Person: Ramanathan, Shriram
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Ramanathan
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Shriram
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Ramanathan, Shriram
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Publication Origins of bad-metal conductivity and the insulator–metal transition in the rare-earth nickelates(Nature Publishing Group, 2014) Ramanathan, Shriram; Jaramillo, Rafael; Ha, Sieu D.; Silevitch, D.M.For most metals, increasing temperature (T) or disorder hastens electron scattering. The electronic conductivity (σ) decreases as T rises because electrons are more rapidly scattered by lattice vibrations. The value of σ decreases as disorder increases because electrons are more rapidly scattered by imperfections in the material. This is the scattering rate hypothesis, which has guided our understanding of metal conductivity for over a century. However, for so-called bad metals with very low σ this hypothesis predicts scattering rates so high as to conflict with Heisenberg’s uncertainty principle1, 2. Bad-metal conductivity has remained a puzzle since its initial discovery in the 1980s in high-temperature superconductors. Here we introduce the rare-earth nickelates (RNiO3, R = rare-earth) as a class of bad metals. We study SmNiO3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni–O covalence explains bad-metal conductivity and the insulator–metal transition. This interaction shifts spectral weight over the large energy scale established by the Ni–O orbital interaction, thus enabling very low σ without violating the uncertainty principle.Publication Synthesis of Vanadium Dioxide Thin Films on Conducting Oxides and Metal–Insulator Transition Characteristics(Elsevier, 2012) Cui, Yanjie; Wang, Xinwei; Zhou, You; Gordon, Roy; Ramanathan, ShriramWe report on growth and physical properties of vanadium dioxide \((VO_2)\) films on model conducting oxide underlayers (Nb-doped \(SrTiO_3\) and \(RuO_2\) buffered \(TiO_2\) single crystals). The \(VO_2\) films, synthesized by rf sputtering, are highly textured as seen from X-ray diffraction. The \(VO_2\) film grown on Nb doped \(SrTiO_3\) shows over two orders of magnitude metal–insulator transition, while \(VO_2\) film on \(RuO_2\) buffered \(TiO_2\) shows a smaller resistance change but with an interesting two step transition. X-ray photoelectron spectroscopy has been performed as a function of depth on both sets of structures to provide mechanistic understanding of the transition characteristics. We then investigate voltage-driven transition in the \(VO_2\) films grown on Nb-doped \(SrTiO_3\) substrate as a function of temperature. The present study contributes to efforts towards correlated oxide electronics utilizing phase transitions.Publication Compositional Tuning of Ultrathin Surface Oxides on Metal and Alloy Substrates Using Photons: Dynamic Simulations and Experiments(American Physical Society, 2010) Chang, Chia-Lin; Sankaranarayanan, Subramanian K. R. S.; Ruzmetov, Dmitry; Engelhard, Mark H.; Kaxiras, Efthimios; Ramanathan, ShriramWe report on the ability to modify the structure and composition of ultrathin oxides grown on Ni and Ni-Al alloy surfaces at room temperature utilizing photon illumination. We find that the nickel-oxide formation is enhanced in the case of oxidation under photo-excitation. The enhanced oxidation kinetics of nickel in 5% Ni-Al alloy is corroborated by experimental and simulation studies of natural and photon-assisted oxide growth on pure Ni(100) surfaces. In case of pure Ni substrates, combined x-ray photoelectron spectroscopy analysis, and atomic force microscope current mapping support the deterministic role of the structure of nickel passive-oxide films on their nanoscale corrosion resistance. Atomistic simulations involving dynamic charge transfer predict that the applied electric field overcomes the activation-energy barrier for ionic migration, leading to enhanced oxygen incorporation into the oxide, enabling us to tune the mixed-oxide composition at atomic length scales. Atomic scale control of ultrathin oxide structure and morphology in the case of pure substrates as well as compositional tuning of complex oxide in the case of alloys leads to excellent passivity as verified from potentiodynamic polarization experiments.Publication High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes(Wiley-Blackwell, 2011) Likovich, Edward Michael; Jaramillo, Rafael; Russell, Kasey; Ramanathan, Shriram; Narayanamurti, VenkateshPublication Dynamic control of light emission faster than the lifetime limit using VO2 phase-change(Nature Pub. Group, 2015) Cueff, Sébastien; Li, Dongfang; Zhou, You; Wong, Franklin J.; Kurvits, Jonathan A.; Ramanathan, Shriram; Zia, RashidModulation is a cornerstone of optical communication, and as such, governs the overall speed of data transmission. Currently, the two main strategies for modulating light are direct modulation of the excited emitter population (for example, using semiconductor lasers) and external optical modulation (for example, using Mach–Zehnder interferometers or ring resonators). However, recent advances in nanophotonics offer an alternative approach to control spontaneous emission through modifications to the local density of optical states. Here, by leveraging the phase-change of a vanadium dioxide nanolayer, we demonstrate broadband all-optical direct modulation of 1.5 μm emission from trivalent erbium ions more than three orders of magnitude faster than their excited state lifetime. This proof-of-concept demonstration shows how integration with phase-change materials can transform widespread phosphorescent materials into high-speed optical sources that can be integrated in monolithic nanoscale devices for both free-space and on-chip communication.Publication Narrow Band Defect Luminescence from AI-doped ZnO Probed by Scanning Tunneling Cathodoluminescence(American Institute of Physics, 2011) Likovich, Edward M.; Jaramillo, Rafael; Russell, Kasey; Ramanathan, Shriram; Narayanamurti, VenkateshWe present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor.Publication Synthesis and Variable Temperature Electrical Conductivity Studies of Highly Ordered TiO2 Nanotubes(Springer Verlag, 2009) Asmatulu, Ramazan; Karthikeyan, Annamalai; Bell, David C.; Ramanathan, Shriram; Aziz, MichaelRafts of aligned, high aspect ratio TiO2 nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225-400 °C. Length, outer diameter and wall thickness of the nanotubes were approximately 60-80 µm, 160 nm and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO2 nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment attemperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250 - 350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.Publication Nanoscale Imaging and Control of Resistance Switching in VO[sub]2 at Room Temperature(American Institute of Physics, 2010) Kim, Jeehoon; Ko, Changhyun; Frenzel, Alex J.; Ramanathan, Shriram; Hoffman, JennyWe demonstrate controlled local phase switching of a VO[sub]2 film using a biased conducting atomic force microscope tip. After application of an initial, higher ‘training’ voltage, the resistance transition is hysteretic with IV loops converging upon repeated voltage sweep. The threshold Vset to initiate the insulator-to-metal transition is on order ∼ 5 V at room temperature, and increases at low temperature. We image large variations in Vset from grain to grain. Our imaging technique opens up the possibility for an understanding of the microscopic mechanism of phase transition in VO[sub]2 as well as its potential relevance to solid state devices.