Browsing by Subject "local-field correction"
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Item Open Access Ground-state properties of double-wire semiconducting systems(Bilkent University, 1997) Mutluay Müstecaplıoğlu, NihalWith the recent advances in nanometer-scale semiconductor device fabrication technology, it became experimentally possible to produce strongly confined electron systems. Quantum wires are among these systems, and are attracting increasing interest due to their potential applications in solid-state device technology such as high-speed transistors, efficient photodetectors and lasers. Quantum wires are quasi-one-dimensional systems where electrons are free to move in one dimension, but their motion is restricted in the remaining two dimensions. Various models for qucisi-one-dimensional structures have been proposed in the literature, such as cylindrical, square-well and parabolic confinements. in this thesis, we examine ground-state correlations in double-quantum-wire systems within the self-consistent scheme of Singwi et ai, namely the STLS approximation. The model we adopt consists of two parallel cylindrically-confined quantum wires. The cases when both wires have electrons as charge carriers and when one wire has electrons while the other has holes are considered. Under the assumption that only one subband is occupied in each quantum wire and there is no tunneling between them, we calculate the local-field factors and static correlation functions. Ground-state energy and collective modes are discussed within the RPA, Hubbard and STLS approximations in order to compare the results. Charge-density-wave instabilities in these structures are examined at small and finite q values. Our numerical results are given for systems where the carrier densities and the radii of both wires are equal. As the charge carrier density is lowered, we observe that the importance of local field corrections increases so that the RPA or Hubbard approximations do not give reliable results in this region. We find that the interwire correlations become quite important for electron-hole systems. Taking into account the exchange-correlation hole around electrons, STLS provides a much better description to this many-body problem compared to the previous models.Item Open Access Many-body properties of one-dimensional systems with contact interaction(Bilkent University, 1999) Demirel, EkremThe one-dimensional electron systems are attracting a lot of interest because of theoretical and technological implications. These systems are usually fabricated on two-dimensional electron systems by confining the electrons in one of the remaining free directions by using nanolithographic techniques. There are also naturally occuring orgnanic conductors such as TTF-TCNQ whose conductivity is thought to be largely one-dimensional. The one-dimensional electron systems are important theoretically since they constitute one of the simplest many-body systems of interacting fermions with properties very different from three- and two-dimensional systems. The one-dimensional electron gas with a repulsive contact interaction model can be a useful paradigm to investigate these peculiar many-body properties. The system of bosons are also very interesting because of the macroscopic effects such as Bose-Einstein condensation and superfluidity. Another motivation to study one-dimensional Bose gas is the theoretical thought that one-dimensional electron gas gives boson gas characteristics. This work is based on the study of correlation effects in one-dimensional electron and boson gases with repulsive contact interactions. The correlation effects are described by a localfield correction which takes into account the short-range correlations. We use Vashishta-Singwi approach to calculate static correlation effects in onedimensional electron and boson gases. We find that Vashishta-Singwi approach gives better results than the other approximations. We also study the dynamical correlation effects in a one-dimensional electron gas with contact interaction within the quantum version of the self-consistent scheme of Singwi et al. (STLS) We calculate frequency dependent local-field corrections for both density and spin fluctuations. We investigate the structure factors, spin-dependent pair-correlation functions, and collective excitations. We compare our results with other theoretical approaches.