Doctoral Dissertations

Date of Award

8-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Janice Musfeldt

Committee Members

Charles Feigerle, Ziling Xue, David Mandrus

Abstract

Multifunctional materials provide a platform to showcase the interplay between charge, structure, and magnetism. Complex oxides, with their delicate balance of energy scales, present a unique opportunity to unravel the cross-coupling between these interactions. Moreover, spin-orbit coupling and broken symmetries give rise to new exotic physical properties in these systems. In this dissertation, I present a spectroscopic investigation of two model complex oxides under external stimuli of temperature, magnetic field, and chemical substitution. Studying these different phase transitions from a microscopic viewpoint enhances fundamental understanding of how phase transitions occur.

Multiferroic magnetoelectrics are interesting materials in that the application of electric (magnetic) fields can change their magnetic (electric) state. We combine high field optical spectroscopy and first principles calculations to analyze the electronic structure of nickel tellurium oxide across the 53 K, 9 T, and 52 T magnetic transitions to reveal that the color properties are sensitive to magnetic order due to field-induced changes in the crystal field environment. We also reveal broadband nonreciprocal effects in nickel tellurium oxide in two different and underexplored symmetry orientations at much higher energies than typically observed.

Infrared spectroscopy is a powerful tool to investigate a wide range of physics problems such as spin-phonon coupling, broken symmetries, the onset of metallicity, and the role of spin-orbit coupling on the lattice. When a magnetic field is iv applied along the easy axis of antiferromagnets, second-order spin-flop transitions are revealed via broken symmetry and large spin-orbit coupling, different than the conventional first-order mechanism. We measure the infrared vibrational properties of nickel tellurium oxide and demonstrate that the phonon anomalies are consistent with a second-order mechanism. Chemical substitution can also induce quantum phase transitions. As an example, we measure the infrared response of several different niobium-substituted europium titanium oxide systems to reveal a semiconducting to metallic state crossover. Together, these findings provide insight on how spin, charge, and the lattice couple. They also explore the fundamental mechanisms of how quantum phase transitions occur in systems with low symmetry and large spin-orbit coupling.

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