragonal state they are governed by intra- band scattering, highlighting the differences between superconductivity inside and outside of the nematic state and suggesting a change in Cooper pairing. Other superconducting properties, such as the coherence lengths, anisotropy and gaps, also change dramatically across the nematic end point. Despite these changes and differences, the excess conductiv- ity outside of the superconducting state shows that the superconducting fluctuations remain two-dimensional across the entire phase diagram. Resistive transitions also become far broader near the nematic end point, where the excess conductivity and Ginzburg number are also enhanced. This may suggest that superconducting fluctuations are stronger at the nematic end point.</p>
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