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Frenzel, Alex J.

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Frenzel

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Alex J.

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Frenzel, Alex J.

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2010 - 20152

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Author's Publications: search.filters.isAuthorOfPublication.391d65b0-b996-4aee-8c62-8eae9c950b5d

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    Terahertz Electrodynamics of Dirac Fermions in Graphene
    (2015-05-13) Frenzel, Alex J.; Hoffman, Jenny; Gedik, Nuh; Sachdev, Subir
    Charge carriers in graphene mimic two-dimensional massless Dirac fermions with linear energy dispersion, resulting in unique optical and electronic properties. They exhibit high mobility and strong interaction with electromagnetic radiation over a broad frequency range. Interband transitions in graphene give rise to pronounced optical absorption in the mid-infrared to visible spectral range, where the optical conductivity is close to a universal value $\sigma_0 = \pi e^2/2h$. Free-carrier intraband transitions, on the other hand, cause low-frequency absorption, which varies significantly with charge density and results in strong light extinction at high carrier density. These properties together suggest a rich variety of possible optoelectronic applications for graphene. In this thesis, we investigate the optoelectronic properties of graphene by measuring transient photoconductivity with optical pump-terahertz probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations are accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Our measurements also reveal the non-monotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions.
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    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, Jenny
    We 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.