Browsing by Subject "Electron gas."

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    Many-body effects in selected two-dimensional systems
    (2009) Subaşı, A. Levent
    In this thesis we study many-body effects in three distinct two-dimensional systems. The two dimensional electron gas is a model system yielding to basic analytical and computational theoretical ideas of many-body physics and at the same time allows faithful experimental realizations. In connection to the recently observed metal-insulator transition in this system, the spin susceptibility is a relevant observable. The behavior of the spin polarization in a parallel magnetic field is studied using a parametrized ground-state energy expression from accurate quantum Monte Carlo simulations and compared with approximate theories. The critical field to fully polarize the system is calculated. A qualitative difference for the ferromagnetic transition is found for an interval of density values. Next, we consider exciton condensation in an electron-hole bilayer system with density imbalance. Electrons and holes attracting via Coulomb interaction pair up to form spatially separated excitons and condense at low temperatures. Using mean- field theory we establish the phase diagram at zero temperature for different electron and hole densities by comparing the energy of the normal phase with that of the condensed phase. In the last chapter, the two-dimensional Bose-Fermi atomic gas mixture which is composed of a condensed boson component and a spin polarized Fermi component at zero temperature is studied. Confinement in the third direction affects the density profiles and can induce collapse of the mixture and spatial separation of components.
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    Many-body properties of one-dimensional systems with contact interaction
    (1999) Demirel, Ekrem
    The 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.
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    Phonon renormalization effects in low dimensional electron-hole systems
    (1995) Güven, Kaan
    Ia)vv (linuMisioiial sciniromluclor st i iid iii4's lia\4‘ 1и‘(М1 an (‘Xt(Misiv(‘ i4\soa,rrh iwcix ill (4)iul(4iscd iiiatlcr physics. In j^aii iculai', iiindi (‘llorl has 1)(Ч'п dcvolnd to lhc‘ study ol ciuasi-one-diiiK'iisional siMniconductor sli-iiclures in гесчмП, yc'ars. The IK'W physic'al pluMioiiK'iia. in\'ol\4*d in tlu'sc* sysitMUs arisin,^ diu‘ to th(‘ Г(‘(1п(чм1 dimensionality point to various |)ot(Milial applicalions for iuture teclinologi(.‘s. y\ltliough the theory is lainiliar with tin* "oiK'-dimensionar’ pi’oblem for a long time, the realization of such structures (also known as (juanlum wires) extends onl}^ to ¿1 decade belonx Ilowciver, the по\ч'1 рго(1 исГкя1 t(‘chni([n('s led to a rapid increase in the experimental studies which, in turn, required a mor(' realistic and comi)rc*hensive theory to anal^'/c' and int('rpr('t tln^ obtaiiKnl data. This tlu'sis work intends to make a contribution in i1k‘ diiH’ctioii of th(‘S(' imj>rovem(‘nts. Our study is based on a (juasi-one-diiiKMisional eh'ctron-hole syst em as realized in pliotoexcit(‘d quantum wires, interacting with tlu' bulk bO-])honons. VVe investigate the polaronic corrections to the band gap and the c:arri(n* eilective mass, cirid the dependence of this correction itself to carrier density, temperature, and quantum finite size effects. VVe apply two different formal approaches; tho' p(n·Lurl.)гıtion theory and the vaiiational iiK'thod. 'I'he latter enables the investigation of dynamical screening elfects, thereby clarifying tin? (luestion of validity of the static screi'iiing approximation in one dimension. Our results have shown that dynamical screening is r<'levant in low dimensions. The dielectric function, which is a key quantity in describing tin' many-particle properties, is analyzf'd under diihn(*nt tip'proximations such as the 1 lartree-Fock ap[)roximation, random-|)hase api)roximation. and the more advaiuaxl local-field correction. Several confinement |)otentials (inlinit(‘ well, paralrolic, cj lindrical) are presented. I'kxplicit results iirc.' ol)tained hn' a (¡a.As (|uan(iim wiiv. VVe compare the results of tin' |)olar(niic corn'cticnis with tluit of tin? (‘xchangecorndalion induced corrections. W'e loiiinl that they are comi)arable in orrler of magnitudi', indicating that cari'i('r-|)lu)non int('iactions are more enhanced as the dimensionality reduces, and Ik.micc' should be treated on tin etpial footing along with the carrier-carri(‘r interact i(jiis. WV* make comparison with the pohironic corrections in two diiiK'iisiinial sysUnus. kdiially, we brielly discuss the renormalization due to confined phuiions as well.