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Open Access Publications from the University of California

Scholarly Works (3 results)

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

This doctoral work presents structural studies of metal-to-insulator phase transitions in neuromorphic devices under voltage operation to gain a deeper insight into mesoscale structural properties as they relate to neuromorphic computing. The primary technique used is dark-field X-ray microscopy (DFXM), which provides spatial mapping using diffraction contrast to distinguish structural phases as well as orientation modulations. This work investigates two opposing neuromorphic devices: La0.7Sr0.3MnO3 (LSMO) and VO2 which display insulator-to-metal (IMT) and metal-to-insulator (MIT) transitions with increasing voltage respectively. Clear device-lattice reorientation and strain originating from applied voltages in LSMO devices are observed and characterized. Strain measurements from micro-diffraction indicate a poorly defined barrier due to strain gradients across the length of the device. Utilizing DFXM, it is shown that there is significant reorientation associated with device operation and possibly driven by substrate-film interactions.

Structural studies of VO2 devices give insight into the memristive properties of VO2 which govern its neuromorphic properties. Key mesoscale features in the switching of VO2 devices are identified which provide information for the construction of energy-efficient neuromorphic devices. Metallic phase formation beneath electrodes before device switching which is maintained through voltage application presents methods for local tuning of synaptic weights. Additionally, the memristive behavior appears to be held in nucleation sites appearing as local modulations to the transition temperature. A heterogeneous filament structure is observed with monoclinic-like pockets appearing within the rutile filament with sizes just below the optical resolution limit. Although phase persistence appears in the devices, the interaction between the film and substrate plays an important role in filament formation and structure.

A strong structural effect on the substrate, originating from the phase transition in the film, appears under device operation. VO2 films locally strain the underlying substrate indicating a complex interaction between substrates and films overlooked until now. It is shown that under filament formation in VO2 films, the filament becomes imprinted on the substrate, identifiable using DFXM leveraging a substrate Bragg peak. Additionally, imprinting of barrier formation in STO/LSMO systems appears as local changes to the mosaicity of the STO substrate. Overall, this thesis is a comprehensive local structural study of neuromorphic devices illustrating complex memory retention means and previously unseen film effects on the substrate.

    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2024)
    Electrical triggering of a metal-insulator transition (MIT) often results in the formation of characteristic spatial patterns such as a metallic filament percolating through an insulating matrix or an insulating barrier splitting a conducting matrix. When MIT triggering is driven by electrothermal effects, the temperature of the filament or barrier can be substantially higher than the rest of the material. Using X-ray microdiffraction and dark-field X-ray microscopy, we show that electrothermal MIT triggering leads to the development of an inhomogeneous strain profile across the switching device, even when the material does not undergo a pronounced, discontinuous structural transition coinciding with the MIT. Diffraction measurements further reveal evidence of unique features associated with MIT triggering including lattice distortions, tilting, and twinning, which indicate structural nonuniformity of both low- and high-resistance regions inside the switching device. Such lattice deformations do not occur under equilibrium, zero-voltage conditions, highlighting the qualitative difference between states achieved through increasing temperature and applying voltage in nonlinear electrothermal materials. Electrically induced strain, lattice distortions, and twinning could have important contributions in the MIT triggering process and drive the material into nonequilibrium states, providing an unconventional pathway to explore the phase space in strongly correlated electronic systems.
      Cover page: Local strain inhomogeneities during electrical triggering of a metal-insulator transition revealed by X-ray microscopy.
      • Article
      • Peer Reviewed
      LBL Publications (2023)
      Most resonant inelastic x-ray scattering (RIXS) studies of dynamic charge order correlations in the cuprates have focused on the high-symmetry directions of the copper oxide plane. However, scattering along other in-plane directions should not be ignored as it may help understand, for example, the origin of charge order correlations or the isotropic scattering resulting in strange metal behavior. Our RIXS experiments reveal dynamic charge correlations over the qx-qy scattering plane in underdoped Bi2Sr2CaCu2O8+δ. Tracking the softening of the RIXS-measured bond-stretching phonon, we show that these dynamic correlations exist at energies below approximately 70 meV and are centered around a quasi-circular manifold in the qx-qy scattering plane with radius equal to the magnitude of the charge order wave vector, qCO. This phonon-tracking procedure also allows us to rule out fluctuations of short-range directional charge order (i.e., centered around [qx = ±qCO, qy = 0] and [qx = 0, qy = ±qCO]) as the origin of the observed correlations.
        Cover page: Low-energy quasi-circular electron correlations with charge order wavelength in Bi2Sr2CaCu2O8+δ.
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