Browsing by Subject "Superconductor."

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Open Access
Impurity effects on superconductors and the electron-phonon interaction
(2000) Savran, Kerim
In this thesis effects of impurities on superconductors and electron-phonon interactions in metals are studied. The first part deals with the effect of magnetic impurities on superconductors. In particular, we focus on the experimental observation that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in the crystalline or the amorphous state. Based on the recent theory of Kim and Overhauser, it is shown that as the disorder in the system increases, the initial slope of the Tc depression decreases by a factor when the mean free path I becomes smaller than the BCS coherence length which is in agreement with experimental findings. Additionally, the transition temperature Tc for a superconductor, which is in a pure crystalline state, drops sharply from about 50% of Tco (transition temperature of a pure system) to zero near the critical impurity concentration. This pure limit behavior was found in crystalline Cd by Roden and Zimmermeyer. In the second part, the effect of weak localization on electron-phonon interactions in metals is investigated. As weak localization leads to the same correction term to both conductivity and electron-phonon coupling constant A (and Xtr), the temperature dependence of the thermal electrical resistivity is decreasing as the conductivity is decreasing due to weak localization. Consequently, the temperature coefficient of resistivity (TCR) decreases, while t he residual resistivit}' increases. As the coupling constant A approaches zero, only the residual resistivity part remains and accordingly TCR becomes negative. In other words, the Mooij rule turned out to be a manifestation of weak localization correction to the conductivity and the electron-phonon interaction.
Open Access
İnvestigation, modeling, and applications feasibility of the thermal crosstalk in high Tc transition edge bolometer arrays
(2006) Bozbey, Ali
So far, the high Tc transition edge bolometer (TEB) devices are mostly used as single pixel detectors. Recently, there are a number of groups working on the 2-4 pixel array applications of the high Tc TEB. Though the target spectrum of the TEB is far IR and mm-waves, we are using a near IR laser source in our investigation due to practical reasons since the response analysis is similar. We have designed and implemented 4-pixel Y Ba2Cu3O7−δ (YBCO) edge transition bolometer arrays. The crosstalk study was made possible through the illumination of the sense-devices and measuring the voltage response of the blocked read-out device in the same array. This was done using a silver coated shadow mask. In order to prevent thermal artifacts created by the mask, the mask was made in free standing configuration on top of the devices. The devices were made of 200 nm and 400 nm thick pulsed laser deposited YBCO films on SrT iO3 and LaAl2O3 substrate materials. In this thesis, we made the qualitative investigation of the dependence of the thermal crosstalk on the various device parameters such as the substrate material, device layout, YBCO film thickness, operating temperature, and modulation frequency. Then, based on the experimental results, we proposed an analytical thermal model. We proposed two models: i) Basic model, which takes into account only the lateral heat diffusion in the substrate for quick design purposes ii)Analytical model, which takes into account the lateral heat diffusion, vertical heat diffusion, and the effect of the leaking laser radiation through the shadow mask, for detailed design purposes and verifying the qualitative analysis. Finally, we proposed and verified possible applications of the thermal crosstalk in TEB arrays. One proposed application of the crosstalk is the electrical free read-out of the sense pixels by utilization of the unique dependence of the magnitude and phase of the response on the thermal crosstalk between bolometer pixels in an array. The qualitative investigation made in this study is the most detailed investigation about the bolometer arrays and the proposed analytical model is the strongest among the reported ones so far in terms of fitting the experimental results, explaining the effects of the various parameters, and designing TEB arrays. The proposed crosstalk based read-out method is expected to decrease the read-out circuitry for possible TEB based applications. Since multilayer process is difficult to make in high Tc superconductors, decreasing the complexity of the read-out circuitry by half is even important and it is the first time that such a method is utilized including bolometer arrays made of different types of materials.
Open Access
Ridge waveguide GaAS
(1994) Türkoğlu, Abdullah Kamuran
The study of solid-state laser structures in low dimensions has gained great deal of attention in recent years. The theory originated in early 1960s developed really fast along with new fabrication methods bringing geometries from macroscopic to sub-micron scale. This, in turn, made it possible to realize more complex semiconductor laser structures having multiple quantum wells in their active region with sub-milliampere threshold currents and tens of mW att/facet optical light outputs. Today, after a long way of effort in the interest for MQW laser structures, quite challenging performances have been achieved.* However, due to complexity encountered during manufacturing and testing processes of these new lasing structures, it seems that overall technique still needs to be improved. In this research conducted at BU Advanced Research Laboratories, design, fabrication and characterization of CaAs/ALGaı-^As Multiple Quantum Well Icisers is aimed. In the subsequent chapters, first the basic theoretical background for QW lasers is summarized, then the method followed during fabrication is reported, and finaly, typical characteristics obtained after test studies are presented.