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Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Merced Electronic Theses and Dissertations (2024)

Oxide materials, including perovskite oxides, are well known for hosting a range of properties, such as ferroelectricity and superconductivity, which are significant for both fundamental understanding and technological applications. The interplay among structural, orbital, charge, and spin factors in these oxides provides a platform for studying and manipulating material properties that can be incorporated into devices for daily life, scientific studies, and industry. This motivation drives the development of atomic-level understanding of the phenomena exhibited by oxide materials, helping in the formulation of design principles to achieve desired properties. In this dissertation, I will explore different properties, including ferroelectricity, piezoelectricity, and magnetism, exhibited by various oxide materials with complex crystal structures through first principles density functional theory calculations and group theoretic arguments.

Layered perovskites are structures formed by the rearrangement of perovskite structures, with other structural layers interspersed between them. Various families of layered perovskite oxide ferroelectrics demonstrate a coupling between polarization and structural order parameters, particularly involving octahedral rotation distortions. The Aurivillius-phase oxides SrBi2B2O9 (B=Ta, Nb) are recognized for exhibiting such order-parameter couplings and are renowned for their excellent room temperature ferroelectric performance. After providing a brief introduction to the physics of layered oxides in Chapter 1 and the theoretical methods used in our research in Chapter 2, I will discuss the results of our calculations which explore the ferroelectric switching processes of SrBi2B2O9 to identify low-energy switching paths in Chapter 3. This chapter also elucidates the roles of coupled order parameters in determining the complex ferroelectric domain structure in SrBi2 B2 O9.

Non-polar structural distortions, like octahedral rotations, present additional degrees of freedom for manipulating ferroelectric properties in Aurivillius phases compared to traditional ferroelectrics such as BaTiO3 and PbTiO3. In Chapter 4, the results of first-principles calculations on the Aurivillius-phase oxide Bi2WO6 are explored. The chapter elucidates the ferroelectric switching mechanism and inves- tigates the coupling of electrical polarization with the isolated spin of a magnetic dopant (Fe3+) in Bi2WO6. It details the change in spin directionality along the ferroelectric switching pathways and emphasizes the roles of crystalline symmetry in such coupling.

In addition to demonstrating geometry-driven ferroelectricity, layered perovskites like Ruddlesden-Popper phases also exhibit piezoelectricity. However, there has been minimal study of piezoelectricity in Ruddlesden-Popper phases. Chapter 5 will highlight our efforts to establish a microscopic understanding of piezoelectricity by exploring the roles of the interplay between strain and structural distortions on piezoelectric coefficients in representative Ruddlesden-Popper phases, including Ca3Ti2O7, Sr3Zr2O7, and Sr3Sn2O7.

Some layered oxides host intriguing phenomena like frustrated magnetism together with the ferroelectricity. Recent experiments have highlighted the rare earth oxide TbInO3 for its simultaneous manifestation of improper ferroelectricity and frustrated magnetism, making it a potential spin liquid candidate material of interest to the materials science and physics communities. While spin liquids are anticipated to exhibit superconductivity upon suitable doping, the influence of doping on the electrical conductivity of TbInO3 remains an open question. Chapter 6 will unveil the outcomes of our investigations into the doping effect in hexagonal TbInO3. The dissertation will wrap up with a concise exploration of future directions in Chapter 7.

    • Article
    • Peer Reviewed
    LBL Publications (2022)
    Atomic-scale control of spins by electric fields is highly desirable for future technological applications. Magnetically doped Aurivillius-phase oxides present one route to achieve this, with magnetic ions substituted into the ferroelectric structure at dilute concentrations, resulting in spin-charge coupling. However, there has been minimal exploration of the ferroelectric switching pathways in this materials class, limiting predictions of the influence of an electric field on magnetic spins in the structure. Here, we determine the ferroelectric switching pathways of the end member of the Aurivillius phase family, Bi2WO6, using a combination of group theoretic analysis and density functional theory calculations. We find that in the ground state P21ab phase, a two-step switching pathway via C2 and Cm intermediate phases provides the lowest energy barrier. Considering iron substitutions on the W site in Bi2WO6, we determine the spin easy axis. By tracking the change in spin directionality during ferroelectric switching, we find that a 90∘ switch in the polarization direction leads to a 112° reorientation of the spin easy axis. The low-symmetry crystal-field environment of Bi2WO6 and magnetoelastic coupling on the magnetic dopant provide a route to spin control via an applied electric field.
      Cover page: Manipulation of spin orientation via ferroelectric switching in Fe-doped Bi2WO6 from first principles
      • Article
      • Peer Reviewed
      LBL Publications (2023)
      We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (h) Lu0.6Sc0.4FeO3, and we compare our findings with a detailed symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in h-Lu0.6Sc0.4FeO3 are broad and semiconducting, a finding that we attribute to the presence of an A-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the A site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexible defect states.
        Cover page: Real-Space Infrared Spectroscopy of Ferroelectric Domain Walls in Multiferroic h‑(Lu,Sc)FeO3Creative Commons 'BY-NC' version 4.0 license
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2015)
        In the high-temperature (T(c)) cuprate superconductors, a growing body of evidence suggests that the pseudogap phase, existing below the pseudogap temperature T*, is characterized by some broken electronic symmetries distinct from those associated with superconductivity. In particular, recent scattering experiments have suggested that charge ordering competes with superconductivity. However, no direct link of an interplay between the two phases has been identified from the important low-energy excitations. Here, we report an antagonistic singularity at T(c) in the spectral weight of Bi2Sr2CaCu2O(8+δ) as compelling evidence for phase competition, which persists up to a high hole concentration p ~ 0.22. Comparison with theoretical calculations confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two orders and the complex phase diagram near the pseudogap critical point.
          Cover page: Direct spectroscopic evidence for phase competition between the pseudogap and superconductivity in Bi2Sr2CaCu2O8+δ
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