Browsing by Author "Hardy, Will J."

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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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    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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    Shot noise detection in hBN-based tunnel junctions
    (AIP Publishing, 2017) Zhou, Panpan; Hardy, Will J.; Watanabe, Kenji; Taniguchi, Takashi; Natelson, Douglas; Applied Physics Program; Smalley-Curl Institute
    High quality Au/hBN/Au tunnel devices are fabricated using transferred atomically thin hexagonal boron nitride as the tunneling barrier. All tunnel junctions show tunneling resistance on the order of several kΩ/μm2. Ohmic I-V curves at small bias with no signs of resonances indicate the sparsity of defects. Tunneling current shot noise is measured in these devices, and the excess shot noise shows consistency with theoretical expectations. These results show that atomically thin hBN is an excellent tunnel barrier, especially for the study of shot noise properties, and this can enable the study of the tunneling density of states and shot noise spectroscopy in more complex systems.
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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.