The Interplay of Pressure and Doping Effects on BaFe2As2 Superconductors

The study of iron-based superconductors exists at the intersection of some of the most fascinating disciplines in physical sciences, with avenues to reach illuminating conclusions in the study of magnetism, quantum physics, materials science, and applications at the forefront of technical engineering. The youth of the field is apparent, having only been discovered in 2008, with many open questions as to how the many facets of their normal-state and superconducting properties intermingle, and behave under the extreme conditions made imperative by their significant promise of applicability in magnet technologies.

This thesis hones in on the study of BaFe2As2 and the way in which isovalent doping with phosphorous, charge doping with nickel, and the application of intermediate amounts of hydrostatic pressure (1.2 GPa) affect it’s superconducting critical current density (Jc) and superconducting critical temperature (Tc). This avenue of study already provides a multitude of interesting results.

We uncover a significantly orientation dependent sharp peak in the Ni- doping dependent phase diagram, coincident with a large increase in the susceptibility of Jc to applied pressure. Pressure applied at the optimal doping of Ba(Fe0.95Ni0.05)2As2 result in increases of 300 %, drastically up from 50 % Ba(Fe0.95Ni0.048)2As2, exemplifying a behaviour correlated with the posited quantum critical point (QCP) at this doping. The fact that this striking behaviour only manifests in a field oriented perpendicular to the materials ab-plane at ambient pressure, reinforced by the application of Dew-Hughes and strong pinning models, makes a strong case for it being a phenomena dependent on the coexistence of both δTc and δl forms of pinning.

The behaviour of the phosphorous doped material is immediately contrast- ing, with changes in Jc completely correlated with changes in Tc. The behaviour of Tc under applied pressures illuminates a non-linear response at low pressures due to the difference in compressibility of orthogonal lattice parameters along the c and a axes. This effect results in huge suppression of Tc with the optimally doped sample displaying a shift from Tc = 29 to 15.5 K at a rate of 35 K/GPa. Further analysis shows that this may be re- lated to a softening in the structural lattice induced by the P-doping.

This work exemplifies the complex nature of the interacting material and superconducting properties of BaFe2As2, and will prove useful to any future research attempting to optimise BaFe2As2 for future application as well as filling out the body of work necessary to understand the nature of superconductivity in iron-based superconductors.