published_or_final_version / Physics / Master / Master of Philosophy

Many-body Coulomb interactions make understanding the complete excitation spectra of conjugated polymers a formidable task, and a variety of sophisticated experimental and theoretical techniques have been used in an attempt to elucidate their complicated electronic structure. It is thus crucial to have a intuitive, physical picture in order to interpret the wide range of available experimental data. We present the results of calculations within an exciton basis which allows for a simple pictorial description of all linear and nonlinear excitations in conjugated polymers, and settles a number of longstanding controversies. The exciton basis further allows us to justify the application of single configuration interaction (SCI) techniques to the understanding of nonlinear optical experiments in the low energy region. We show that SCI can give a clear, self-consistent picture of the photoexcitations in poly(para-phenylene vinylene), a conjugated polymer which has attracted much recent interest.

This dissertation describes experimental and theoretical studies of carrier tunneling in semiconductor heterostructures and optical properties of neutron irradiated quantum wells. Unambiguous experimental evidence for the dramatic dependence of hole tunneling rates on in-plane momentum in (Ga,In)As/(Al,In)As asymmetric double quantum wells (ADQWs) is presented. Holes generated near the bandedge tunnel on hundred picosecond time scales, whereas holes excited with large excess energy tunnel on subpicosecond time scales. The mechanism responsible for this increase of three orders of magnitude in the hole tunneling rate is nonresonant delocalization of hole wavefunctions by band mixing in the valence band. The carrier density and temperature dependencies of tunneling dynamics are presented. A simple kinetic model developed for electron LO-phonon assisted tunneling shows good qualitative agreement with experimental data. Exciton tunneling in wide gap, II-VI semiconductors was studied using (Zn,Cd)Se/ZnSe ADQW. The strong Coulomb interaction in II-VI semiconductors makes the tunneling process significantly different from that in III-VI ADQWs. Fast (1 ps) and complete recovery of the narrow well exciton absorption was observed after resonant femtosecond pulse excitation. The observed dynamics contradict the theory of independent electron and hole tunneling. The theory of exciton tunneling was developed. Theoretical analysis shows that tunneling of the exciton as a whole entity with the emission of only one LO-phonon is very slow. Instead, the exciton tunnels via an indirect state in a two-step process whose efficiency is dramatically enhanced by the Coulomb interaction. The optical properties of neutron irradiated GaAs/Ga,Al)As multiple quantum wells are investigated. Sharp room temperature exciton features and a 21 ps carrier lifetime are demonstrated in neutron irradiated multiple quantum wells. Carrier lifetime reduction is consistent with the presence of EL2 defects that are efficiently generated by fast neutrons. The influence of the gamma rays accompanying neutron irradiation is discussed. Neutron irradiation provides a straightforward way to control the carrier lifetime in semiconductor heterostructures with minor deterioration of their excitonic properties.

Based on a model Hamiltonian appropriate for the description of fermionic systems such as semiconductor lasers, we describe a natural asymptotic closure of the BBGKY hierarchy in complete analogy with that derived for classical weak turbulence. The main features of the interaction Hamiltonian are the inclusion of full Fermi statistics containing Pauli blocking and a simple, phenomenological, uniformly weak two particle interaction potential equivalent to the static screening approximation. The resulting asymytotic closure and quantum kinetic Boltzmann equation are derived in a self consistent manner without resorting to a priori statistical hypotheses or cumulant discard assumptions. We find a new class of solutions to the quantum kinetic equation which are analogous to the Kolmogorov spectra of hydrodynamics and classical weak turbulence. They involve finite fluxes of particles and energy across momentum space and are particularly relevant for describing the behavior of systems containing sources and sinks. We explore these solutions by using differential approximation to collision integral. We make a prima facie case that these finite flux solutions can be important in the context of semiconductor lasers. We show that semiconductor laser output efficiency can be improved by exciting these finite flux solutions. Numerical simulations of the semiconductor Maxwell Bloch equations support the claim.

In this thesis, we have systematically investigated the stability, surface dynamics, electronic transport, and growth of metal nanowires using a semiclassical free energy functional based on the mean-field interacting electron model, which is simple and general enough. In this model, the ionic degrees of freedom of the wire are modeled as an incompressible fluid, and the conducting electrons are treated as a Fermi gas confined within the wire with Dirichlet boundary conditions. In equilibrium, we prove that the electron-electron interaction is a second-order effect to the total grand canonical free energy, while the shell-correction to the noninteracting grand canonical free energy is a first-order effect. To leading order, the electron-electron interactions just renormalize the Weyl parameters, such as the average energy density, surface tension and mean curvature energy, but not the mesoscopic shell effect. This finding for open mesoscopic systems is a generalization of the well-known Strutinsky theorem for finite-Fermion systems. It is for this reason that self-consistent jellium calculations obtain essentially identical equilibrium mesoscopic effects as calculations based on the free-electron model. However, for systems out of equilibrium, the electron-electron interaction plays important roles. First of all, the Strutinsky theorem breaks down in the non-equilibrium case. Secondly, the gauge invariance condition is violated if the electron-electron interaction is not adequately included. We first derive a thermodynamic phase diagram for jellium nanowires, which predicts that cylindrical wires with certain "magic" conductance values are stable with respect to small perturbations up to remarkably high temperatures and high applied voltage. We have shown that Jahn-Teller-distorted wires can be stable. The derived sequence of stable cylindrical and elliptical geometries explains the experimentally observed shell and supershell structures for alkali metals. Highly deformed wires can explain additional conductance peaks in low temperature experiments on alkali metals and in gold. We then study the surface dynamic properties of different phases. Both surface phonons and surface self-diffusion of atoms are included in the linearized surface dynamics. It is found that inertial dynamics (phonons) always dominate the long-wavelength behavior at small time scales, including the critical points. (Abstract shortened by UMI.)

This thesis presents the techniques and results of our x-ray structural studies of strained Ga$ sb{1-x}$In$ sb{x}$As epilayers grown on GaAs (001) by metallorganic chemical vapor deposition. / By combining conventional x-ray techniques with newly developed glancing incidence and reflectivity measurements, we study both the out-of-plane and the in-plane structure. We also obtain direct information on the mechanisms of the structural relaxation which occurs in these systems. The techniques we have used are based on using a conventional x-ray source and could be widely used to characterize and study growth processes and sample quality. / Using the conventional characterization of the positions, widths, and intensities of Bragg peaks lattice parameters, domain sizes and strains have been evaluated. Studying the shape of the Bragg peak shows that the simple theoretical models based on the existence of a critical thickness due to dislocation can not be used to explain the structural relaxation observed. Our results based on thin (500 $ pm$ 12A, $x=0.19 pm 0.003$) and thick (40000 $ pm$ 1000A, $x=0.16 pm 0.01$) epilayers require a complicated microstructure in a transition region between the substrate and the surface of the epilayers.

The dynamics of a driven interface, with conservation of total volume under the interface, has been studied using a conserved Kardar-Parisi-Zhang-like equation. Dynamic renormalization group analyses have been performed on the nonlinear, far from equilibrium system in all practically interesting dimensions. The dynamic scaling form, which completely determines the fractal properties of the interface morphology, has been derived and found to be in extremely good agreement with numerical simulations. A new universality class of the growth regime, characterized by a novel superscaling relation, is obtained. For substrate dimension d = 1, the interface morphology is significantly less rough than that observed in the nonconserved system; at the critical dimension $d sb{c}$ = 2, the interface is found to be logarithmically rough. The dynamic roughening transition of the conserved driven interface has also been studied by taking into account a lattice pinning potential. This conserved system exhibits a true phase transition, rather than crossover behavior, as has been observed for nonconserved driven interfaces. The conserved nonlinear driving force is favorable for smoothing the interface, implying that the roughening transition shifts to higher temperatures as that driving force is increased. The nature of the phase transition remains the same as the equilibrium transition; the critical properties are controlled by a Kosterlitz-Thouless fixed point.

In the present thesis, we study the change in structural properties induced by ion beam irradiation of Ni/Fe and Co/Cu bilayers using various x-ray scattering techniques. These bilayers exhibit interesting GMR and magnetotransport properties. / We show that an N-step model is useful in simulating any given electron density profile. We test four different interface profile functions in fitting the reflectivity and conclude that the error-function profile best describes our samples. Different types of interfaces are introduced, namely graded and rough interfaces, together with a discussion of their representation and their effect on both specular reflectivity and non-specular x-ray intensity. / We develop a data acquisition and processing method in order to separate the specular and diffuse components of x-ray scattering and to obtain the normalized reflectivity. A computer program in C was developed to calculate the x-ray reflectivity (XRR) and diffuse scattering intensity and to fit the theoretical calculation to the experimental data using a non-linear least-squares fitting method. / By fitting the XRR data of six bilayers of Ni/Fe and Cu/Co of different thicknesses and deposition sequence, the electron density profiles are constructed for different irradiation doses, &phis;. The intermixing at interfaces is found to increase with increasing &phis;. No change in the bulk materials electron density is observed upon irradiation of four single layers of these materials. / A more detailed study is performed on Si/Ni(500A)/Fe(500A) bilayers. From diffuse-scan fits we find that as &phis; increases the interfaces become rougher, more jagged and the height-height correlation length of the roughness decreases. The intermixing can be approximated using the ballistic model of ion mixing. / Using high-angle x-ray diffraction (XRD) measurements, the samples are found to be polycrystalline with a strong texture of fcc Ni(111) and bcc Fe(101) parallel to the substrate surface. Both plane-view and cross-sectional transmission electron microscopy (TEM, XTEM) images show that in-plane and out-of-plane grain sizes increase with &phis;, in good agreement with out-of-plane grain sizes calculated from Bragg peaks. The high-angle x-ray Bragg peak positions agree well with selected-area electron diffraction (SAED) rings. The iron oxide parameters obtained from XTEM and SAED patterns agree well with XRR results.

We investigate the equilibrium properties and the relaxational dynamics of random heteropolymers in three dimensions. We proceed by studying an isolated random heteropolymer without an explicit solvent before studying different types of random heteropolymers in a monomeric solvent. Both equilibrium and relaxational data were obtained by performing extensive off-lattice molecular dynamics simulations. An equilibrium "phase diagram" is determined in all cases which facilitates the determination of a characteristic temperature separating extended states from collapsed states of the heteropolymer. This temperature is an increasing function of the strength of the random interactions and of the solvent density. It allows us to determine the appropriate initial and final temperatures to be used for the quenches from an extended coil to a collapsed globule. In particular for long isolated chains the heteropolymer collapse is a process which involves two distinct time regimes; both are energetically of stretched exponential form. Our simulation clearly shows the nature of the relaxation process where the formation of locally collapsed clusters takes place first followed by a global aggregation of the local blobs leading to the two collapse time regimes. Finally, we determine the effect of an explicit solvent on the collapse dynamics of a random heteropolymer. We bridge the difference between implicit and explicit solvents in two steps. First, we consider a heteropolymer with random interaction between the monomers, immersed in a purely repulsive solvent and we find that the rate of collapse increases with the strength of the random interaction but remains almost independent of the solvent density for higher values of the strength of the random interaction. Secondly, we look at a random heteropolymer with random interaction between the monomers and the solvent particles and we find that the rate of collapse increases slightly with density but is almost constant at low dens

We propose a model based on a Ginzburg-Landau approach to study a strain relief mechanism at a free interface of a non-hydrostatically stressed solid, commonly observed in thin-film growth. The evolving instability, known as the Grinfeld instability, is of high technological importance. It can be associated with the dislocation-free island-on-layer growth mode in epitaxy which is an essential process used in the semiconductor industry. / In our model, the elastic field is coupled to a scalar order parameter in such a way that the solid supports shear whereas the liquid phase does not. Thus, the order parameter has a transparent meaning in the context of liquid-solid phase transitions. / We show that our model reduces in the appropriate limits to the sharp-interface equation, which is the traditional formulation of the problem. Inherent in our description is the proper treatment of non-linearities which avoids the numerical deficiencies of previous approaches and allows numerical studies in two and three dimensions. / To test our model, we perform a numerical linear stability analysis and obtain a dispersion relation which agrees with analytical results. We study the non-linear regime by measuring the Fourier transform of the height-height correlation function. We observe that, as strain is relieved, interfacial structures, corresponding to different wave numbers, coarsen. Furthermore, we find that the structure factor shows scale invariance. We expect that our result on transient coarsening phenomena can be measured through microscopy or x-ray diffraction.