HIGH PRESSURE DRIVEN EVOLUTION OF CHARGE AND STRUCTURAL ORDER IN NEMATIC SUPERCONDUCTOR, Ba1−xSrxNi2As2

Abstract

The desire for a complete understanding of high temperature unconventional superconductivityhas illustrated a necessity for the study of non-magnetic sources of superconducting enchantment, such as nematically driven fluctuations and charge order fluctuations. BaNi2As2, a non-magnetic counterpart to high Tc superconductor BaFe2As2, shows a six-fold superconducting enhancement neighboring charge and nematic orders, positioning it as an excellent candidate for studying the interactions between charge order, nematic order, and enhanced superconductivity. In this thesis, I will present X-ray diffraction and electrical transport evidence for the development of complex charge order within the system as functions of isovalent chemical substitution via Ba1−xSrxNi2As2 and applied hydrostatic pressure. The discovery of three separate charge orders will be detailed: an incommensurate charge order at Q = 0.28 and two commensurate charge orders at Q = 0.33 and Q = 0.5. X-ray diffraction measurements of the Q = 0.28 charge order will be used to show a strong correlation between it and a previously established nematic order for Ba1−xSrxNi2As2. Applied pressure of BaNi2As2 up to 10.4 GPa will detail the development of all three charge orders and be used to show a correlation between pressure and isovalvent substitution in BaNi2As2. The critical substitution of 71% Sr and the critical pressure of 9 ± 0.5 GPa will be directly compared by X-ray measurements of their lattice parameters, revealing a collapsed tetragonal phase. This phase is shown to be analogous to the collapsed tetragonal phase of the Fe-pnictide superconductors, likely playing a key role seen at the critical substitution and pressure of BaNi2As2.