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Open Access Publications from the University of California

Scholarly Works (12 results)

  • Article
  • Peer Reviewed
UC Irvine Previously Published Works (2013)

Projection neurons (PNs), located in the antennal lobe region of the insect brain, play a key role in processing olfactory information. To explore how activity is regulated at the level of single PNs within this central circuit we have recorded from these neurons in adult Drosophila melanogaster brains. Our previous study demonstrated that PNs express voltage-gated calcium currents with a transient and sustained component. We found that the sustained component is mediated by cac gene-encoded Cav2-type channels involved in regulating action potential-independent release of neurotransmitter at excitatory cholinergic synapses. The function of the transient calcium current and the gene encoding the underlying channels, however, were unknown. Here we report that the transient current blocked by prepulse inactivation is sensitive to amiloride, a vertebrate Cav3-type channel blocker. In addition PN-specific RNAi knockdown of α1T, the Drosophila Cav3-type gene, caused a dramatic reduction in the transient current without altering the sustained component. These data demonstrate that the α1T gene encodes voltage-gated calcium channels underlying the amiloride-sensitive transient current. Alterations in evoked firing and spontaneous burst firing in the α1T knockdowns demonstrate that the Cav3-type calcium channels are important in regulating excitability in adult PNs.

    Cover page: Cav3-type α1T calcium channels mediate transient calcium currents that regulate repetitive firing in Drosophila antennal lobe PNs.
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2009)

    Voltage-gated calcium channels containing alpha1 subunits encoded by Ca(v)2 family genes are critical in regulating release of neurotransmitter at chemical synapses. In Drosophila, cac is the only Ca(v)2-type gene. Cacophony (CAC) channels are localized in motor neuron terminals where they have been shown to mediate evoked, but not AP-independent, release of glutamate at the larval neuromuscular junction (NMJ). Cultured embryonic neurons also express CAC channels, but there is no information about the properties of CAC-mediated currents in adult brain nor how these channels regulate transmission in central neural circuits where fast excitatory synaptic transmission is predominantly cholinergic. Here we report that wild-type neurons cultured from late stage pupal brains and antennal lobe projection neurons (PNs) examined in adult brains, express calcium currents with two components: a slow-inactivating current sensitive to the spider toxin Plectreurys toxin II (PLTXII) and a fast-inactivating PLTXII-resistant component. CAC channels are the major contributors to the slow-inactivating PLTXII-sensitive current based on selective reduction of this component in hypomorphic cac mutants (NT27 and TS3). Another characteristic of cac mutant neurons both in culture and in whole brain recordings is a reduced cholinergic miniature excitatory postsynaptic current frequency that is mimicked in wild-type neurons by acute application of PLTXII. These data demonstrate that cac encoded Ca(v)2-type calcium channels regulate action potential (AP)-independent release of neurotransmitter at excitatory cholinergic synapses in the adult brain, a function not predicted from studies at the larval NMJ.

      Cover page: Cav2-type calcium channels encoded by cac regulate AP-independent neurotransmitter release at cholinergic synapses in adult Drosophila brain.Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2023)
      The past decade has witnessed dramatic progress related to various aspects of emergent topological polar textures in oxide nanostructures displaying vortices, skyrmions, merons, hopfions, dipolar waves, or labyrinthine domains, among others. For a long time, these nontrivial structures (the electric counterparts of the exotic spin textures) were not expected due to the high energy cost associated with the dipolar anisotropy: the smooth and continuous evolution of the local polarization to produce topologically protected structures would result in a large elastic energy penalty. However, it was discovered that the delicate balance and intricate interplay between the electric, elastic, and gradient energies can be altered in low-dimensional forms of ferroelectric oxide nanostructures. These can be tuned to manipulate order parameters in ways once considered impossible. This review addresses the historical context that provided the fertile background for the dawning of the polar topological era. This has been possible thanks to a fruitful, positive feedback between theory and experiment: advances in materials synthesis and preparation (with a control at the atomic scale) and characterization have come together with great progress in theoretical modeling of ferroelectrics at larger length and timescales. An in-depth scientific description to formalize and generalize the prediction, observation, and probing of exotic, novel, and emergent states of matter is provided. Extensive discussions of the fundamental physics of such polar textures, a primer explaining the basic topological concepts, an explanation of the modern theoretical and computational methodologies that enable the design and study of such structures, what it takes to achieve deterministic, on-demand control of order-parameter topologies through atomically precise synthesis, the range of characterization methods that are key to probing these structures, and their thermodynamic field-driven (temperature-driven, stress-driven, etc.) susceptibilities are included. The new emergent states of matter join together with exotic functional properties (such as chirality, negative capacitance, and coexistence of phases) that, along with their small size and ultrafast dynamical response, make them potential candidates in multifunctional devices. Finally, some open questions and challenges for the future are presented, underlining the interesting future that is anticipated in this field.
        Cover page: Topological phases in polar oxide nanostructuresCreative Commons 'BY-NC-SA' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2018)
        Chirality is a geometrical property by which an object is not superimposable onto its mirror image, thereby imparting a handedness. Chirality determines many important properties in nature-from the strength of the weak interactions according to the electroweak theory in particle physics to the binding of enzymes with naturally occurring amino acids or sugars, reactions that are fundamental for life. In condensed matter physics, the prediction of topologically protected magnetic skyrmions and related spin textures in chiral magnets has stimulated significant research. If the magnetic dipoles were replaced by their electrical counterparts, then electrically controllable chiral devices could be designed. Complex oxide BaTiO3/SrTiO3 nanocomposites and PbTiO3/SrTiO3 superlattices are perfect candidates, since "polar vortices," in which a continuous rotation of ferroelectric polarization spontaneously forms, have been recently discovered. Using resonant soft X-ray diffraction, we report the observation of a strong circular dichroism from the interaction between circularly polarized light and the chiral electric polarization texture that emerges in PbTiO3/SrTiO3 superlattices. This hallmark of chirality is explained by a helical rotation of electric polarization that second-principles simulations predict to reside within complex 3D polarization textures comprising ordered topological line defects. The handedness of the texture can be topologically characterized by the sign of the helicity number of the chiral line defects. This coupling between the optical and novel polar properties could be exploited to encode chiral signatures into photon or electron beams for information processing.
          Cover page: Emergent chirality in the electric polarization texture of titanate superlattices
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Spintronic elements based on spin transfer torque have emerged with potential for on-chip memory, but they suffer from large energy dissipation due to the large current densities required. In contrast, an electric-field-driven magneto-electric storage element can operate with capacitive displacement charge and potentially reach 1-10 µJ cm-2 switching operation. Here, magneto-electric switching of a magnetoresistive element is shown, operating at or below 200 mV, with a pathway to get down to 100 mV. A combination of phase detuning is utilized via isovalent La substitution and thickness scaling in multiferroic BiFeO3 to scale the switching energy density to ≈10 µJ cm-2 . This work provides a template to achieve attojoule-class nonvolatile memories.
            Cover page: Ultralow Voltage Manipulation of Ferromagnetism
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2022)
            Resonant elastic X-ray scattering (REXS) offers a unique tool to investigate solid-state systems providing spatial knowledge from diffraction combined with electronic information through the enhanced absorption process, allowing the probing of magnetic, charge, spin, and orbital degrees of spatial order together with electronic structure. A new promising application of REXS is to elucidate the chiral structure of electrical polarization emergent in a ferroelectric oxide superlattice in which the polarization vectors in the REXS amplitude are implicitly described through an anisotropic tensor corresponding to the quadrupole moment. Here, we present a detailed theoretical framework and analysis to quantitatively analyze the experimental results of Ti L-edge REXS of a polar vortex array formed in a PbTiO3/SrTiO3 superlattice. Based on this theoretical framework, REXS for polar chiral structures can become a useful tool similar to x-ray resonant magnetic scattering (XRMS), enabling a comprehensive study of both electric and magnetic REXS on the chiral structures.
              Cover page: Chiral structures of electric polarization vectors quantified by X-ray resonant scattering
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO3) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO3 films, to large (10's of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.
                Cover page: The role of lattice dynamics in ferroelectric switching
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2022)
                An escalating challenge in condensed-matter research is the characterization of emergent order-parameter nanostructures such as ferroelectric and ferromagnetic skyrmions. Their small length scales coupled with complex, three-dimensional polarization or spin structures makes them demanding to trace out fully. Resonant elastic x-ray scattering (REXS) has emerged as a technique to study chirality in spin textures such as skyrmions and domain walls. It has, however, been used to a considerably lesser extent to study analogous features in ferroelectrics. Here, we present a framework for modeling REXS from an arbitrary arrangement of charge quadrupole moments, which can be applied to nanostructures in materials such as ferroelectrics. With this, we demonstrate how extended reciprocal space scans using REXS with circularly polarized x rays can probe the three-dimensional structure and chirality of polar skyrmions. Measurements, bolstered by quantitative scattering calculations, show that polar skyrmions of mixed chirality coexist, and that REXS allows valuation of relative fractions of right- and left-handed skyrmions. Our quantitative analysis of the structure and chirality of polar skyrmions highlights the capability of REXS for establishing complex topological structures toward future application exploits.
                  Cover page: Structural Chirality of Polar Skyrmions Probed by Resonant Elastic X-Ray ScatteringCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2020)
                  Magnetoelectric coupling at room temperature in multiferroic materials, such as BiFeO3, is one of the leading candidates to develop low-power spintronics and emerging memory technologies. Although extensive research activity has been devoted recently to exploring the physical properties, especially focusing on ferroelectricity and antiferromagnetism in chemically modified BiFeO3, a concrete understanding of the magnetoelectric coupling is yet to be fulfilled. We have discovered that La substitutions at the Bi-site lead to a progressive increase in the degeneracy of the potential energy landscape of the BiFeO3 system exemplified by a rotation of the polar axis away from the 〈111〉pc towards the 〈112〉pc discretion. This is accompanied by corresponding rotation of the antiferromagnetic axis as well, thus maintaining the right-handed vectorial relationship between ferroelectric polarization, antiferromagnetic vector and the Dzyaloshinskii-Moriya vector. As a consequence, La-BiFeO3 films exhibit a magnetoelectric coupling that is distinctly different from the undoped BiFeO3 films.
                    Cover page: Manipulating magnetoelectric energy landscape in multiferroics
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2020)
                    Magnetoelectric coupling at room temperature in multiferroic materials, such as BiFeO3, is one of the leading candidates to develop low-power spintronics and emerging memory technologies. Although extensive research activity has been devoted recently to exploring the physical properties, especially focusing on ferroelectricity and antiferromagnetism in chemically modified BiFeO3, a concrete understanding of the magnetoelectric coupling is yet to be fulfilled. We have discovered that La substitutions at the Bi-site lead to a progressive increase in the degeneracy of the potential energy landscape of the BiFeO3 system exemplified by a rotation of the polar axis away from the 〈111〉pc towards the 〈112〉pc discretion. This is accompanied by corresponding rotation of the antiferromagnetic axis as well, thus maintaining the right-handed vectorial relationship between ferroelectric polarization, antiferromagnetic vector and the Dzyaloshinskii-Moriya vector. As a consequence, La-BiFeO3 films exhibit a magnetoelectric coupling that is distinctly different from the undoped BiFeO3 films.
                      Cover page: Manipulating magnetoelectric energy landscape in multiferroics
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