Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Electrostatic modeling of contacts considering integer quantum Hall systems Deniz Eksi, 1* Ozge Kilicoglu 1 and Afif Siddiki 2,3 1 Trakya University, Department of Physics, 22030 Edirne, Turkey 2 Istanbul University, Department of Physics, 34134 Vezneciler / Istanbul, Turkey 3 Harvard University, Department of Physics, 02138 Cambridge / MA, USA Abstract-In this work, we calculate electron and current density distributions in the edge and bulk of 2D samples, namely focusing on ideal and non-ideal contacts. A three dimensional Poisson equation is solved selfconsistently to obtain the potential profile in the absence of an external magnetic field for gate and etching defined devices. In the presence of a perpendicular magnetic field, we obtained the locations of the incompressible strips, taking into account electron-electron interaction within the Thomas-Fermi theory of screening. Using a local version of Ohm’s law, together with a relevant conductivity model, we also calculate the current distribution. We observed that the incompressible strips can be on the edge or at the center of bulk under the presence of different magnetic fields. Regardless of what system one is interested in theoretically, the metallic contacts deposited on the two dimensional charge systems are the most important ingredients of the measurement. Up to now no comprehensive study of contacts is accessible in the literature that both take into account interactions, device geometry, impurities and formation incompressible and compressible regions, which exist due to Landau quantization. Ι It is a formidable task to model the sudden change of density of states (DOS) near the contacts, while at the metallic region the DOS is approximately infinite, whereas at the 2D system the DOS is either constant (without magnetic field B) or varies between no DOS at the Fermi energy (incompressible [1,2]) and high DOS (compressible, with a degeneracy of eB/h). Here, we present our self-consistently calculated results which takes into account Landau quantization and direct Coulomb interactions at a mean-field approximation level [3]. As seen from in Figure1 e, the ideal contacts are modeled as equipotential lines, where no density poor region resides just in front of the contacts. In Figure1 a and c, the nonideal contacts are described by density fluctuations near the transition region, as indicated by experiments [4,5] which essentially influence the current distribution drastically. We investigated theoretically the current and density distributions considering high B fields and intermediate electron density concentrations in the close proximity of the contacts, together with the entire sample. Our results point that, the assumption of ideal contacts cannot be justified, when considering density inhomogeneities, hence, models based on ideal contacts has to be revised [6,7]. Moreover, we present clearly the formation of hotspots depending on the current amplitude and field direction. The authors would like to acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK) for supporting under grant no 109T083. * Corresponding author: denizeksi@gmail.com Figure 1: The color coded ( x, y) ν for = Ω 3.72, 3.02, 2.26 and k BT E 0 F = 0.058, 0.094, 0.11 (a,c,e) and corresponding current distribution indicated by the arrows. The calculations are done for a fixed external current driven in the y direction −4 j x, y = 0,84× 10 A m . The 2DES is 150 nm below the ( ( ) ) surface. [1] D. B. Chklovskii, B. I. Shklovskii, and L. I. Glazman, Phys. Rev. B 46, 4026 (1992). [2] A. M. Chang, Solid State Commun. 74, 871 (1990). [3] A. Siddiki and R. R. Gerhardts, Phys. Rev. B 70, 195335 (2004). [4] E. Ahlswede, J. Weis, K. von Klitzing, and K. Eberl, Physica E 12, 165 (2002). [5] O. Goektas, Ph.D. thesis, Stuttgart University, 2009. [6] B. I. Halperin, Phys. Rev. B 25, 2185 (1982). [7] M. Büttiker, Phys. Rev. Lett. 57, 1761 (1986). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 238
Poster Session, Tuesday, June 15 Theme A1 - B702 Effects of Artificial Pinning Centers on NdBa 2 Cu 3 O 7-δ Thin Films Prepared by PLD Erdal Sönmez 1* , Mücahit Yılmaz 2 , Mehmet Yılmaz 1 , Refik Dilber 1 , Bahattin Düzgün 1 and Mehmet Ertuğrul 3 1 K. K. Education Faculty, Department of Physics, Ataturk University, Erzurum, Turkey 2 Education Faculty, Department of Physics, Selcuk University, Konya, Turkey 3 Engineering Faculty, Department of Electric-Electronic, Ataturk University, Erzurum, Turkey Abstract— In this study, the effect of artificial pinning centers (APC) on the critical properties of NdBa 2 Cu 3 O 7-δ (Nd-123) superconductive thin films grown on Ni-W metallic substrates was investigated. Nd-123 superconductive ceramic materials were prepared by solid state reaction technique and then this ceramic material was used as target for the producing Nd-123 superconductive thin films with pulsed laser deposition (PLD). APCs were prepared with the deposition of Nd 2 O 3 by PLD. The obtained superconductive thin films were characterized by x-ray diffraction (XRD) analysis, morphological investigations of scanning electron microscopy (SEM) and atomic force microscopy (AFM) images, and electrical analysis such as R-T and I-V measurements. As a result of these analyses, it was determined that APCs had important role for the flux pinning and it provide the increasing about 25% for the critical current value (J c ). For many practical applications of high temperature superconductors, high critical current densities (J c ), especially in the presence of large magnetic fields, are required. High temperature superconductivity (HTS) coated conductors should employ the optimized flux pinning centers such as intrinsic or extrinsic imperfections, which can prevent the motion of flux lines, in order to achieve high J c values in the presence of high magnetic fields [1-3]. Due to the short coherence length in high temperature superconductors, nanosized non-superconducting regions are necessary to act as effective pinning centers in NdBa 2 Cu 3 O 7-δ (Nd-123) [4-5]. In the study, we have grown epitaxial Nd-123 films via pulsed laser deposition. Stoicheiometric Nd-123 powder was synthesized by using commercially available powders of Nd 2 O 3 , BaCO 3 , and CuO with purities of over 99,9% via solid-state synthesis at 920 ◦C for 24 h. After, Nd-123 films were prepared with a configuration of Ni–3 at.%W (50μm)/MgO (30nm)/STO(200nm)/CeO 2 (30nm) via pulsed laser deposition using a Kr-F excimer laser (wave length θ=248nm) at a repetition rate of 10 Hz at a substrate temperature (Ts) of 790 0 C in 1%O 2 /Ar gas at a deposition pressure of 800 mTorr. The laser energy density and substratetarget distance were 1,5 J cm −2 and 6,5 cm, respectively. Besides, to control the density of Nd 2 O 3 nanoislands that they will serve as artificial pinning centers (APCs), 1, 5, 10, 25, 50 and 100 laser pulses for Nd 2 O 3 target ablation were selected. Afterwards the films were cooled to room temperature within an hour in 500 Torr of oxygen and after depositing sputtered Ag electrodes onto the films. Transport properties of the samples were measured using the standard four-probe method. Phase and textural analysis was undertaken using X-ray diffraction (XRD). Surface morphology of films were investigated by using scanning electron microscopy (SEM) and atomic force microscopy (AFM) images. (a) (b) Figure 2. AFM images of samples a)APCs b)3-D image of Nd-123 thin film Typical AFM image of samples prepared by the 25-pulse PLD process is shown in Fig. 2 (a). The Nd 2 O 3 nanoisland were formed randomly on the substrates, and each nanoisland was isolated and homogeneously distributed. Fig. 2 (b) reveals that roughness of the film is 11 nm. Figure 3. Temperature versus J c curve of the Nd-123 films at 0 T APCs were successfully introduced into Nd-123 thin films by the deposition of uniformly distributed Nd 2 O 3 nano-island grown on substrate. The density and size of nano-islands were controlled by the deposition parameters and AFM. The critical current of the Nd-123 thin films grown on substrates with nano-islands was enhanced. We reached that best performance in Nd-123 thin film prepared with 25 pulses on substrate. This condition has been observed by the direct AFM that Nd 2 O 3 nano-islands effectively generated APCs inside Nd-123 film. Fig. 3. show that APCs had important role for the flux pinning and it provide the increasing about 25% for the critical current value (J c ). *Corresponding author: esonmez@atauni.edu.tr Figure 1. X-ray diffraction of the Nd-123 thin films [1] M. Murakami et al., 1989. Jpn J. Appl. Phys. 28 1189. [2] S. Neesleshwar et. al., 1999. Mat. Sci. & Eng. B. 65 164-169. [3] K. J. Song et. al., 2006. Physica C 445-448, 656-659. [4] P. Mele et. al., 2005. Physica C 426-431, 1108-1112. [5] P. Mele et. al., 2006. Physica C 445-448, 648-651. 6th Nanoscience and Nanotechnology Conference, İzmir, 2010 239
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