Browsing by Author "Savran, Kerim"
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Item Open Access A critique of the two level approximation(Springer New York, 2006) Hakioǧlu, Tuğrul; Savran, Kerim; Meşe, E.The conditions in favor and necessity of a realistic multileveled description of a decohering quantum system is examined. Under these conditions approximate techniques to simplify a multileveled system by its first two levels is unreliable and a realistic multilevel description in the formulation of decoherence is unavoidable. In this regard, the first crucial observation is that, the validity of the two level approximation of a multileveled system is not controlled purely by sufficiently low temperatures. That the type of system-environment coupling and the environmental spectrum have a dominant role over the temperature is demonstrated. Particularly, zero temperature quantum fluctuations induced by the Caldeira-Leggett type linear coordinate coupling can be influential in a wide energy range in the systems allowed transitions. The second crucial observation is that the decoherence times being among the system's short time scales are found to be dominated not by the resonant but non-resonant processes. © 2006 Springer Science+Business Media, Inc.Item Open Access Decoherence in open quantum systems : a realistic approach(Bilkent University, 2006) Savran, KerimDecoherence mechanisms of opcu quantum systems ın intcraction with an enviromental bath is investigated using the master equation formalism. Widely used two-level approximation is questioned.Item Open Access Fabry-Perot-type resonances in metallic photonic crystals(IEEE, 1998) Serpengüzel, Ali; Savran, KerimA metallic photonic crystals (MPC) with a face-centered cubic (FCC) Bravais lattice is investigated in the microwave regime. It was found that the transmission below and above the stop band is unity, which is quite remarkable since the metallic surface would have reflected 99.97% of the incident radiation in the given microwave frequency range. The effect of the incidence angle on the transmission of the MPC was also studied.Item Open Access Impurity effects on superconductors and the electron-phonon interaction(Bilkent University, 2000) Savran, KerimIn 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.