Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 4Citation - Scopus: 4Photonic Crystal Textiles for Heat Insulation(American Institute of Physics, 2023) Çetin, Zebih; Tunçtürk, Yiğit; Sözüer, Hüseyin SamiIn this work, we have studied transmission properties of a photonic crystal-like structure that can be woven into fabrics. An interesting possibility emerges when considering the potential energy savings through suppression of radiation. It is a well-established fact that every object at a finite temperature inherently emits electromagnetic waves. Within the specific context of the human body, radiation takes on a crucial role as a fundamental mechanism governing heat dissipation. Thus, exploring ways to manage or mitigate this radiation could offer innovative approaches to optimize energy consumption and enhance heat regulation. It is well known that a photonic crystal can block electromagnetic energy with a specific frequency that is falling into a photonic bandgap. By using the numerical method called a finite-difference time domain, we have shown that this property of a periodic structure can be used to make textiles to save energy that is used to heat a human body environment. Numerical calculations have shown that by using the proposed photonic crystal structure, 53 % of electromagnetic energy is reflected. Although we mainly focused on textiles, it is worth highlighting that the same fundamental principle can be extended to diverse fields; for example, this structure can be integrated with construction materials and effectively function as a radiation heat insulator. © 2023 Author(s).Article Citation - WoS: 2Citation - Scopus: 2The Effect of Thickness of Silver Thin Film on Structural and Optical Properties of Porous Silicon(World Scientific Publishing Co. Pte Ltd, 2017) Çetinel, A.; Özdoğan, M.; Utlu, G.; Artunç, N.; Şahin, G.; Tarhan, EnverIn this study, porous silicon (PS) samples were prepared on n-type silicon (100) wafers by electrochemical etching method, varying the current density from 20 to 100mA/cm2 and keeping constant HF concentration (10%) and etching time of 15min. Then, Ag thin films, which have 10, 50 and 100nm film thicknesses, were deposited on PS layers by using thermal evaporation to investigate the influence of Ag film thickness on structural and optical properties of PS. The structural and optical properties of PS and Ag deposited PS layers have been investigated by XRD, FE-SEM, Raman and photoluminescence (PL) spectroscopy. FE-SEM XRD and Raman analyzes indicate that average pore size and porosity of PS layers increase with the increasing current density. Further, Ag nanoparticles have embedded in pore channel. PL measurement reveals that higher porosity of PS would be better to form the Ag-PS nano-composite material leading to stronger PL band. The PL spectra of PS and Ag-PS samples indicate that PL bands show blue shift with increasing current density and film thickness. Consequently, it has been found that the structural and optical properties of PS depend on current density and Ag film thickness individually.Article Citation - WoS: 12Citation - Scopus: 12Ultra-Thin Znse: Anisotropic and Flexible Crystal Structure(Elsevier Ltd., 2017) Bacaksız, Cihan; Şenger, Ramazan Tuğrul; Şahin, HasanBy performing density functional theory-based calculations, we investigate the structural, electronic, and mechanical properties of the thinnest ever ZnSe crystal [11]. The vibrational spectrum analysis reveals that the monolayer ZnSe is dynamically stable and has flexible nature with its soft phonon modes. In addition, a direct electronic band gap is found at the gamma point for the monolayer structure of ZnSe. We also elucidate that the monolayer ZnSe has angle dependent in-plane elastic parameters. In particular, the in-plane stiffness values are found to be 2.07 and 6.89 N/m for the arm-chair and zig-zag directions, respectively. The angle dependency is also valid for the Poisson ratio of the monolayer ZnSe. More significantly, the in-plane stiffness of the monolayer ZnSe is the one-tenth of Young modulus of bulk zb-ZnSe which indicates that the monolayer ZnSe is a quite flexible single layer crystal. With its flexible nature and in-plane anisotropic mechanical properties, the monolayer ZnSe is a good candidate for nanoscale mechanical applications.Article Citation - WoS: 45Citation - Scopus: 43Tuning the Magnetic Anisotropy in Single-Layer Crystal Structures(American Physical Society, 2015) Torun, Engin; Şahin, Hasan; Bacaksız, Cihan; Senger, Ramazan Tugrul; Peeters, François M.The effect of an applied electric field and the effect of charging are investigated on the magnetic anisotropy (MA) of various stable two-dimensional (2D) crystals such as graphene, FeCl2, graphone, fluorographene, and MoTe2 using first-principles calculations. We found that the magnetocrystalline anisotropy energy of Co-on-graphene and Os-doped-MoTe2 systems change linearly with electric field, opening the possibility of electric field tuning MA of these compounds. In addition, charging can rotate the easy-axis direction of Co-on-graphene and Os-doped-MoTe2 systems from the out-of-plane (in-plane) to in-plane (out-of-plane) direction. The tunable MA of the studied materials is crucial for nanoscale electronic technologies such as data storage and spintronics devices. Our results show that controlling the MA of the mentioned 2D crystal structures can be realized in various ways, and this can lead to the emergence of a wide range of potential applications where the tuning and switching of magnetic functionalities are important.Article Citation - WoS: 18Citation - Scopus: 20Impact of Incorporated Oxygen Quantity on Optical, Structural and Dielectric Properties of Reactive Magnetron Sputter Grown High-? Hfo2/Hf Thin Film(Elsevier Ltd., 2014) Cantaş, Ayten; Aygün, Gülnur; Turan, RaşitHigh-κ hafnium-oxide thin films have been fabricated by radio frequency (rf) reactive magnetron sputtering technique. To avoid formation of an undesired interfacial suboxide layer between Si and high-κ film, prior to HfO2 deposition, a thin Hf buffer layer was deposited on p-type (1 0 0) Si substrate at room temperature. Effect of oxygen gas quantity in the O2/Ar gas mixture was studied for the optical and structural properties of grown HfO2 high-κ thin films. The grown thin oxide films were characterized optically using spectroscopic ellipsometer (SE) in detail. Crystal structure was studied by grazing incidence X-ray diffractometer (GIXRD) technique, while bonding structure was obtained by Fourier transform infrared spectroscopy (FTIR) analyses. In agreement with GIXRD and FTIR analyses, SE results show that any increment above ideal quantity of oxygen content in the gas mixture resulted in decrements in the refractive index and thickness of HfO2 dielectric film, while increments in SiO2 thickness. It is apparent from experimental results that oxygen to argon gas ratio needs to be smaller than 0.2 for a good film quality. The superior structural and optical properties for grown oxide film were obtained for O2/Ar gas ratio of about 0.05-0.1 combined with ∼30 W constant rf sputtering power. © 2014 Elsevier B.V. All rights reserved.Article Citation - WoS: 42Citation - Scopus: 43Counterintuitive Consequence of Heating in Strongly-Driven Intrinsic Junctions of Bi2 Sr2 Cacu2 O 8+? Mesas(American Physical Society, 2010) Kurter, Cihan; Özyüzer, Lütfi; Proslier, T.; Zasadzinski, John F.; Hinks, David G.; Gray, Kenneth E.Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi2 Sr2 CaCu2 O 8+δ (Bi2212) mesas have been commonly interpreted as superconducting energy gaps but here we show they are a result of strong self-heating. This conclusion follows directly from a comparison to the equilibrium gap measured by tunneling in single break junctions on equivalent crystals. As the number of junctions in the mesa, N, and thus heating increase, the peak voltages decrease and the peak width abruptly sharpens for N≥12. Clearly these widely variable features vs N cannot all represent the equilibrium properties. Our data imply that the sharp peaks represent a transition to the normal state. That it occurs at the same dissipated power for N=12-30 strongly implicates heating as the cause. Although peak sharpening due to heating is counterintuitive, as tunneling spectra usually broaden at higher temperatures, a lateral temperature gradient, leading to coexistence of normal hot spots and superconductive regions, qualitatively explains the behavior. However, a more uniform temperature profile cannot be ruled out. As the peak's width and voltage in our shortest mesa (N=6) are more consistent with the break junction data, we propose a figure of merit for Bi2212 mesas, the relative conductance peak width, such that small values signal a crossover into the strong self-heating regime. © 2010 The American Physical Society.Conference Object Citation - WoS: 36Citation - Scopus: 40Thermal Management in Large Bi2212 Mesas Used for Terahertz Sources(Institute of Electrical and Electronics Engineers Inc., 2009) Kurter, Cihan; Gray, Kenneth E.; Zasadzinski, John F.; Özyüzer, Lütfi; Koshelev, A. E.; Li, Q.; Yamamoto, T.; Kadowaki, K.; Kwok, W. K.; Tachiki, M.; Welp, U.We present a thermal analysis of a patterned mesa on a Bi 2Sr2CaCu2O8 (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ∼800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40 × 300 × 1.2 μm3) and power dissipation that result in self-heating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, Rqp(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular R qp (T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ∼60 K.Article Citation - WoS: 25Citation - Scopus: 26Structural and Optical Characteristics of Tantalum Oxide Grown by Pulsed Nd:yag Laser Oxidation(AVS Science and Technology Society, 2006) Atanassova, Elenada A.; Aygün, Gülnur; Turan, Raşit; Babeva, T.Tantalum pentoxide (Ta2 O5) thin films (20-50 nm) have been grown by 1064 nm Nd:YAG laser oxidation of Ta film deposited on Si. The chemical bonding, structure, and optical properties of the films have been studied by Fourier transform infrared spectroscopy, x-ray diffraction, and reflectance measurements at normal light incidence in the spectral range of 350-800 nm. The effect of the substrate temperature (250-400 °C) during oxidation and its optimization with respect to the used laser beam energy density (3.2-3.4 J cm2 per pulse) is discussed. It is established that the substrate temperature is a critical factor for the effectiveness of the oxidation process and can be used to control the composition and amorphous status of the films. The film density explored by refractive index is improved with increasing film thickness. The refractive index of the layers grown under the higher laser beam energy density and at substrate temperature of 350-400 °C was found to be close to the value of bulk Ta2 O5. The films are amorphous at substrate temperature below 350 °C and possessed an orthorhombic (Β- Ta2 O5) crystal structure at higher temperatures. The thinner layers crystallize at a little higher temperature.Article Citation - WoS: 2Citation - Scopus: 2Origin of a Localized Vibrational Mode in a Gasb Substrate With a Mbe-Grown Znte Epilayer(IOP Publishing Ltd., 2006) Kim, Hyunjung; Tarhan, Enver; Chen, G.; Ramdas, A. K.; Sciacca, M. D.; Gunshor, R. L.A localized vibrational mode (LVM) with a remarkable fine structure is observed in the infrared transmission spectrum of a ZnTe epilayer grown with molecular beam epitaxy (MBE) on a GaSb substrate. On the basis of the Zn and Te deposited on the GaSb substrate during the MBE growth of ZnTe, and assuming diffusion of Zn and Te into GaSb, the LVM is attributed to Zn, substitutionally replacing either the cation, Ga (ZnGa), or the anion, Sb (Zn Sb). The frequency of the LVM and its fine structure can then be interpreted in terms of the infrared active modes of 64Zn substituting for Sb as an anti-site impurity and treating the centre as an XY4 quasimolecule. With X≡64Zn and Y≡ 69Ga and 71Ga, occupying the nearest-neighbour sites reflecting all the possible combinations and permutations as well as the natural isotopic abundance of Ga, the fine structure of the LVM can be accounted for quantitatively.Article Citation - WoS: 74Citation - Scopus: 77Full Bulk Spin Polarization and Intrinsic Tunnel Barriers at the Surface of Layered Manganites(Nature Publishing Group, 2005) Freeland, John W.; Gray, Kenneth E.; Özyüzer, Lütfi; Berghuis, Peter; Badica, E.; Kavich, Jerald J.; Zheng, Hong; Mitchell, John F.The affect of full bulk spin polarization and intrinsic tunnel barriers on the surface of layered manganites were studied using a combination of surface-sensitive x-ray and tunnelling process. The surface bilayer of air-cleaved layered manganites forms an antiferromagnetic insulating nanoskin composed of a single bilayer unit. It was shown that for the quasi-two-dimensional bilayer manganites the outermost Mn-O bilayer is affected while the next bilayer displays the full spin polarization of the bulk. The results show that the outermost bilayer act as an intrinsic barrier between the fully spin-polarized bilayer beneath and a subsequently deposited ferromagnetic counterelectrode.