Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Author: search.filters.author.Özyüzer, LütfiSubject: search.filters.subject.Superconductors

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Now showing 1 - 6 of 6
  • Master Thesis
    Fabrication and Characterization of Superconducting Bi2212 Bolometer for the Detection of Thz Waves
    (Izmir Institute of Technology, 2014) Kurt, Metin; Özyüzer, Lütfi; Aygün, Gülnur; Kurt, Metin; Aygün Özyüzer, Gülnur; Özyüzer, Lütfi; 01. Izmir Institute of Technology; 04.05. Department of Pyhsics; 04. Faculty of Science
    Since terahertz (THz) waves can pass through materials like clothing, plastic, wood, ceramic, leather and without harm to the body, it can be used for characterization, detection and 3D imaging of these materials. THz application area expands day by day such as high-speed wireless communications, medical imaging, security in airports and shopping centres and detection of chemical and biological materials. Rapidly increasing applications of the electromagnetic waves (EM) in the under developed terahertz frequency (0.1-10 THz) range requires a well understandings of efficient terahertz wave detection. An intense, coherent and continuous electromagnetic wave source is obtained by High-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal. At the same time Bi2212 single crystal can detect THz waves in suitable conditions. Nowadays, different types of bolometers are used for detection of THz waves. But they have many limitations like slow response time which is the most significant problem and costly cryogenic spending. In this study, single crystal of Bi2212 is cleaved to layer by layer by scotch tape until the necessary is reached thickness. Afterwards, it is pasted on a sapphire substrate and the scotch tape is etched with the aid of chloroform solution and ultrasonic cleaner. After the crystal fabrication, the exact thickness of the crystal were obtained using atomic force microscopy. Then, the samples were annealed at 400oC for 1 hour in order to adjust the oxygen doping level and then deposited with 150 nm Au layer by thermal evaporation. Afterwards they were annealed again at 425oC for 30 minutes to decrease the contact resistivity. After clean room process, our log-periodic antenna design was formed on the crystal by using e-beam lithography and Ar-ion beam etching step by step. Finally, four probe wires were connected to the two contact paths and log-periodic antenna by silver epoxy. The temperature dependence of a-b axis resistivity (R-T) for Bi2212 single crystals were performed.
  • Master Thesis
    Area Dependence of Josephson Critical Current Density in Superconducting Bi2sr2cacu2o8+d Mesas for Terahertz Emission
    (Izmir Institute of Technology, 2013) Sağlam, Hilal; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There are numerous application fields of terahertz waves such as airport screening of passengers for weapons, explosives, drugs, secure wireless communications, cancer detection, etc. high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal has been observed as an intense, coherent, continuous electromagnetic wave source in terahertz frequency region. Bi2212, which is highly anisotropic high-Tc superconductor, is considered as a stack of intrinsic Josephson junctions (IJJs) on atomic scale. In this study, we have fabricated triple mesa structures on a same chip with various mesa areas (300x50, 200x50, 100x50 μm2). Firstly, single crystal of Bi2212 is glued onto a sapphire substrate from its smooth a-b surface by silver epoxy. After deposition of 100 nm Au layer, rectangular mesa structures were fabricated on the surface of an under-doped Bi2212 crystal by using e-beam lithography and Ar-ion etching step by step. On account of the difficulties in making a contact on small area of the mesa, CaF2 insulating layer deposition was performed. After that, a gold stripe with the width of 30 μm was created by lift-off technique on the mesa and CaF2 layer. Finally, three gold probe wires were connected to the two contact paths and mesa by silver epoxy. After the mesa fabrication, the exact dimensions of the mesas were obtained using atomic force microscope. To obtain the electrical characterization, c-axis resistance versus temperature (R-T), and current-voltage behavior (I-V) were measured. From I-V characteristics, critical current of each mesa structure having different dimension was obtained, after that, we have calculated the critical current densities of each mesa structure and then we have studied change in Josephson critical current density of mesas with different dimensions. We can conclude from the I-V measurements of the mesas that the Josephson critical current density is decreasing when the area of mesa is increasing. Furthermore, the backbending voltage points are increasing since heating effects dominate for the large areas of the mesa structures.
  • Master Thesis
    Growth of Magnetron Sputtered Superconductor Mgb2 Thin Films
    (Izmir Institute of Technology, 2006) Ulucan, Savaş; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The discovery of superconductivity in the intermetalic compound MgB2 (39 K) in 2001 raised the great interest for the both science and technology applications. It has the highest Tc value among the intermetalic compounds. MgB2 has many properties make it very attractive for superconducting applications; these are large coherence length, high critical current density (Jc), high critical magnetic field (Bc) values. There are several methods developed to produce high quality MgB2 superconducting thin films. Magnetron sputtering system is a widely used method to deposit thin films. In this study, an MgB2/Mg target was produced by using MgB2 and Mg powders with a hot press technique. Prepared sputtering target used to grow MgB2 superconducting thin films on Al2O3 polycrystal and LaAlO3 single crystal substrate by a high vacuum magnetron sputtering system. To enhance the superconducting properties of as-grown films and to increase the crystal quality of the as-grown film an ex-situ anneal process was examined. X-Ray Diffraction (XRD) method was used to obtain crystal structure of the grown films. To observe the surface morphology of the films and to measure the thickness of the films Scanning Electron Microscopy (SEM) images were taken.Electron Dispersive X-Ray Spectroscopy (EDX) technique was used to identify the chemical contents of the films. Low temperature electrical measurement was done under various magnetic fields to observe the superconducting behavior of prepared films. The effects of ex-situ annealing process were also investigated. It was found that ex-situ annealing process develops the structural and electrical properties of MgB2 thin films.
  • Master Thesis
    Spin Polarized Tunneling Spectroscopy of Intercalated Bi2sr2cacu2o8+
    (Izmir Institute of Technology, 2006) Özdemir, Mehtap; Özyüzer, Lütfi; Özdemir, Mehtap; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There has been continuing interest in decades in heterostructure tunnel junctions that combine ferromagnetic material (F), insulator (I) and superconductor (S). This junction has been known to a good probe to analyze the physical properties of ferromagnetic and superconducting materials. In order to understand the influence of the spin injection on the c-axis tunneling characteristics of a Bi-2212 single crystal as a function of temperature and magnetic field, two sets of samples have been prepared; in one set, the surface of HgBr2 intercalated Bi-2212 crystals have been deposited with merely Au while the other set has ferromagnetic multilayer (Au/Co/Au) on top of the crystal. The micron sized mesa arrays have been patterned using photolithography and ion beam etching techniques. The surface topography and height of the mesa were investigated using atomic force microscopy. Tunneling characteristics were examined by means of the novel technique, point contact tunneling (PCT), and experiment were performed wide range of temperatures from 4.2 K to 300 K and that of magnetic field from 0 G to 2600 G. The results of spin polarized tunneling measurements compared with spin degenerate tunneling. Magnetic field dependence hysteretic I-V curves with multiple branches were examined to show the suppression of the superconductivity with spin polarized current. The distinct temperature dependence of depression in tunneling conductance near Fermi level was discussed for spin degenerate and polarized currents above Tc. The zero bias conductance and resistivity versus temperature plots were investigated to show the existence of the pseudogap in tunneling spectroscopy of investigated samples.
  • Master Thesis
    Microstructural, Electrical and Mechanical Properties of Mgb2/Mg Metal Matrix Composites
    (Izmir Institute of Technology, 2004) Eğilmez, Mehmet; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The recent discovery of superconducting MgB2 (2001) has risen a great interest because of its relatively high Tc (39 K). Furthermore, simple crystal structure, large coherence lengths, high critical current densities (Jc) and critical fields (Bc2), promise that MgB2 will be a good material for large-scale applications based on production of superconducting wires. However, MgB2 is not a stand-alone material to produce wires because of its brittle nature. The potential of MgB2/Mg composites for producing superconducting wires, is investigated. MgB2/Mg composites were prepared using metal matrix composite fabrication technique, which is an alternative to conventional wire fabrication methods such as powder in tube. MgB2 and Mg powders were mixed at different weight fractions and uniaxially pressed in a cylindrical dye under the pressure of 0.5 GPa and 1GPa for two hours at different temperatures in the air. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Dispersive X-Ray Spectroscopy (EDX) techniques were used for phase identification and microstructural studies. D.C resistivity measurements were carried on the pellet segments of the MgB2/Mg composites in the temperature range 20-300 K. Magnetization measurements were done on the composites using a vibrating sample magnetometer to characterize the magnetic properties of the samples. The effects of pressing temperature on the mechanical properties of MgB2/Mg composites were also investigated. For this purpose, compressive mechanical testing were performed to measure elastic modulus and strength at failure values of the composites. It was found that the relative weight fractions of Mg and the pressing temperature have some considerable effect on the electrical, magnetic, microstructural and the mechanical properties of the composites.
  • Master Thesis
    Fabrication of Double Mesa Structures From Superconducting Bi2sr2cacu2o8+d by E-Beam Lithography for Terahertz Emission
    (Izmir Institute of Technology, 2011) Demirhan, Yasemin; Demirhan, Yasemin; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    Terahertz radiation is part of the electromagnetic spectrum lying between microwaves and the far-IR. This region has frequencies ranging from 0.1 - 10 THz and wavelengths from 3 mm to 0.03 mm. We work on a frequency tunable solid state device to meet the needs of continuous, coherent, powerful terahertz emission sources that fill practically important terahertz gap. Among the cuprates, single crystal of Bi2Sr2CaCu2O8+ (Bi2212) is a potential candidate of compact solid state devices designed for generating electromagnetic waves in terahertz frequency range. Bi2212 crystals are consisting of natural arrays in the form of identical layers called intrinsic Josephson junctions (IJJ). In this study, under optimized doping conditions we aimed to investigate powerful terahertz emission. THz emission in the μW range can be obtained fabricating rectangular-shaped mesa structures on the Bi2212 crystal. In the experimental procedure, in order to obtain various doping levels, we annealed the high temperature superconducting Bi2212 single crystals at various temperatures in vacuum or under argon flow. By using the thermal evaporation, optic and electron beam lithography, ion beam etching techniques single and double mesa structures were fabricated. For the e-beam lithography process, we optimized and improved the required parameters. After the mesa fabrication, the exact dimensions of the mesas were obtained using surface profilometer and atomic force microscope. In order to characterize the Bi2212 mesas,by three probe contact c-axis resistance versus temperature (R-T), and current-voltage behavior (I -V) were measured in a He flow cryostat. Some of the hysteretic quasiparticle branches are seen in the I-V characteristics of Bi2212 crystals.