Second Harmonic Generation Imaging of a Magnetic Topological Insulator

A topological insulator (TI) is a type of quantum material that is insulating in the bulk but metallic on the surface. Due to the unique spin properties of the surface electrons, TIs have attracted much interest for their potential applications in spin-based electronics and quantum computers. Even more exotic effects occur when TIs are brought in contact with magnetic materials. This thesis represents a study of two-layer thin films consisting of the TI Bi2Se3 and the magnetic insulator EuS using a nonlinear optical technique called second harmonic generation (SHG) imaging. SHG imaging can probe the crystal and magnetic structure at the surfaces and interfaces of inversion symmetric materials. At the interface between the EuS and Bi2Se3 layers, magnetic domains are expected to form. The domain boundaries are predicted to host chiral edge states, which are dissipationless currents that flow in one direction around a domain. Using SHG imaging, our goal was to visualize the magnetic domains in this magnetic topological insulator system. Even though we have yet to observe any evidence of magnetic domains in this material, we also performed SHG imaging on monolayer MoS2 and were able to visualize distinct crystal grains. Our SHG imaging setup that was improved upon during this thesis has the potential to reveal information about other interesting quantum materials

Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy

Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl

Structural investigation of the bilayer iridate Sr_3Ir_2O_7

A complete structural solution of the bilayer iridate compound Sr_3Ir_2O_7 presently remains outstanding. Previously reported structures for this compound vary and all fail to explain weak structural violations observed in neutron scattering measurements as well as the presence of a net ferromagnetic moment in the basal plane. In this paper, we present single crystal neutron diffraction and rotational anisotropy second harmonic generation measurements unveiling a lower, monoclinic symmetry inherent to Sr_3Ir_2O_7. Combined with density functional theory, our measurements identify the correct structural space group as No. 15 (C2/c) and provide clarity regarding the local symmetry of Ir^(4+) cations within this spin-orbit Mott material

Z2_2 topology and superconductivity from symmetry lowering of a 3D Dirac Metal Au2_2Pb

3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties It has been suggested that structurally distorting a DSM can create a Topological Insulator (TI), but this has not yet been experimentally verified. Furthermore, quasiparticle excitations known as Majorana Fermions have been theoretically proposed to exist in materials that exhibit superconductivity and topological surface states. Here we show that the cubic Laves phase Au$_2$Pb has a bulk Dirac cone above 100 K that gaps out upon cooling at a structural phase transition to create a topologically non trivial phase that superconducts below 1.2 K. The nontrivial Z$_2$ = -1 invariant in the low temperature phase indicates that Au$_2$Pb in its superconducting state must have topological surface states. These characteristics make Au$_2$Pb a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states

Structural investigation of the bilayer iridate Sr_3Ir_2O_7

A complete structural solution of the bilayer iridate compound Sr_3Ir_2O_7 presently remains outstanding. Previously reported structures for this compound vary and all fail to explain weak structural violations observed in neutron scattering measurements as well as the presence of a net ferromagnetic moment in the basal plane. In this paper, we present single crystal neutron diffraction and rotational anisotropy second harmonic generation measurements unveiling a lower, monoclinic symmetry inherent to Sr_3Ir_2O_7. Combined with density functional theory, our measurements identify the correct structural space group as No. 15 (C2/c) and provide clarity regarding the local symmetry of Ir^(4+) cations within this spin-orbit Mott material