Browsing by Subject "electron-phonon interaction"

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Open Access
Approximation methods in the polaron theory: applications to low dimensionally confined polarons
(1996) Senger, R. Tuğrul
The pelaron problem has been of interest in condensed matter physics cind held theory tor cibout half a century. Within the framework of Vcist variety of theoreticcil approximations, the bulk polaron properties have been extensively (explored and fairly well understood in the literature. In the last two deccides, with the impressive progress achieved in the mici-ofabrication technology, it became possible to ol)t£iin low dimensional microstructures, in which the charge ca.rriers are confined in one or more spatial dii'ections. Consequently, there has appeared (|uite a large interest in phonon coupling-induced effects and polaronic properties of low dimensionally confined electrons. In this context, this thesis work is devoted to the study of low dimensional optical polaron properties, with the application of several different formal approaches common in the literature, such as perturbation theory, variatioiicil principles and Feynman path integral formalism. The model we adopt in this work consists of an electron, confined within an external potential (quantuni well), and interacting via the Fröhlich Harniltonian with the bulk LO-phonons of the relevant well material. Therefore, our primary concern is to give a clear view of only the bulk phonon effects on an electron in confined media, and we disregard all other complications that may come about from screening effects, phonon confinement, etc. Under these assumptions, we calculate the ground state energy, the effective mass, and some other quantities of polaron in several confinement geometries. We also provide a broad interpolating overview to the one polaxon problem in the overall range of electron-phonon coupling constant and in a general type of confinement, which can be conformed from one geometriccd configuration to another. Another interesting theme of the polaron theory, magneto-polaron, is considered in the context of the confinement effect on the polaron, brought about by the rncignetic field. A detailed analysis is given in the case, where the effect of electron-phonon coupling is dominated over by the magnetic field counterpart of the problem.
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
Phonon anomalies in high temperature superconductors
(1996) Türeci, E Hakan
Anomalously large low temperature phonon anharmonicities can lead to static as well as dynamical changes in the low temperature properties of the vibrational system. In this work, we focus our attention on the low temperature lattice anharmonicity and its effect on the electron-phonon ground state. We are, in particular, motivated by certain high temperature superconductors. The third and fourth order anharmonic coupling constants for YBCO, LBCO and several other superconducting compounds are extracted from their measured elastic constants using the anharmonic elastic continuum model. The coupling constants are then used to extract the average anharmonic potential energy for the transverse and longitudinal modes in the Cu-0 planes. We find that, anharmonic contribution to the lattice potential relative to the harmonic one is unusually high for all examined high Tc compounds. The presence of anharmonic phonons elicits non-perturbative dynamical effects in the ground state of the electron-phonon system. Phonon correlations induced by anharmonic effects enhance the electron-phonon interaction which then create a self-consistent mechanism to act back on the ground state of the electron-phonón system. In result, strong momentum correlations are created and the ground state comprises fluctuating polarons. The zero point fluctuations and other ground state properties are obtained by self-consistent numerical calculations. The influence of low temperature phonon anharmonicity on the superconducting properties in the intermediate coupling range is also investigated. It is shown that, the otherwise bare electron-phonon coupling is strengthened in the presence of correlated polarons and the zero point fluctuations are enhanced. Within this frame, it is plausible to achieve superconducting transition temperatures as high as %20 of the characteristic vibrational energy scale. The non-perturbative, self-consistent formalism thus introduced also offers an account for the recently observed temperature anomalies near Tc in the Debye-Waller factor and dynamical pair correlations of certain high temperature superconductors.