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Yazar: Karakurt, İsmail ×

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    Nonreciprocal switching of VO2 thin films on microstructured surfaces
    (Optical Soc Amer, 2010-05-15) Karakurt, İsmail; Adams, Charles H; Leiderer, Paul; Boneberg, Johannes; Haglund, Richard F.
    We demonstrate that the combination of near-field focusing and a switchable oxide layer permits construction of a modulator with different optical switching thresholds in two opposing directions. For that purpose VO2 layers are deposited onto spherical micrometer-sized particles. The phase transition in VO2 is induced by a nanosecond-pulsed Nd:YAG laser from two different directions. The measured thresholds differ by a factor of 2.4, consistent with calculated differences in the intensities in the two directions.
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    Magnetoresistivity of a weakly-screened, low-density, two-dimensional electron liquid
    (Physical Soc Japan, 2013-07) Karakurt, İsmail; Dahm, Arnold J.
    We probe the strength of electron-electron interactions at intermediate magnetic fields in the classical regime using magnetoconductivity measurements of two-dimensional, non-degenerate electrons on liquid helium. We span both the independent-electron regime, where the data are qualitatively described by the self-consistent Born approximation (SCBA), and the strongly-interacting electron (Drude) regime. We observe a crossover from SCBA to Drude theory at finite magnetic fields as a function of electron density. The SCBA magnetoresistance is found to be density dependent. Our data confirm the theory for magnetoresistivity in a weakly screened, two-dimensional electron gas, and demonstrate that electron-electron interactions are important to very low densities.
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    Electrochromic switching of WO3 nanostructures and thin films
    (Springer-Verlag, 2006-04) Karakurt, İsmail; Boneberg, Johannes; Leiderer, Paul
    We present transmission measurements through tungsten tri-oxide nanostructures and thin films prepared by sol-gel process on micro-contact printed substrates. Identical electrochromic switching times are found for both the nanostructures and the bulk films with equal thicknesses upon intercalation of H+ ions. We attribute the large change in the transmission through nanostructures at 632 nm, which can not be solely explained by absorption, to diffraction effects.
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    Transmission increase upon switching of VO2 thin films on microstructured surfaces
    (American Institute of Physics, 2007-08-27) Karakurt, İsmail; Boneberg, Johannes; Leiderer, Paul; Lopez, Rene; Halabica, Andrej; Haglund, Richard F.
    The authors compare transmission measurements of near-infrared light through VO2 thin films on smooth substrates and on ordered arrays of silica microspheres. When the samples are heated above the critical temperature for the semiconductor-metallic phase transition, smooth thin films show reduced transmission independent of thickness; however, the VO2 film deposited on the microspheres may show either reduced or enhanced transmission, depending on VO2 film thickness. They show that this at a first glance, unexpected behavior is directly related to the change of scattering efficiency upon the phase transition. This suggests that optical transmission through thin-film microstructures could be tuned by an appropriate combination of microsphere ordering and VO2 film thickness.
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    Size-dependent self-organization of colloidal particles on chemically patterned surfaces
    (SP Zhejiang University Press, 2006-04-14) Karakurt, İsmail; Leiderer, Paul; Boneberg, Johannes
    A study of the self-organization of colloidal particles during the evaporation of particle solutions on chemically patterned surfaces is presented. On a surface with hydrophilic and hydrophobic regions, colloidal particles form compact structures on the hydrophilic sites. When a colloidal solution containing a mixture of particles with a variation in size is used, the number density of each type of particle deposited on the hydrophilic islands after evaporation decreases with increasing particle size. This makes it possible to produce a concentration gradient of the particles on islands of different sizes. It is shown that this technique could allow for particle separation.
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    Tunneling rates of single electrons on liquid helium in an extracting field
    (Springer/Plenum Publishers, 2009-02) Karakurt, İsmail
    We calculated the tunneling rates of single electrons from the quasi-stationary states on both liquid He-4 and He-3 in an extracting electric field. The rates were obtained from the widths of the resonance lineshapes of the asymptotic amplitude of the wave function for the electron. The calculations were carried out in the limit of strong tunneling which leads to tunneling rates of the order of 1 GHz, and is of recent interest in a proposed quantum-computer system using the electronic states of electrons on helium as qubits. We find that the resonances can, in general, be described by Fano lineshapes. Our results, in addition to presenting quantitative information involving the read-out operations of qubits, clarify that when tunneling is weak the resonances are sharp and more accurately Lorentzian and that the observed Fano lineshapes result from strong tunneling leading to fat resonances and hence asymmetry.
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    Particle size and shell thickness dependence of the light intensity enhancement in the cap layers of Ag, Au, Al and SiO2@TiO2 core-shell nanostructures
    (Wiley-V C H Verlag GMBH, 2018-03-22) Başcura, Erdem Berkay; Karakurt, İsmail
    We present a systematic study of the light intensity enhancement in the shell layers of half burrried Ag, Au, Al and SiO2 spherical nanoparticles capped with TiO2. We calculated the light intensity profiles near these structures when they are illuminated by a UV light at 380 nm. The calculations cover the particle-size range between 20 and 180 nm, and the cap thickness range between 6 and 80 nm. Our data is the first report on the intensity enhancement in the shell layers of TiO2-capped Al nanoparticles and Al@TiO2 core-shell nanostructures. We obtain, in the cap layers, an average intensity enhancement of similar to 3.7, similar to 6.1, similar to 2.4 and similar to 2.7 for a cap thickness of similar to 30nm above Ag, Al, Au and SiO2 particles. Taking into account the increase in the active surface area, these numbers can be multiplied by a factor of two, for each capped-nanoparticle system. The optimal Ag, Au, Al and SiO2-particle sizes for a 30-nm cap-thickness are found to be 120 nm, 80 nm, 140 nm, and 80 nm. While half-buried and capped-nanoparticle systems resemble the typical core-shell nanoparticles, the intensity enhancements are not equivalent. We compare the intensity increases in these two systems at the optimal sizes for Ag and Al particles for a 30-nm shell thickness.
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    Time-dependent lateral diffusion in WO3 thin films
    (Elsevier B.V., 2023-10) Karakurt, İsmail; Başar, Günay; Leiderer, Paul; Parlatan, Şeyma; Arapoğlu, Nihan
    We have studied the anomalous lateral diffusion process in thin tungsten trioxide films by optical means. The diffusion process seems to start at imperfections within the film a few seconds after the H+ ion intercalation begins, and progresses parallel to the surface of the film. We measured the mean square displacement of the diffusion front and used its time-dependence to calculate the instantaneous diffusion coefficient. The anomalous exponents are found to be 2.24 and 2.92 for 400 nm and 270 nm thick films, respectively. We explain the observed large diffusion coefficients and depth dependence of the expansion of the film by interfacial job-sharing diffusion of electrons and protons. Raman measurements were also carried out on virgin films, and on films after the lateral diffusion. Although the observed spectrum after the lateral diffusion is, in general, consistent with the literature for H+ intercalated films, we observe an additional strong band at 855 cm?1. This lateral diffusion process is observed to be irreversible; therefore, it has to be avoided in electrochromic switching devices based on WO3.