Publications by authors named "Alex Frano"

10 Publications

  • Page 1 of 1

Low-temperature emergent neuromorphic networks with correlated oxide devices.

Proc Natl Acad Sci U S A 2021 Aug;118(35)

Department of Physics, University of California San Diego, La Jolla, CA 92093;

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.
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http://dx.doi.org/10.1073/pnas.2103934118DOI Listing
August 2021

Short-Range Nematic Fluctuations in Sr_{1-x}Na_{x}Fe_{2}As_{2} Superconductors.

Phys Rev Lett 2021 Mar;126(10):107001

Department of Physics, University of California, Berkeley, California 94720, USA.

Interactions between nematic fluctuations, magnetic order and superconductivity are central to the physics of iron-based superconductors. Here we report on in-plane transverse acoustic phonons in hole-doped Sr_{1-x}Na_{x}Fe_{2}As_{2} measured via inelastic x-ray scattering, and extract both the nematic susceptibility and the nematic correlation length. By a self-contained method of analysis, for the underdoped (x=0.36) sample, which harbors a magnetically ordered tetragonal phase, we find it hosts a short nematic correlation length ξ∼10  Å and a large nematic susceptibility χ_{nem}. The optimal-doped (x=0.55) sample exhibits weaker phonon softening effects, indicative of both reduced ξ and χ_{nem}. Our results suggest short-range nematic fluctuations may favor superconductivity, placing emphasis on the nematic correlation length for understanding the iron-based superconductors.
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http://dx.doi.org/10.1103/PhysRevLett.126.107001DOI Listing
March 2021

Perovskite neural trees.

Nat Commun 2020 05 7;11(1):2245. Epub 2020 May 7.

School of Materials Engineering, Purdue University, West Lafayette, IN, 47907, USA.

Trees are used by animals, humans and machines to classify information and make decisions. Natural tree structures displayed by synapses of the brain involves potentiation and depression capable of branching and is essential for survival and learning. Demonstration of such features in synthetic matter is challenging due to the need to host a complex energy landscape capable of learning, memory and electrical interrogation. We report experimental realization of tree-like conductance states at room temperature in strongly correlated perovskite nickelates by modulating proton distribution under high speed electric pulses. This demonstration represents physical realization of ultrametric trees, a concept from number theory applied to the study of spin glasses in physics that inspired early neural network theory dating almost forty years ago. We apply the tree-like memory features in spiking neural networks to demonstrate high fidelity object recognition, and in future can open new directions for neuromorphic computing and artificial intelligence.
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http://dx.doi.org/10.1038/s41467-020-16105-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7206050PMC
May 2020

Charge ordering in superconducting copper oxides.

J Phys Condens Matter 2019 Dec 12;32(37):374005. Epub 2019 Dec 12.

Department of Physics, University of California, San Diego, CA 92093, United States of America.

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.
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http://dx.doi.org/10.1088/1361-648X/ab6140DOI Listing
December 2019

Local Orthorhombicity in the Magnetic C_{4} Phase of the Hole-Doped Iron-Arsenide Superconductor Sr_{1-x}Na_{x}Fe_{2}As_{2}.

Phys Rev Lett 2017 Nov 30;119(18):187001. Epub 2017 Oct 30.

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

We report on temperature-dependent pair distribution function measurements of Sr_{1-x}Na_{x}Fe_{2}As_{2}, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic C_{4} phase. Quantitative refinements indicate that the instantaneous local structure in the C_{4} phase comprises fluctuating orthorhombic regions with a length scale of ∼2  nm, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic C_{4} phase and the neighboring C_{2} and superconducting phases.
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http://dx.doi.org/10.1103/PhysRevLett.119.187001DOI Listing
November 2017

Correlated states in β-LiIrO driven by applied magnetic fields.

Nat Commun 2017 10 16;8(1):961. Epub 2017 Oct 16.

Department of Physics, University of California, Berkeley, CA, 94720, USA.

Magnetic honeycomb iridates are thought to show strongly spin-anisotropic exchange interactions which, when highly frustrated, lead to an exotic state of matter known as the Kitaev quantum spin liquid. However, in all known examples these materials magnetically order at finite temperatures, the scale of which may imply weak frustration. Here we show that the application of a relatively small magnetic field drives the three-dimensional magnet β-LiIrO from its incommensurate ground state into a quantum correlated paramagnet. Interestingly, this paramagnetic state admixes a zig-zag spin mode analogous to the zig-zag order seen in other Mott-Kitaev compounds. The rapid onset of the field-induced correlated state implies the exchange interactions are delicately balanced, leading to strong frustration and a near degeneracy of different ground states.Materials with a Kitaev spin liquid ground state are sought after as models of quantum phases but candidates so far form either zig-zag or incommensurate magnetic order. Ruiz et al. find a crossover between these states in β-LiIrO under weak magnetic fields, indicating strongly frustrated spin interactions.
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http://dx.doi.org/10.1038/s41467-017-01071-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5643435PMC
October 2017

Modular soft x-ray spectrometer for applications in energy sciences and quantum materials.

Rev Sci Instrum 2017 Jan;88(1):013110

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

Over the past decade, the advances in grating-based soft X-ray spectrometers have revolutionized the soft X-ray spectroscopies in materials research. However, these novel spectrometers are mostly dedicated designs, which cannot be easily adopted for applications with diverging demands. Here we present a versatile spectrometer design concept based on the Hettrick-Underwood optical scheme that uses modular mechanical components. The spectrometer's optics chamber can be used with gratings operated in either inside or outside orders, and the detector assembly can be reconfigured accordingly. The spectrometer can be designed to have high spectral resolution, exceeding 10 000 resolving power when using small source (∼1μm) and detector pixels (∼5μm) with high line density gratings (∼3000 lines/mm), or high throughput at moderate resolution. We report two such spectrometers with slightly different design goals and optical parameters in this paper. We show that the spectrometer with high throughput and large energy window is particularly useful for studying the sustainable energy materials. We demonstrate that the extensive resonant inelastic X-ray scattering (RIXS) map of battery cathode material LiNiCoMnO can be produced in few hours using such a spectrometer. Unlike analyzing only a handful of RIXS spectra taken at selected excitation photon energies across the elemental absorption edges to determine various spectral features like the localized dd excitations and non-resonant fluorescence emissions, these features can be easily identified in the RIXS maps. Studying such RIXS maps could reveal novel transition metal redox in battery compounds that are sometimes hard to be unambiguously identified in X-ray absorption and emission spectra. We propose that this modular spectrometer design can serve as the platform for further customization to meet specific scientific demands.
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http://dx.doi.org/10.1063/1.4974356DOI Listing
January 2017

Ubiquitous interplay between charge ordering and high-temperature superconductivity in cuprates.

Science 2014 Jan 19;343(6169):393-6. Epub 2013 Dec 19.

Joseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544, USA.

Besides superconductivity, copper-oxide high-temperature superconductors are susceptible to other types of ordering. We used scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of charge ordering in the high-temperature superconductor Bi2Sr2CaCu2O(8+x). Depending on the hole concentration, the charge ordering in this system occurs with the same period as those found in Y-based or La-based cuprates and displays the analogous competition with superconductivity. These results indicate the similarity of charge organization competing with superconductivity across different families of cuprates. We observed this charge ordering to leave a distinct electron-hole asymmetric signature (and a broad resonance centered at +20 milli-electron volts) in spectroscopic measurements, indicating that it is likely related to the organization of holes in a doped Mott insulator.
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http://dx.doi.org/10.1126/science.1243479DOI Listing
January 2014

Layer selective control of the lattice structure in oxide superlattices.

Adv Mater 2014 Jan 24;26(2):258-62. Epub 2013 Oct 24.

Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569, Stuttgart, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie, Wilhelm-Conrad-Röntgen-Campus BESSY II, Albert-Einstein-Str. 15, D-12489, Berlin, Germany.

A combined synchrotron X-ray diffraction and transmission electron microscopy study reveals a structural phase transition controlled by the overall thickness of epitaxial nickelate-aluminate superlattices. The transition between uniform and twin-domain states is confined to the nickelate layers and leaves the aluminate layers unaffected.
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http://dx.doi.org/10.1002/adma.201303483DOI Listing
January 2014

Orbital reflectometry of oxide heterostructures.

Nat Mater 2011 Mar 6;10(3):189-93. Epub 2011 Feb 6.

The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could not thus far be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ~3% in the occupation of Ni e(g) orbitals in adjacent atomic layers of a LaNiO(3)-LaAlO(3) superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.
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http://dx.doi.org/10.1038/nmat2958DOI Listing
March 2011
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