UC Irvine

This thesis covers some of our recent experiments using high-resolution zero-area loop magneto-optic Sagnac interferometry to study the nature of time reversal symmetry breaking in emerging phases of several topological material systems. The materials studied include the topological superconductors FeTe$_{1-x}$Se$_x$, non-collinear ferromagnetic Weyl semimetal CeAlSi, and charge density wave kagome superconductor CsV$_3$Sb$_5$. The first few chapters cover the theory of MOKE and basic principle of operation of the magneto-optic Sagnac interferometer used in X Lab. Chapter 4 documents the development of a highly-compact scanning optical probe, which is constructed to extend the compatibility of the Sagnac interferometer to a convenient cryostat with 9 T field and 1.8–400 K temperature range. The probe additionally allows for integration of applied strain in-situ with Sagnac interferometry in the same compact design. Chapter 5 covers the measurements of TRSB in Fe-Chalcogenide topological superconductors FeTe$_{1-x}$Se$_x$. This chapter includes our published work on identifying TRSB in the material's surface and provides a brief summary of a recent unpublished collaboration with Boston College in the last section. Chapter 6 covers the experiments on CeAlSi where we use scanning Sagnac interferometry to image the domains of CeAlSi in fields larger than which were unattainable in previous magnetic scanning experiments, in hopes of shining light on the mechanisms behind the hysteretic “Loop Hall Effect”. Chapter 7 covers the high-resolution Kerr measurements and polarization rotation measurements on CsV$_3$Sb$_5$ to test for signatures of a TRSB charge density wave order, and resolve contradicting results of MOKE measurements performed by several different research groups.