In Search of the Kitaev Interactions in Low-Dimensional Yttrium-Doped Calcium Iridates: Ca(5-x)Y(x)Ir3O12

Abstract

The goal of this thesis is to search a potential experimental realization of the low-dimensional Kitaev chain in calcium iridates, satisfying the two criteria proposed by G. Jackeli and G. Khaliullin. These criteria state the necessity for a material with a ground state characterized by J_eff = 1/2 and a 90 deg metal-oxygen-metal (TM-O-TM) bond geometry to facilitate the emergence of bond-dependent Kitaev interactions. Our initial potential candidate, Ca2IrO4, could not be synthesized under any attempted conditions. Structural and magnetic analyses revealed an antiferromagnetic transition consistent with Ca5Ir3O12, a calcium-deficient version of Ca2IrO4, with a mixed valence state of Ir^{4.67+}. This deviation from the ideal pure Ir^{4+} valence state hindered the realization of a pure Kitaev chain. We then proposed the second potential candidate, Ca(5-x)Y(x)Ir3O12. By introducing x=2 (i.e. 40%) yttrium doping, the valence of Ca5Ir3O12 can be tuned from Ir^{4.67+} to Ir^{4+}. The highest achieved doping level, x=1.52(2), resulted in an Ir^{4.16+} oxidation state, very close to the desired pure Ir^{4+} chain. Further structural analysis revealed deviations from the ideal TM-O-TM bond angles necessary for realizing pure Kitaev interactions. Regardless, Ca(5-x)Y(x)Ir3O12 samples exhibit intriguing properties. First, the achieved doping levels exceed those in previously studied doped Ca5Ir3O12. Second, electronic characterizations reveal consistency with a spin-orbit-driven state in all synthesized Ca(5-x)Y(x)Ir3O12 samples. Third, magnetic susceptibility data demonstrate that as the doping level increases to the range of 5-20%, the magnetic transition observed in the parent compound at around 7K completely vanishes. With further doping to 25-30%, a new magnetic transition emerges at around 11-13K. This research opens up additional avenues for further investigation of Ca(5-x)Y(x)Ir3O12, as well as potential candidates for realizing Kitaev interactions.