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Open Access Publications from the University of California

Scholarly Works (41 results)

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

A primary focus of modern condensed matter physics concerns the study of strongly correlatedmaterials in which enhanced electron interactions give rise to emergent phenomena, such as high-temperature superconductivity. The strong electromagnetic coupling between electrons and photons makes x-rays a highly sensitive probe for investigating the emergent properties of these materials. The advent of modern synchrotron light sources significantly enhances these capabilities through the production of high-brilliance x-ray beams that offer full control over the incident photon energy, thereby enabling the detection of signals emanating from phases in which only a small number of valence electrons contribute to the scattering signal via resonant enhancement. This dissertation demonstrates how these synchrotron-based x-ray scattering and spectroscopy techniques can be utilized to study a wide range of material characteristics and electronic properties in modern condensed matter systems. The x-ray techniques are applied to the characterization of two strongly correlated systems, PrxY1−xBa2Cu3O7 and FeGe. In PrxY1−xBa2Cu3O7, x-ray absorption spectroscopy and resonant inelastic x-ray scattering are used to elucidate the effects of the substituted Pr ion on the electronic structure, non-resonant inelastic x-ray scattering techniques are employed to determine the shape of the active Cu 3d orbital hole that is responsible for the anisotropic behavior observed in transport measurements, and resonant elastic x-ray scattering techniques are used to investigate a novel three-dimensional charge order that is stabilized by the Pr substitution and is demonstrated to compete directly with the high-temperature superconducting phase. Resonant elastic x-ray scattering techniques are further utilized to probe the role of the Ge honeycomb lattice in the formation of charge order in the kagome metal FeGe, which is supported by density-functional theory calculations. Understanding and controlling the correlated phenomena in these and other quantum materials has significant implications for various technologies, including quantum computing, energy storage, and next-generation electronics.

    Cover page: Investigation of Charge Ordering in the Strongly Correlated Materials PrxY1−xBa2Cu3O7 and FeGe by Synchrotron-based X-ray Techniques
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2024)

    Strongly correlated electron physics host complex phenomena. Often such complex phenomenon are found close to Anti-ferromagnetic phase by tuning one parameter with which the spin degrees of freedom couple to. Examples of such complex phenomena include pseudo-gap phase, spin density wave, charge density wave, superconductivity and many more. Furthermore, phase competition and phase coexistence is ubiquitous in these materials. Here we focus on such strongly coupled antiferromagnets and utilize coherent resonant scattering to study their real space structures. We will see that the real space evolution of meso scale structure encodes many crucial information about the physics of the complex system. Finally we end our discussion with proposing modification to existing coherent scattering experiments to study dynamic equilibrium states of metastable matter.

      Cover page: Nucleation, Fabrication, and Competition in Antiferromagnetic Textures Studied with Coherent Resonant Soft X-ray Scattering
      • 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 Berkeley Previously Published Works (2020)
        Charge order has recently been identified as a leading competitor of high-temperature superconductivity in moderately doped cuprates. We provide a survey of universal and materials-specific aspects of this phenomenon, with emphasis on results obtained by scattering methods. In particular, we discuss the structure, periodicity, and stability range of the charge-ordered state, its response to various external perturbations, the influence of disorder, the coexistence and competition with superconductivity, as well as collective charge dynamics. In the context of this journal issue which honors Roger Cowley's legacy, we also discuss the connection of charge ordering with lattice vibrations and the central-peak phenomenon. We end the review with an outlook on research opportunities offered by new synthesis methods and experimental platforms, including cuprate thin films and superlattices.
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2021)
          Neuromorphic computing-which aims to mimic the collective and emergent behavior of the brain's neurons, synapses, axons, and dendrites-offers an intriguing, potentially disruptive solution to society's ever-growing computational needs. Although much progress has been made in designing circuit elements that mimic the behavior of neurons and synapses, challenges remain in designing networks of elements that feature a collective response behavior. We present simulations of networks of circuits and devices based on superconducting and Mott-insulating oxides that display a multiplicity of emergent states that depend on the spatial configuration of the network. Our proposed network designs are based on experimentally known ways of tuning the properties of these oxides using light ions. We show how neuronal and synaptic behavior can be achieved with arrays of superconducting Josephson junction loops, all within the same device. We also show how a multiplicity of synaptic states could be achieved by designing arrays of devices based on hydrogenated rare earth nickelates. Together, our results demonstrate a research platform that utilizes the collective macroscopic properties of quantum materials to mimic the emergent behavior found in biological systems.
            Cover page: Low-temperature emergent neuromorphic networks with correlated oxide devices
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2019)
            Gestational hypoxia is a risk factor in the development of pulmonary hypertension in the newborn and other sequela, however, the mechanisms associated with the disease remain poorly understood. This review highlights disruption of metabolism by antenatal high altitude hypoxia and the impact this has on pulmonary hypertension in the newborn with discussion of model organisms and human populations. There is particular emphasis on modifications in glucose and lipid metabolism along with alterations in mitochondrial function. Additional focus is placed on increases in oxidative stress and the progression of pulmonary vascular disease in the newborn and on the need for further exploration using a combination of contemporary and classical approaches.
              Cover page: Gestational Hypoxia and Programing of Lung Metabolism
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              Honeycomb iridates such as γ-Li2IrO3 are argued to realize Kitaev spin-anisotropic magnetic exchange, along with Heisenberg and possibly other couplings. While systems with pure Kitaev interactions are candidates to realize a quantum spin-liquid ground state, in γ-Li2IrO3 it has been shown that the presence of competing magnetic interactions leads to an incommensurate spiral spin order at ambient pressure below 38 K. We study the pressure sensitivity of this magnetically ordered state in single crystals of γ-Li2IrO3 using resonant x-ray scattering (RXS) under applied hydrostatic pressures of up to 3 GPa. RXS is a direct probe of electronic order, and we observe the abrupt disappearance of the qsp=(0.57,0,0) spiral order at a critical pressure Pc=1.4 GPa with no accompanying change in the symmetry of the lattice.
                Cover page: Resonant x-ray scattering reveals possible disappearance of magnetic order under hydrostatic pressure in the Kitaev candidate γ-Li2IrO3
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2014)
                Dichlorodiphenyltrichloroethane (DDT) has been used extensively to control malaria, typhus, body lice and bubonic plague worldwide, until countries began restricting its use in the 1970s. Its use in malaria control continues in some countries according to recommendation by the World Health Organization. Individuals exposed to elevated levels of DDT and its metabolite dichlorodiphenyldichloroethylene (DDE) have an increased prevalence of diabetes and insulin resistance. Here we hypothesize that perinatal exposure to DDT disrupts metabolic programming leading to impaired metabolism in adult offspring. To test this, we administered DDT to C57BL/6J mice from gestational day 11.5 to postnatal day 5 and studied their metabolic phenotype at several ages up to nine months. Perinatal DDT exposure reduced core body temperature, impaired cold tolerance, decreased energy expenditure, and produced a transient early-life increase in body fat in female offspring. When challenged with a high fat diet for 12 weeks in adulthood, female offspring perinatally exposed to DDT developed glucose intolerance, hyperinsulinemia, dyslipidemia, and altered bile acid metabolism. Perinatal DDT exposure combined with high fat feeding in adulthood further impaired thermogenesis as evidenced by reductions in core temperature and in the expression of numerous RNA that promote thermogenesis and substrate utilization in the brown adipose tissue of adult female mice. These observations suggest that perinatal DDT exposure impairs thermogenesis and the metabolism of carbohydrates and lipids which may increase susceptibility to the metabolic syndrome in adult female offspring.
                  Cover page: Perinatal Exposure of Mice to the Pesticide DDT Impairs Energy Expenditure and Metabolism in Adult Female Offspring
                  • Article
                  • Peer Reviewed
                  UC Davis Previously Published Works (2018)

                  Background

                  Obesity induces significant changes in lipid mediators, however, the extent to which these changes persist after weight loss has not been investigated.

                  Subjects/methods

                  We fed C57BL6 mice a high-fat diet to generate obesity and then switched the diet to a lower-fat diet to induce weight loss. We performed a comprehensive metabolic profiling of lipid mediators including oxylipins, endocannabinoids, sphingosines and ceramides in key metabolic tissues (including adipose, liver, muscle and hypothalamus) and plasma.

                  Results

                  We found that changes induced by obesity were largely reversible in most metabolic tissues but the adipose tissue retained a persistent obese metabolic signature. Prostaglandin signaling was perturbed in the obese state and lasting increases in PGD2, and downstream metabolites 15-deoxy PGJ2 and delta-12-PGJ2 were observed after weight loss. Furthermore expression of the enzyme responsible for PGD2 synthesis (hematopoietic prostaglandin D synthase, HPGDS) was increased in obese adipose tissues and remained high after weight loss. We found that inhibition of HPGDS over the course of 5 days resulted in decreased food intake in mice. Increased HPGDS expression was also observed in human adipose tissues obtained from obese compared with lean individuals. We then measured circulating levels of PGD2 in obese patients before and after weight loss and found that while elevated relative to lean subjects, levels of this metabolite did not decrease after significant weight loss.

                  Conclusions

                  These results suggest that lasting changes in lipid mediators induced by obesity, still present after weight loss, may play a role in the biological drive to regain weight.
                    Cover page: Obesity-induced changes in lipid mediators persist after weight loss
                    • Article
                    • Peer Reviewed
                    UC Davis Previously Published Works (2021)
                    The prolonged, postweaning fast of northern elephant seal (Mirounga angustirostris) pups is characterized by a reliance on lipid metabolism and reversible, fasting-induced insulin resistance, providing a unique model to examine the effects of insulin on lipid metabolism. We have previously shown that acute insulin infusion induced a shift in fatty acid metabolism dependent on fasting duration. This study complements the previous study by examining the effects of fasting duration and insulin infusion on circulating levels of oxylipins, bioactive metabolites derived from the oxygenation of polyunsaturated fatty acids. Northern elephant seal pups were studied at two postweaning periods (n = 5/period): early fasting (1-2 wk postweaning; 127 ± 1 kg) and late fasting (6-7 wk postweaning; 93 ± 4 kg). Different cohorts of pups were weighed, sedated, and infused with 65 mU/kg of insulin. Plasma was collected prior to infusion (T0) and at 10, 30, 60, and 120 min postinfusion. A profile of ∼80 oxylipins was analyzed by UPLC-ESI-MS/MS. Nine oxylipins changed between early and late fasting and eight were altered in response to insulin infusion. Fasting decreased prostaglandin F2α (PGF2α) and increased 14,15-dihydroxyicosatrienoic acid (14,15-DiHETrE), 20-hydroxyeicosatetraenoic acid (20-HETE), and 4-hydroxy-docosahexaenoic acid (4-HDoHE) (P < 0.03) in T0 samples, whereas insulin infusion resulted in an inverse change in area-under-the-curve (AUC) levels in these same metabolites (P < 0.05). In addition, 12-12-hydroperoxyeicosatetraenoic acid (HpETE) and 12-HETE decreased with fasting and insulin infusion, respectively (P < 0.04). The oxylipins altered during fasting and in response to insulin infusion may contribute to the manifestation of insulin resistance and participate in the metabolic regulation of associated cellular processes.
                      Cover page: Oxylipin responses to fasting and insulin infusion in a large mammalian model of fasting-induced insulin resistance, the northern elephant seal
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