Browsing by Author "Ji, Heng"

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    Hydrogen Diffusion and Stabilization in Single-Crystal VO2 Micro/Nanobeams by Direct Atomic Hydrogenation
    (American Chemical Society, 2014) Lin, Jian; Ji, Heng; Swift, Michael W.; Hardy, Will J.; Peng, Zhiwei; Fan, Xiujun; Nevidomskyy, Andriy H.; Tour, James M.; Natelson, Douglas; Smalley Institute for Nanoscale Science and Technology
    We report measurements of the diffusion of atomic hydrogen in single crystalline VO2 micro/nanobeams by direct exposure to atomic hydrogen, without catalyst. The atomic hydrogen is generated by a hot filament, and the doping process takes place at moderate temperature (373 K). Undoped VO2 has a metal-to-insulator phase transition at ∼340 K between a high-temperature, rutile, metallic phase and a low-temperature, monoclinic, insulating phase with a resistance exhibiting a semiconductor-like temperature dependence. Atomic hydrogenation results in stabilization of the metallic phase of VO2 micro/nanobeams down to 2 K, the lowest point we could reach in our measurement setup. Optical characterization shows that hydrogen atoms prefer to diffuse along the c axis of rutile (a axis of monoclinic) VO2, along the oxygen “channels”. Based on observing the movement of the hydrogen diffusion front in single crystalline VO2 beams, we estimate the diffusion constant for hydrogen along the c axis of the rutile phase to be 6.7 × 10–10 cm2/s at approximately 373 K, exceeding the value in isostructural TiO2 by ∼38×. Moreover, we find that the diffusion constant along the c axis of the rutile phase exceeds that along the equivalent a axis of the monoclinic phase by at least 3 orders of magnitude. This remarkable change in kinetics must originate from the distortion of the “channels” when the unit cell doubles along this direction upon cooling into the monoclinic structure. Ab initio calculation results are in good agreement with the experimental trends in the relative kinetics of the two phases. This raises the possibility of a switchable membrane for hydrogen transport.
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    Hydrogen doping and the metal-insulator phase transition in vanadium dioxide
    (2015-04-22) Ji, Heng; Natelson, Douglas; Du, Rui-rui; Biswal, Sibani L
    Strongly correlated systems represent a major topic of study in condensed matter physics. Vanadium dioxide, a strongly correlated material, has a sharp metal-to-insulator phase transition at around 340 K (67 °C), a moderate temperature which can be easily achieved. Its potential as a functional material in optical switches and semiconductor applications has attracted a great deal of attention in recent years. In this thesis, after a detailed introduction of this material and the methods we used to grow VO2 in our lab (Chapter 1), I will discuss our attempts to modulate the electronic properties and phase transition of single-crystal VO2 samples. It started with a plan to use ionic liquid to apply an electrostatic gate to this material. Although modulation of the resistance was observed, we also discovered an unexpected electrochemical reaction, leading to a suspicion that hydrogen doping is the reason for the change of properties of VO2 (Chapter 2). Next, a series of experiments were performed to systematically study the mechanism of this hydrogen doping process and to characterize the hydrogenated VO2. Our collaborators also provided supporting simulation results to interpret these phenomena from a theoretical point of view, as well as results from synchrotron x-ray diffraction and neutron diffraction experiments. From all these studies, we confirmed the existence of the hydrogen intercalation in VO2 (Chapter 3), and further, plotted the phase diagram as a function of temperature and hydrogen concentration (Chapter 5). We also found that this diffusion process prefers the rutile crystal structure of VO2 (i.e. metallic phase) and specifically, its c-axis (Chapter 4). Finally, the low-temperature electric transport properties of the hydrogenated VO2 material have been studied for the first time, and interesting magneto-resistance responses will be discussed (chapter 6).
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    In Situ Diffraction Study of Catalytic Hydrogenation of VO2: Stable Phases and Origins of Metallicity
    (American Chemical Society, 2014) Filinchuk, Yaroslav; Tumanov, Nikolay A.; Ban, Voraksmy; Ji, Heng; Wei, Jiang; Swift, Michael W.; Nevidomskyy, Andriy H.; Natelson, Douglas
    Controlling electronic population through chemical doping is one way to tip the balance between competing phases in materials with strong electronic correlations. Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high-temperature, tetragonal, metallic state (T) and a low-temperature, monoclinic, insulating state (M1), driven by electronﾖelectron and electronﾖlattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the detailed effects of intercalated H on the crystal and electronic structure of the resulting hydride have not been previously reported. Here we present synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states and discuss the physical basis by which hydrogen stabilizes conducting phases, in the context of the metalﾖinsulator transition.
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    Nanostructure investigations of nonlinear differential conductance in NdNiO3 thin films
    (American Physical Society, 2014) Hardy, Will J.; Ji, Heng; Mikheev, Evgeny; Stemmer, Susanne; Natelson, Douglas; Rice Quantum Institute
    Transport measurements on thin films of NdNiO3 reveal a crossover to a regime of pronounced nonlinear conduction below the well-known metal-insulator transition temperature. The evolution of the transport properties at temperatures well below this transition appears consistent with a gradual formation of a gap in the holelike Fermi surface of this strongly correlated system. As T is decreased below the nominal transition temperature, transport becomes increasingly non-Ohmic, with a model of Landau-Zener breakdown becoming most suited for describing I(V) characteristics as the temperature approaches 2 K.
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    Sequential insulator-metal-insulator phase transitions of VO2 triggered by hydrogen doping
    (American Physical Society, 2017) Chen, Shi; Wang, Zhaowu; Fan, Lele; Chen, Yuliang; Ren, Hui; Ji, Heng; Natelson, Douglas; Huang, Yingying; Jiang, Jun; Zou, Chongwen
    As a typical correlated oxide, V O 2 has attracted significant attentions due to its pronounced thermal-driven metal-insulator transition. Regulating electronic density through electron doping is an effective way to modulate the balance between competing phases in strongly correlated materials. However, the electron-doping triggered phase transitions in V O 2 as well as the intermediate states are not fully explored. Here, we report a controlled and reversible phase transition in V O 2 films by continuous hydrogen doping. Metallic and insulating phases are successively observed at room temperature as the doping concentration increases. The doped electrons linearly occupy V 3 d -O 2 p hybridized orbitals and consequently modulate the filling of the V O 2 conduction band edge states, resulting in the electron-doping driven continuous phase transitions. These results suggest the exceptional sensitivity of V O 2 electronic properties to electron concentration and orbital occupancy, providing key information for the phase transition mechanism.
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    Thermally driven analog of the Barkhausen effect at the metal-insulator transition in vanadium dioxide
    (AIP Publishing LLC., 2014) Huber-Rodriguez, Benjamin; Kwang, Siu Yi; Hardy, Will J.; Ji, Heng; Chen, Chih-Wei; Morosan, Emilia; Natelson, Douglas
    The physics of the metal-insulator transition (MIT) inﾠvanadiumﾠdioxide remains a subject of intense interest. Because of the complicating effects of elastic strain on the phase transition, there is interest in comparatively strain-free means of examining VO2ﾠmaterial properties.ﾠWe report contact-free, low-strain studies of the MIT through an inductive bridge approach sensitive to the magnetic response of VO2ﾠpowder.ﾠRather than observing the expected step-like change inﾠsusceptibilityﾠat the transition, we argue that theﾠmeasuredﾠresponse is dominated by an analog of theﾠBarkhausen effect,ﾠdue to the extremely sharp jump in the magnetic response of each grain as a function of time as theﾠmaterialﾠis cycled across the phase boundary. This effect suggests that futureﾠmeasurementsﾠcould access the dynamics of this and similar phase transitions.
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    Very large magnetoresistance in Fe0.28TaS2 single crystals
    (American Physical Society, 2015) Hardy, Will J.; Chen, Chih-Wei; Marcinkova, A.; Ji, Heng; Sinova, Jairo; Natelson, D.; Morosan, E.
    Magnetic moments intercalated into layered transition metal dichalcogenides are an excellent system for investigating the rich physics associated with magnetic ordering in a strongly anisotropic, strong spin-orbit coupling environment. We examine electronic transport and magnetization in Fe0.28TaS2, a highly anisotropic ferromagnet with a Curie temperature TC∼68.8 K. We find anomalous Hall data confirming a dominance of spin-orbit coupling in the magnetotransport properties of this material, and a remarkably large field-perpendicular-to-plane magnetoresistance (MR) exceeding 60% at 2 K, much larger than the typical MR for bulk metals, and comparable to state-of-the-art giant MR in thin film heterostructures, and smaller only than colossal MR in Mn perovskites or high mobility semiconductors. Even within the FexTaS2 series, for the current x=0.28 single crystals the MR is nearly 100× higher than that found previously in the commensurate compound Fe0.25TaS2. After considering alternatives, we argue that the large MR arises from spin-disorder scattering in the strong spin-orbit coupling environment, and suggest that this can be a design principle for materials with large MR.