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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2021)

Nonlinear optical properties of materials offer a diverse and powerful experimental probe into symmetry, electronic properties, and band structure. When paired with variable-wavelength light sources, these techniques are enhanced with the capability of resolving energy-dependent features. These properties are restricted to space groups that break inversion symmetry, but this dependence on crystal structure provides an opportunity to gain information about the material, and can help distinguish various electronic effects within a crystal. This point is crucial to this work, where bulk and surface optical properties in the Weyl semimetal RhSi are completely disentangled through symmetry considerations alone, with no need to rely on conjectures or assumptions about spectral features.

Weyl semimetals are a class of materials of special interest in condensed matter physics community within the last 10 years because their electronic band structures contain features which are analogous to Weyl fermions, a class of chiral fundamental particles which have not been observed in nature. While studied extensively experimentally, certain properties of Weyl semimetals have eluded measurement and theories have gone unconfirmed. One issue is that most Weyl semimetals only resemble an ideal model over a small energy range, limiting the optical sources available to study them within a relevant photon energy. Weyl semimetals may also contain trivial bands away from the Weyl fermion structures whose properties can be difficult to disentangle from those of interest.

In this work, nonlinear optical properties of the Weyl semimetal RhSi are measured. In particular, the theory of the Quantized Circular Photogalvanic Effect [35] in Weyl semimetals, a bulk property, is tested via measurements of terahertz generation and radiation. Precise numerical agreement with the theory is ruled out for the material, although we find the qualitative features of the prediction to hold true. Similar measurements are made in order to examine surface states on RhSi which have proven difficult to experimentally study with traditional transport and optical techniques. The bulk and surface properties are distinguished by considering experimental geometry in concert with crystal symmetry and orientation.

    Cover page: Nonlinear Optical Properties of the Chiral Weyl Semimetal RhSi
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2020)
    Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is a topological invariant known as the Berry monopole charge. The circular photogalvanic effect (CPGE), whereby circular polarized light generates a helicity-dependent photocurrent, is a notable example of a macroscopic property that emerges directly from the topology of the Weyl semimetal band structure. Recently, it was predicted that the amplitude of the CPGE associated with optical transitions near a Weyl node is proportional to its monopole charge. In chiral Weyl systems, nodes of opposite charge are nondegenerate, opening a window of wavelengths where the CPGE resulting from uncompensated Berry charge can emerge. Here, we report measurements of CPGE in the chiral Weyl semimetal RhSi, revealing a CPGE response in an energy window that closes at 0.65 eV, in agreement with the predictions of density functional theory.
      Cover page: Helicity-dependent photocurrents in the chiral Weyl semimetal RhSi
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2021)
      Despite the fundamental nature of the edge state in topological physics, direct measurement of electronic and optical properties of the Fermi arcs of topological semimetals has posed a significant experimental challenge, as their response is often overwhelmed by the metallic bulk. However, laser-driven currents carried by surface and bulk states can propagate in different directions in nonsymmorphic crystals, allowing for the two components to be easily separated. Motivated by a recent theoretical prediction G. Chang et al., Phys. Rev. Lett. 124, 166404 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.166404, we have measured the linear and circular photogalvanic effect currents deriving from the Fermi arcs of the nonsymmorphic, chiral Weyl semimetal RhSi over the 0.45-1.1 eV incident photon energy range. Our data are in good agreement with the predicted spectral shape of the circular photogalvanic effect as a function of photon energy, although the direction of the surface photocurrent departed from the theoretical expectation over the energy range studied. Surface currents arising from the linear photogalvanic effect were observed as well, with the unexpected result that only two of the six allowed tensor element were required to describe the measurements, suggesting an approximate emergent mirror symmetry inconsistent with the space group of the crystal.
        Cover page: Direct Measurement of Helicoid Surface States in RhSi Using Nonlinear OpticsCreative Commons 'BY-NC' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2020)
        Nematic order is the breaking of rotational symmetry in the presence of translational invariance. While originally defined in the context of liquid crystals, the concept of nematic order has arisen in crystalline matter with discrete rotational symmetry, most prominently in the tetragonal Fe-based superconductors where the parent state is four-fold symmetric. In this case the nematic director takes on only two directions, and the order parameter in such 'Ising-nematic' systems is a simple scalar. Here, using a spatially resolved optical polarimetry technique, we show that a qualitatively distinct nematic state arises in the triangular lattice antiferromagnet Fe1/3NbS2. The crucial difference is that the nematic order on the triangular lattice is a [Formula: see text] or three-state Potts-nematic order parameter. As a consequence, the anisotropy axes of response functions such as the resistivity tensor can be continuously reoriented by external perturbations. This discovery lays the groundwork for devices that exploit analogies with nematic liquid crystals.
          Cover page: Three-state nematicity in the triangular lattice antiferromagnet Fe1/3NbS2
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation1. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties2, 3 as well as for various applications including magneto-optical recording4. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts5-9. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure10-14. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of M ~0.002 µB/Mn, the non-collinear AF metal Mn3Sn15 exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Néel state16 may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.
            Cover page: Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal
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