Date of Award
Doctor of Philosophy
Haidong Zhou, Norman Mannella, Steven Johnston
The interplay between charge, structure, magnetism, and orbitals leads to rich physics and exotic cross-coupling in multifunctional materials. Superlattices provide a superb platform to study the complex interactions between different degrees of freedom. In this dissertation, I present a spectroscopic investigation of natural and engineered superlattices including FexTaS2 and (LuFeO3)m/(LuFe2O4)1 under external stimuli of temperature and magnetic field as well as chemical substitution. Studying the phase transitions, symmetry-breaking, and complex interface interactions from a microscopic viewpoint enhances fundamental understanding of coupling mechanism between different order parameters and the exciting properties.
In FexTaS2, we use optical spectroscopies to analyze the electronic properties. Strikingly, Fe intercalation dramatically changes the metallic character, revealing two separate free carrier responses in the Fe monolayer and TaS2 slabs, respectively. Signatures of chirality are deeply embedded in the electronic structures. These include a transition of electron density pattern from triangular to Kagome to honeycomb, a hole to electron pocket crossover at the K-point, and low energy excitations between spin split bands that cross the Fermi surface. These findings advance the understanding of intercalation and symmetry-breaking on the fundamental excitations in metallic chalcogenides, while at the same time, raise important questions about how the embedded metal monolayer affects vibrational properties due to the free-carrier response screened the infrared-active phonons.
To address these issues, we extended this work using Raman scattering spectroscopy to reveal the vibrational properties. We particular focus on the coherent excitations in the Fe monolayer. The results reveal both in- and out-of-plane vibrational excitations at low frequencies in the intercalated Fe monolayer. Extending the measurements to other intercalated chalcogenides such as Cr1/3NbS2 and RbFe(SO4)2 reveals structural-property relations, which confirms the intercalated monolayer excitations are general and intrinsic. Furthermore, the intercalated monolayer excitations have a trend that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. A model for how mass ratio affects the frequencies of the monolayer excitations is developed as well,which excellently fits to our experimental trend. These findings suggest that external stimuli such as pressure and strain may be able to tune these intercalated monolayer excitations.
In the (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices (m= 3, 7, and 9), we combined optical spectroscopy, magnetic circular dichroism, and first-principle calculations to uncover the origin of high temperature magnetism and charge-ordering states in a site-specific manner. Analysis of the dichroic spectra reveals optical hysteresis loops for different Fe sites. The site-specific coercivity vs. temperature curves are extracted from the optical hysteresis, which demonstrates that bulk magnetism derives principally from the LuFe2O4 layers. Magnetism emanating from the LuFe2O4 layer becomes more robust as the (3, 1) to (7, 1) to (9, 1) series progresses - a trend that correlates with increasing Lu-layer distortion. To understand this relationship more deeply, we extract the spectral signature of the interface for the (LuFeO3)m/(LuFe2O4)1 series (m = 3, 7 and 9). While the overall contribution of spin-down channel excitations is persistent over the sequence, enhanced Lu-layer distortion at the interface increases the contribution of the Fe2+ to Fe3+charge-transfer excitation in the spin-up channel. This amplifies LuFe2O4 layer magnetization and pinpoints the role of Fe2+. Key to this discovery is the ability of magneto-optical spectroscopy to provide direct, microscopic, site-specific information about interface magnetism in a two-dimensional material with multiple magnetic centers. Comparison of the theoretically predicted magnetic circular dichroism with the experimental spectrum also establishes the non-polar self-doped structure as the precise charge-ordering arrangement within the LuFe2O4 layer of the (3, 1) superlattice, thus resolving controversy regarding the many different isoenergetic charge states. In addition to introducing a remarkably powerful and versatile spectroscopic decomposition technique for revealing microscopic spin and charge character at the interface of a multiferroic superlattice with many different iron centers in a site-selective manner, this work provides a pathway to link bulk and interface properties in other engineered materials.
Fan, Shiyu, "Spectroscopic properties of ferroic superlattices. " PhD diss., University of Tennessee, 2021.