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Browsing Science by Author "Aas, Shahnaz"

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    Geometric band properties in strained monolayer transition metal dichalcogenides using simple band structures
    (American Institute of Physics, 2019) Aas, Shahnaz; Bulutay, Ceyhun
    Monolayer transition metal dichalcogenides (TMDs) bare large Berry curvature hotspots readily exploitable for geometric band effects. Tailoring and enhancement of these features via strain is an active research direction. Here, we consider spinless two- and three-band and spinful four-band models capable to quantify the Berry curvature and the orbital magnetic moment of strained TMDs. First, we provide a k⋅p parameter set for MoS2, MoSe2, WS2, and WSe2 in the light of the recently released ab initio and experimental band properties. Its validity range extends from the K valley edge to about one hundred millielectron volts into valence and conduction bands for these TMDs. To expand this over a larger part of the Brillouin zone, we incorporate strain to an available three-band tight-binding Hamiltonian. With these techniques, we demonstrate that both the Berry curvature and the orbital magnetic moment can be doubled compared to their intrinsic values by applying typically a 2.5% biaxial tensile strain. These simple band structure tools can find application in the quantitative device modeling of the geometric band effects in strained monolayer TMDs.
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    Graphene-Based Adaptive Thermal Camouflage
    (American Chemical Society, 2018) Salihoğlu, Ömer; Uzlu, H. B.; Yakar, Ozan; Aas, Shahnaz; Balci, Osman; Kakenov, Nurbek; Balci, S.; Olcum, S.; Süzer, Şefik; Kocabas, Coşkun
    In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin (<50 μm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.
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    Strain dependence of photoluminescence and circular dichroism in transition metal dichalcogenides: a k. p analysis
    (OSA - The Optical Society, 2018) Aas, Shahnaz; Bulutay, Ceyhun
    Within a two-band k p method we analyze different types of strain for the k valley optical characteristics of a freestanding monolayer mos2 Mose2 Ws2 andwse2. we predict that circular polarization selectivity for energies above the direct transition onset deteriorates/improves by tensile/compressive strain. wide range of available strained-sample photoluminescence data can be reasonably reproduced by this simple bandstructure combined with accounting for excitons at a variational level. according to this model strain impacts optoelectronic properties through its hydrostatic component.Whereas the shear strain only causes a rigid wavevector shift of the valley. furthermore Under the stress loading of flexible substrates the presence of poisson's effect or the lack of it are examined individually for the reported measurements.