UCLA

Research on two-dimensional (2D) van der Waal (vdW) materials with structural benefits and topological materials with advantages due to unique electronic band structures has rapidly grown over the past 20 years in anticipation of them being a robust platform for uncovering emerging phenomena and implementing device structures. In addition, scholars have found that materials having both these properties simultaneously produce enhanced topological states due to their large surface to volume ratio. Furthermore, dedicated efforts over the past few years have led to extensions that encompass magnetism. The interplay between the topological band structures and their magnetic order is expected to demonstrate exceptional physical characteristics. The purpose of this thesis is to explore magnetic topological and 2D vdW materials via optical spectroscopic methods for analyzing their vibration dynamics with their symmetric group(s). First, MnBi2nTe3n+1 (n=1,2,3,4) magnetic topological heterostructures are examined by cryogenic polarized-Raman spectroscopy. MnBi2Te4 is the first-ever intrinsic antiferromagnetic (AFM) topological insulator arising by intercalating Mn-Te chains into Bi2Te3 crystals. Two resonances at 66 and 112 cm-1 show abnormal broadening in Raman linewidths below the Néel temperature because of spin-phonon coupling. In addition, the MnBi4Te7 heterostructure has an out-of-plane interlayer force constant three times weaker than Bi2Te3 by Davydov splitting of the A1g mode at cryogenic temperature. Second, we explore ferromagnetic (FM)-EuCd2As2, an excellent candidate for investigating Weyl physics due to its minimal number of Weyl points, via polarized magneto-Raman spectroscopy. Enhanced Raman intensities are observed below the Curie temperature due to the spin-phonon coupling. In addition, we demonstrate that A-mode peaks are manipulated by magneto helical-Raman spectroscopy because of the magneto-optic effect. Third, we also investigate electronic transition peaks of FM-EuCd2As2, which is a family of rare earth compounds of Eu. These lanthanide compounds have strong spin-orbit coupling due to the protected 4f-orbital block, and thus, the electronic transition state exists near phononic excitation level. We show the Zeeman effect coupled to the direction of cross-circular polarization configuration with the external magnetic fields. In addition, anomalies in intensity are observed in the low-frequency region between anti-Stokes and Stokes. Lastly, we seek to enhance the second harmonic generation (SHG) for 2D vdW MoS2 materials by intercalating a foreign species between the MoS2 layer, called bulk monolayer (BM)- MoS2. Due to its centrosymmety, the even-number of susceptibility term vanishes in bulk MoS2 materials. The foreign species causes reduced interlayer coupling due to an increased gap between the layers, which breaks the inversion symmetry of the materials and in turn, leads to an enhanced SHG signal. We find that BM-MoS2 has a signal 126 times higher than its monolayer and 21 times higher than a GaAs wafer.