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Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2023)

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.

    Cover page: Magneto-optical studies of novel condensed matter phases with Sagnac interferometry
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2023)
    Kagome metals AV3Sb5 (A = K, Cs, Rb) provide a rich platform for intertwined orders, where evidence for time-reversal symmetry breaking, likely due to the long-sought loop currents, has emerged in STM and muon spin relaxation experiments. An isotropic component in the spontaneous optical rotation has also been reported and was interpreted as the magneto-optic Kerr effect. Intriguingly, the observed rotations differ by five orders of magnitude between different wavelengths and samples, suggesting more intricate physics. Here we report optical rotation and polar Kerr measurements in CsV3Sb5 crystals at the same wavelength. We observe large isotropic components of 1 milliradian in the optical rotation that do not respond to applied magnetic fields, while the spontaneous Kerr signal is less than 20 nanoradians. Our results prove unambiguously that the reported isotropic rotation is not from time-reversal symmetry breaking but represents the long-sought specular optical rotation and indicates a new intertwined order.
      Cover page: Unconventional specular optical rotation in the charge ordered state of Kagome metal CsV3Sb5Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2020)
      Engineering magnetic anisotropy in two-dimensional systems has enormous scientific and technological implications. The uniaxial anisotropy universally exhibited by two-dimensional magnets has only two stable spin directions, demanding 180° spin switching between states. We demonstrate a previously unobserved eightfold anisotropy in magnetic SrRuO3 monolayers by inducing a spin reorientation in (SrRuO3)1/(SrTiO3) N superlattices, in which the magnetic easy axis of Ru spins is transformed from uniaxial 〈001〉 direction (N < 3) to eightfold 〈111〉 directions (N ≥ 3). This eightfold anisotropy enables 71° and 109° spin switching in SrRuO3 monolayers, analogous to 71° and 109° polarization switching in ferroelectric BiFeO3. First-principle calculations reveal that increasing the SrTiO3 layer thickness induces an emergent correlation-driven orbital ordering, tuning spin-orbit interactions and reorienting the SrRuO3 monolayer easy axis. Our work demonstrates that correlation effects can be exploited to substantially change spin-orbit interactions, stabilizing unprecedented properties in two-dimensional magnets and opening rich opportunities for low-power, multistate device applications.
        Cover page: Correlation-driven eightfold magnetic anisotropy in a two-dimensional oxide monolayer
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