UC Irvine

According to Landau’s theory of phase transition, a second-order phase transition is accompanied by spontaneous symmetry breaking characterized by an order parameter. This thesis focuses on using magneto-optical Kerr effect (MOKE) to characterize the time-reversal symmetry (TRS) breaking in novel phases of matters including unconventional superconductivity, charge density wave and quantum magnetism. Chapter 1 gives a brief theoretical introduction of MOKE, followed by description of Sagnac interferometry in Chapter 2. The rest of the thesis will focus on three projects involving high resolution MOKE measurements.Bi/Ni bilayer is a unique system where unconventional superconductivity that breaks TRS arises at the interface between two non-superconducting elements. However, the origin of the superconductivity is under debate, the formation of superconducting NiBi3 impurity has been observed in samples grown by certain methods and is believed to cause the observed TRS breaking superconductivity. A test for the TRS breaking superconductivity in NiBi3 single crystal with high resolution MOKE measurements is presented in Chapter 3. Sr2RuO4 is an unconventional superconductor with TRS breaking and unknown pairing symmetry, despite having been heavily studied for almost three decades. Recently studies on Sr2RuO4 under uniaxial strain provided new experimental evidence, including a split of the Tc and onset temperature of TRS breaking phase under strain observed in μSR measurements. A study to test such strain-induced double transition scenario by combining MOKE measurements with application of uniaxial strain is presented in Chapter 4. Recently discovered Kagome metal CsV3Sb5 holds an unconventional charge density wave order below 94K and a superconducting ground state below 2.5K, providing an ideal playground to study intertwined orders. Evidence for TRS breaking has been found in its charge density wave phase by various experimental probes, which are believed to have connections to the long-sought loop current phase first proposed for cuprate superconductors. A comprehensive MOKE study on CsV3Sb5 to test the TRS breaking charge density wave phase is presented in Chapter 5.