Synthesis, Characterization, and Scattering Measurements of the Perovskite and Infinite Layer Nickelates

Abstract

227 pagesMaterials hosting unconventional and high Tc superconductivity demonstrate phase diagrams of extreme complexity where superconductivity cooperates and competes with other ground states involving entangled charge, lattice, and spin degrees of freedom. Joining the Fe-based and Cu-based families of superconductors, the newly realized infinite layer nickelates, RENiO2, RE=La, Nd, Pr, offer a novel platform to study the nature of high Tc superconductivity and the normal state from which it emerges. However, this class of materials poses significant synthesis challenges which must be overcome to investigate their intrinsic ground state properties. In this thesis we discuss the synthesis of perovskite and infinite layer nickelates by reactive oxide molecular-beam epitaxy and activated hydrogen reduction. First, we examine the growth of the perovskite nickelate NdNiO3 by ozone assisted molecular-beam epitaxy and describe a novel reduction procedure, using atomic hydrogen, for transforming the perovskite to the infinite layer phase. We illustrate the utility of atomic hydrogen reduction for producing highly crystalline undoped NdNiO2 by a variety of experimental probes. Second, we explore the properties of high quality NdNiO3 thin films in the ultrathin limit, revealing a previously undetected thickness driven structural transformation and suppression of the metal-to-insulator transition in films under tensile strain. Finally, we re-examine the presence of charge density wave order in NdNiO2, arguing that the previously reported 3a0 ordering can be attributed to interstitial oxygen ordering rather than a correlation driven density wave. These results serve to clarify some of the essential features of the phase diagrams of both the perovskite and infinite layer nickelates