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Thesis advisor: Snapper / Thesis advisor: Hoveyda / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Ziqiang . Wang / Thesis advisor: Jiadong . Zang / Magnetism is an old field in condensed matter physics, but it is still vibrant and full of excitement. Regardless of deep fundamental physics therein, it also has broad application in engineering technology, modern hard disk drive as an example. Magnetic skyrmion, a vortex-like structure in two-dimensional magnetic systems, has been discovered in various magnetic materials, among which chiral magnets are a family of candidates. The skyrmions are characterized by nonzero topological charges. The vortex-like structure of skyrmions makes skyrmion materials good candidates of new generation of data storage device. So understanding the transport properties of the skyrmion materials is important for the possible application in the future. The Hall effect is a key aspect of electron transports. The topological Hall effect, which is one component in the total Hall effect, only depends on the magnetic structures, and the topological Hall conductivity is proportional to the topological charge. It thus serves as the transport signature of magnetic skyrmions. The major mission of this thesis is to investigate the topological charge distribution in realistic models and uncover the relationship between the existence of skyrmions and other chiral excitations. The organization of the thesis is the following. The first chapter is the introduction. A historical survey about magnetic skyrmions and chiral magnets is presented firstly. The magnetic skyrmion is identified by the topological charge. Further, the relationship between the topological hall effect and topological charge is described by the emergent electrodynamics. The importance of the topological charge in chiral magnets is explained in this part. Following the importance of the topological charge, the investigation of topological charge in two-dimensional chiral magnets is presented in the second chapter. The Monte Carlo simulation is employed to calculate the topological charge on a square lattice. The results show that the nonzero topological charge is not necessarily correlated to the existence of skyrmions in chiral magnets. To understand the numerical results, simple analysis based on the physical picture of a triangle on the square lattice is performed. Then we calculate the topological charge in continuum model of chiral magnets. At the high temperature limit, the numerical results, picture analysis and the analytic result are consistent. Then, in this chapter, there is a description of the recent experimental work on thin film SrRuO3 which confirmed our theoretical prediction. A discussion on spin chirality, topological charge and Hall conductivity is presented in the end. However, no experiment on chiral magnets has been on a perfect monolayer system. So we extend the investigation of topological charge into three-dimensional situation. This work is introduced in the third chapter. The Monte Carlo simulation and the analytical calculation are presented firstly. A special issue in three-dimensional chiral magnets is the thickness dependence. The Monte Carlo simulation is used to address this issue. A combination of analytical calculation and physical picture of magnons is used to explain the numerical results well. Similar as the second chapter, the experiment on finite thickness SrRuO3 is described. Because the effective Dzyaloshinskii—Moriya interaction is due to the interface effect which cannot be used to judge our numerical results based on homogenous chiral magnets. The Heisenberg interaction in the system described in the previous two chapters is ferromagnetic interaction. More physical results with antiferromagnetic interaction are expected in different magnetic system. In the fourth chapter, a review of the work on a frustrated magnet with hexagonal lattice is introduced. The direction of the DM interaction of the hexagonal lattice is perpendicular to the bonds of nearest magnetic atoms. The topological charge is calculated numerically. A similar thermally driven topology as found in chiral magnets is achieved by investigating the topological charge. Following that, the system with staggered DM interaction is discussed. The study of the topological charge in this system not only gives the evolution of thermally driven topology of the system, but also distinguishes the topological charge and spin chirality based on the antiferromagnetic interaction. Not only thermally driven topology in chiral magnets but also the driven motion of skyrmions are interesting to us. Inspired by the similarity of the vortex state in the Type-II superconductor and skyrmion crystal phase, we investigate the proximity effect between the skyrmion material and non-centrosymmetric s-wave superconductor. The method is to calculate the effective interaction between the Cooper pairs and skyrmions. A field-theoretical approach is employed to this end. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Submitted to the faculty of the University Graduate School in partial fulfillment of the requirements for the degree of Master of Science in the School of Informatics Indiana Univeristy / A QSAR using Comparative Molecular Field Analysis (CoMFA) is developed for a set of 23 catalysts containing bis-oxazoline or phosphino-oxazoline ligands that are known to induce asymmetry during the Diels-Alder reaction of N-2-alkenoyl-1, 3-oxazolidine-2-one with cyclopentadiene. It is shown that extemely high q2 statistics can be derived using standard modeling protocols when internal validation alone is done as well as when an external test set is used. From these models it is shown that approximately 70% of the variance in the observed enantiomeric excess can be attributed to the steric field and the remainder of the variance to the electrostatic field. Suggestions about how to improve the performance of inefficient catalysts are given the thesis.

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Direct chiral separations of the non-steroidal drugs of 2-methylarylpropionic acids (profens) on the chiral stationary phases (CSPs) of amylose tris(3,5-dimethylphenyl-carbamate), Chiralpak AD, and cellulose tris(3,5-dimethylphenylcarbamate), Chiralcel OD, were investigated. Chiralpak AD and Chiralcel OD are CSPs coated on silica gel and have the same type of constituents. However, they have different higher order structures arising from their different arrangements of the glucose units, i.e., the former has an a-(1,4)-D-glucose linkage and the latter has a b-(1,4)-D-glucose linkage. The orders of optimum enantioselectivity of racemic acids were reversed on the two CSPs which demonstrated that the enantioseparating abilities of these CSPs are complementary. This phenomenon also confirmed that the chiral recognition abilities of both CSPs were dependent on their higher order structures. Mechanisms for retention and chiral recognition for the separation of racemic 2-methylarylpropionic acids on Chiralpak AD and Chiralcel OD were explored. In depth studies of the dependence of retention and enantioselectivity on temperature and mobile phase compositions were made. The thermodynamic parameters, the differences in free energy, enthalpy, and entropy of association between enantiomers and the CSP were evaluated. The results indicated that the retention of racemic acids on both CSPs is mainly dependent on the hydrogen bonding interaction between the acid proton of the carboxyl moiety of the analyte and the carbonyl oxygen of the carbamate moiety of the CSP. The chiral recognition mechanism for Chiralpak AD involves: (1) the formation of transient diastereomeric analyte-CSP complexes through hydrogen bonding interactions between the carboxyl and the carbamate moieties of the acid and CSP, respectively; (2) stabilization of these complexes by insertion of the aromatic portion of the analytes into the chiral cavities of the CSP, as well as pi-pi, dipole-dipole, and additional hydrogen bonding interactions between analyte and CSP; and (3) chiral discrimination between enantiomer analytes arising from the additional hydrogen bond between analyte and CSP. For Chiralcel OD, the chiral recognition mechanisms involve: (1) the formation of transient diastereomeric analyte-CSP complexes through hydrogen bonding interactions between the carboxyl and the carbamate moieties of the acid and CSP, respectively; (2) stabilization of these complexes by insertion of the aromatic portion of the analytes into the chiral cavities of the CSP, as well as pi-pi and dipole-dipole interactions between analyte and CSP; and (3) chiral discrimination due to: (a) the difference in the steric fit of enantiomers into the chiral cavity of the CSP (entropy controlled); and (b) dipole-dipole or p-p interactions between enantiomer analytes and CSP (enthalpy controlled). Chromatographic and quantitative thermodynamic data showed that there are at least two different chiral recognition mechanisms for Chiralcel OD. One mechanism was characterized by negative values for the enthalpy and entropy differences of the association between enantiomers and CSP that classifies the enantioseparation to be enthalpy controlled. This behavior was exhibited by racemic 2-methylarylpropionic acids with fused rings that were favorably separated at low temperatures. The other mechanism was associated with positive values for the enthalpy and entropy differences of the association between enantiomers and CSP, and the enantioseparation is said to be entropy controlled. The analytes with "free" phenyl moieties favored high temperatures for their enantioseparations. Both studies on the effects of temperature and mobile phase composition also indicated that the higher order structures of CSPs influence their chiral recognition abilities. / Ph. D.

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As natural chiral materials demonstrate limited circularly dichroic contrasts, enhancement of these polarization dependent signals has long been the focus of chiral metamaterial research. By manipulating the geometric chirality of resonant plasmonic nanostructures, we are capable of enhancing light confinement to amplify chiral modified, nonlinear signals from quantum emitters. The metamaterial demonstrates a linear transmission contrast of 0.5 between left and right circular polarizations and a 20× contrast between second harmonic responses from the two incident polarizations. Nonlinear and linear response images probed with circularly polarized lights show strongly defined contrast. As a second set of experimentation, the chiral center of the metamaterial is opened, providing direct access to place emitters to occupy the most light-confining and chirally sensitive regions. The resulting two-photon emission profiles from circularly polarized excitation displays mirrored symmetry for the two enantiomer structures. The efficiency of the nonlinear signal directly correlates to the chiral resonance of the linear regime. The nonlinear emission signal is enhanced by 40× that of the emitters not embedded in the metamaterial and displays a 3× contrast for the opposite circular polarization. Such manipulations of nonlinear signals with metamaterials open pathways for diverse applications where chiral selective signals are monitored, processed, and analyzed.

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