Browsing by Author "Demirel, AI"

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Electron Gas Statistics in Size-Quantized Thin Films With Non-Parabolic Energy Spectra
(World Scientific Publ Co Pte Ltd, 2005) Eminbeyli, R; Guliyev, B; Demirel, AI; Yaman, A
The electron gas statistics in semiconducting thin films with arbitrary isotropic energy spectra was found in this theoretical work. General expressions for the density of states of the charge carriers and for the chemical potential were deduced according to the two-band Kane model approach. The ratio of the expression obtained for the density of states to that of the expression deduced according to the parabolic approach was shown to be a function of the energy. The Fermi energy of the electron gas was observed to be a function of the thin film thickness, the non-parabolic parameter, the concentration and the temperature. In the strong non-parabolic approach the dependency of the Fermi energy on the thin film thickness was shown to exhibit non-monotonic characteristics.
Granularity Aspect of the High Temperature Superconductivity (Htsc) Materials Made by Metal Organic Chemical Vapour Deposition Method
(Natl inst Science Communication, 2000) Demirel, AI; Yilmaz, R; Lu, AAG; Zahrir, AD
Superconducting YBCO thin films grown by a MOCVD technique have been studied. Granular structure appearance of the high temperature superconducting materials and exponential behaviour of their resistance near the zero resistance region were investigated. Power law fitting for. resistance versus temperature plot was obtained as R=1.6 x 10(-19)T(7.91). Twin boundaries, due to accommodation to the structural symmetry as the a and b axis are differentiated by oxygen uptake are a soul-cc of. a further weak-link picture. The scale range for. a twin boundary is commonly measured to be close to 1000 Angstrom.
An Investigation of Superconducting Ybco Thin Films Grown on Different Substrates
(Iop Publishing Ltd, 2002) Demirel, AI
In this work, superconducting YBa2Cu3O7 (YBCO) thin films were grown on different substrates under the same conditions and their behaviours were investigated. At a higher substrate temperature, because of the occurrence of a chemical reaction between the substrate and film, the film quality was impaired, especially in case of the Al2O3 (sapphire) substrate. Films were also deposited onto other substrates, SrTiO3, MgO and ZrO2, and the effects of these substrates on the film quality were compared by x-ray diffraction.
Magnetic Relaxation and Flux Creep in Superconducting Ybco Thin Films
(World Scientific Publ Co Pte Ltd, 2003) Demirel, AI
In this work, the temperature dependence of the magnetic relaxation in samples of YBa2Cu3O7 (YBCO) thin films of two different qualities grown on MgO by metal organic chemical deposition technique (MOCVD) is measured in the range of 4.2-80 K using an initial applied magnetic field of 0.1 T. The relaxation data are analyzed in terms of a single effective activation energy and then more correctly of a distribution of activation energies. The single activation energies extracted are in broad agreement with published results.
Measurement Method and Results of the Thermal Conductivity Coefficient at High Temperatures
(Iop Publishing Ltd, 2006) Ceferov, EO; Penahov, EZ; Demirel, AI
In this work, a new method of measuring the thermal conductivity coefficient (K) at high temperatures is presented. It is shown that by expressing the thermal distribution along the sample as a series, it is possible to determine. by measuring the temperature at two points of the sample.
Pinning Mechanism and Flux Motion in Yba2cu3o7 Thin Films
(World Scientific Publ Co Pte Ltd, 2004) Demirel, AI
The flux creep phenomena and the J(c) characteristics have been investigated in the YBa2Cu3O7 (YBCO) thin films grown by metal organics chemical vapor deposition (MOCVD) technique. The anisotropy of the current density (J(c)) is discussed on the basis of the intrinsic pinning model and the anisotropic critical magnetic field (H-c2). The properties of the high J(c) and the pinning energy are related with the fine precipitates observed by tunneling electron microscopy (TEM). The strong pinning mechanism affects the amount of the flux motion.
Some Structural and Fluctuation Properties of Superconducting Ybco Thin Films
(Iop Publishing Ltd, 2001) Demirel, AI
The characterization of superconducting thin films was performed by starting with x-ray measurements as this is a necessary technique to single out potential superconductors although it is not sufficient to determine superconductivity. The temperature dependence of the paraconductivity can be described by a power law described by the Aslamasov-Larkin theory. A study of the fluctuation conductivity was performed leading to the determination of the coherence length. The values found are compatible with the two-dimensional nature of these new superconductors.
Tunneling Properties Between Yba2cu3o7-D and Ba1-Xkxbio3 Thin Films
(World Scientific Publ Co Pte Ltd, 2002) Demirel, AI
In this study, a contact between YBa2CU3O7-d thin film and Ba1-xKxBiO3 thin film has been fabricated. The typical I-V characteristics of tunneling are shown in the contact to have different superconducting energy gaps. According to the I-V characteristics, the superconducting energy gaps, 2Delta, are calculated as 1.5 meV and 3 meV for YBa2CU3O7-d and for Ba1-xKxBiO3, respectively, along the c-axis. The contact boundary between these superconducting thin films shows a boundary Josephson junction with tunneling barrier properties.