Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Physics and Astronomy

First Advisor

Dr. Gang Cao

Abstract

Previous studies of iridates have shown that an interplay of strong SOI, Coulomb interaction U, Hund’s rule coupling and crystalline electric fields result in unexpected insulating states with complex magnetic states. The novel Jeff =1/2 insulating state first observed in Sr2IrO4 is a direct consequence of such an intriguing interplay and is one of the central foci of this dissertation study.

The work presented here consists of three projects: (1) Effects of Tb doping on Sr2IrO4 having tetravalent Ir4+(5d5) ions; (2) Emergence of unexpected magnetic states in double-perovskite (Ba1-xSrx)2YIrO6 with pentavalent Ir5+(5d4) ions in the presence of strong SOI, and (3) The coexistence of a charge and magnetic order in a magnetic dimer chain system, Ba5AlIr2O11, which has both tetravalent Ir4+ (5d5) and pentavalent Ir5+ (5d4) ions.

A significant portion of this dissertation will focus on Tb doped Sr2IrO4. A central finding of this work is that slight Tb doping (3%) readily suppresses the antiferromagnetic state but retains the insulating state, indicating an unusual correlation between the magnetic and insulating states as a result of the presence of the strong SOI.

However, SOI is not the only significant phenomenon. The study on the double-perovskite (Ba1-xSrx)2YIrO6 revealed an exotic magnetic ground state, in sharp contrast to the anticipated singlet ground state in the strong SOI limit, raising an urgent question: is SOI as dominant as was initially anticipated in the iridates?

Finally, this study turns to a system containing both Ir4+ and Ir5+ ions, Ba5AlIr2O11. This system features dimer chains of two inequivalent octahedra occupied by tetravalent Ir4+ (5d5) and pentavalent Ir5+ (5d4) ions respectively. Ba5AlIr2O11 undergoes charge and magnetic order transitions at 210 K and 4.5 K, respectively.

SOI-driven physics is a rapidly evolving field with an ever growing list of theoretical proposals which have enjoyed very limited experimental confirmation thus far. This study has revealed a large range of interesting phenomena in the iridates that defy conventional theoretical arguments and that help to fill an experimental void in this field.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2016.488

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