Browsing by Subject "One-dimensional conductors."

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Open Access
Electronic structure of low dimensional semiconductor systems
(1992) Gülseren, Oğuz
Recent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.
Open Access
Many-body interaction effects in quasi-one-dimensional photo-excited electron-hole systems
(1999) Güven, Kaan
The work in this thesis concerns rnany-body interaction effects in a quasi-onedimensional electron-hole plasma, which may be generated under intense photoexcitation in a semiconductor quantum-well wire. In particular, we investigate how these interactions affect the optical properties of the semiconductor quantum wire. We address this question in two parts: First, the band-gap renormalization (BGR) induced by self-energy corrections of electrons and holes is studied. A two subband model arising from the confinement of the quantum wire is developed to include the multisubband effects. The many-body theoretical formalism of electron (hole) self-energy is given within the GW approximation. We use the dielectric function both in full dynamical random-phase approximation, and in cjuasi-static approximation, in order to emphasize the dynamical properties of screening. The dependence of BGR on the e — h plasma density, temperature and wire width is studied. In the second part, the exciton renormalization induced by e — h plasma screening, and Goulomb correlation effects on the optical spectra of a quantum wire are considered. The optical properties are directly associated with the e — h two particle propogator, which obeys the Bethe-Salpeter equation. Based on recent studies, we review the solution of this equation with screened Coulomb interaction. In particular it is shown* that the dynamical treatment of screening produces an optical absorption/luminescence spectra which is consistent with experimental results. We present a discussion on the interplay of excitons and unbound carriers and on the reflection of this interplay to the optical spectra.
Open Access
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.
Open Access
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.
Open Access
Superconducting systems of low dimensionality
(1992) Gedik, M. Zafer
It is possible to call the last five years as the golden age of superconductivity. The two most important developments in the field are the discovery of copper oxide and fullerene superconductors. In this work, some possible pairing mechanisms for these materials ai’e examined by giving emphasis on the reduced dimensionality. First, an older problem, spatially separated electron-hole system, is investigated to identify the possible phases in coupled double quantum well structures in electric field. Secondly, the superconducting transition temperature and response to external magnetic fields of layered systems with varying number of layers are studied by means of a microscopic model and its GinzburgLandau version. It is also shown that an interlayer pairing mechanism, phonon assisted tunneling, can induce superconductivity. Finally, effects of the spherical structure of fullerenes are examined by solving a two fermion problem on an isolated molecule where the particles interact via a short range attractive potential. As a possible mechanism of superconductivity in alkali metal doped fullerenes, coupling between electrons and the radial vibrations of the molecule is investigated.