Düşük boyutlu elektron-deşik sistemlerinde fonon renormalizasyonu

Abstract PHONON RENORMALIZATION EFFECTS IN LOW DIMENSIONAL ELECTRON-HOLE SYSTEMS Kaan Güven M. S. in Physics Supervisor: Assoc. Prof. Bilal Tanatar September 1995 Low dimensional semiconductor structures have been an extensive research area in condensed matter physics. In particular, much effort has been devoted to the study of quasi-one-dimensional semiconductor structures in recent years. The new physical phenomena involved in these systems arising due to the reduced dimensionality point to various potential applications for future technologies. Although the theory is familiar with the ''one-dimensional" problem for a long time, the realization of such structures (also known as quantum wires) extends only to a decade before. However, the novel production techniques led to a rapid increase in the experimental studies which, in turn, required a more realistic and comprehensive theory to analyze and interpret the obtained data. This thesis work intends to make a contribution in the direction of these improvements. Our study is based on a quasi-one-dimensional electron-hole system as realized in photoexcited quantum wires, interacting with the bulk LO-phonons. We investigate the polaronic corrections to the band gap and the carrier effective mass, and the dependence of this correction itself to carrier density, temperature, and quantum finite size effects. We apply two different formal approaches;the perturbation theory and the variational method. The latter enables the investigation of dynamical screening effects, thereby clarifying the question of validity of the static screening approximation in one dimension. Our results have shown that dynamical screening is relevant in low dimensions. The dielectric function, which is a key quantity in describing the many-particle properties, is analyzed under different approximations such as the Hartree-Fock approximation, random-phase approximation, and the more advanced local-field correction. Several confinement potentials (infinite well, parabolic, cylindrical) are presented. Explicit results are obtained for a GaAs quantum wire. We compare the results of the polarouic. corrections with that of the exchange- correlation induced corrections. We (bund that they are comparable in order of magnitude, indicating that carrier- phonon interactions are more enhanced as the dimensionality reduces, and hence should be treated on an equal footing along with the carrier-carrier interactions. We make comparison with the polarouic corrections in two dimensional systems. Finally, we brielly discuss the renormalization dvie to confined phonons as well. Keywords: Low dimensional electron gas, exchange-correlation, electron- phonon interaction, polarons, random-phase approximation, Hartree-Fock approximation, dielectric function.