Doctor of Philosophy, The Ohio State University, 2019, Physics

In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material's paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaF (open full item for complete abstract)

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2018, Physics

Graphene, a two-dimensional carbon honeycomb lattice, has generated immense interest within the condensed matter physics community due to its fascinating electrical, optical, and mechanical properties. The differences in band structure of mono-, bi- and tri-layer graphene give rise to drastically different electronic ground state configurations and competing symmetries (such as spin, valley, orbital, and layer). Recent efforts have significantly improved electronic its charge carrier mobility and enabled the observation of a number of exciting phenomena in monolayer and few layer graphene. In this thesis we present an experimental study of spin transport through monolayer graphene antiferromagnet insulator (AFMI), and quantum Hall (QH) phases in multiple Dirac band trilayer graphene, which provide further insight into both single-particle and many-body physics in these exciting two-dimensional (2D) systems. These projects require samples of exceptional quality. To this end, I, together with Nathaniel Gillgren, developed a dry transfer technique (first pioneered by the Columbia group), in order to fabricate graphene devices encapsulated within hexagonal boron nitride (hBN) layers. Since hBN sheets are atomically flat and host very few trapped charges and defects, they are ideal substrates for graphene devices, which boost charge carrier mobility as high as ˜ 10^5. In my research these ultraclean devices enabled the resolution of symmetry-broken quantum Hall phases and fractional quantum Hall states, as well as the establishment of an antiferromagnetic insulator the affords long distance spin transport. In the first part of thesis, we focus on the observation of tunable symmetries of the integer and fractional quantum Hall (QH) states in ABA-stacked trilayer graphene, which hosts multiple Dirac bands. At finite doping and in the quantum Hall regime, we use transport measurements to map the Landau levels of hBN-encapsulated ABA-stacked trilayer graphene as a functi (open full item for complete abstract)

Committee: Chun Ning Lau (Advisor); Marc Bockrath (Committee Member); Yuan-Ming Lu (Committee Member); Brian Winer (Committee Member); Jeffrey Chalmers J. (Committee Member) Subjects: Condensed Matter Physics; Experiments; Physics

Doctor of Philosophy (PhD), Ohio University, 2006, Physics (Arts and Sciences)

Magnetic nitrides and magnetic heterostructures have attracted considerable attention due to their potential importance to magnetic sensor technology. This project investigates the growth, electronic, magnetic properties of magnetic manganese nitride and manganese nitride based magnetic heterostructures. A series of magnetic materials have been studied. It is composed of three types of magnetic systems: magnetic nitride-manganese nitride, ferromagnetic metal on antiferromagnetic nitride - Fe/Mn3N2; and ferri-magnetic nitride on nitride semiconductor – Mn4N/GaN. These magnetic materials have been grown by molecular beam epitaxy and investigated with various analytical tools. Reflection high energy electron diffraction and X-ray diffraction are used to determine the structures of these materials and their epitaxial relationship with the substrates. Scanning tunneling microscopy is used to study the phase transformation of a thin manganese nitride film, the well-ordered array of MnN-bonded Mn-tetramer clusters on Mn3N2 (001) surface, the step structures and morphology of Mn3N2 (010) surface and Fe thin films on Mn3N2 (010). The row-wise antiferromagnetic Mn3N2 (010) surface has been investigated using spin-polarized scanning tunneling microscopy. It is shown that the magnetic contrast in atomic-scale images is a strong function of the bias voltage around the Fermi level. Atomic force microscopy is also used to investigate the morphology of Fe films on Mn3N2 (010) and Mn4N thin films on GaN(0001).

Committee: Arthur Smith (Advisor) Subjects: