Spin and charge density fluctuations are important excitations in the strongly correlated systems, especially in the recently discovered high temperature superconductors. Several different theories on high temperature superconductors have been proposed based on spin fluctuations. However, experiments have also shown the existence of strong charge fluctuations. It is, therefore, desirable to investigate the physical consequences of the coexistence of strong spin and charge density fluctuations. As a first step toward a full understanding of both spin and charge excitations, a self-consistent theory is established. In this self-consistent theory, there are three important quantities, the spin susceptibility, the charge susceptibility, and the phonon Green's function. These three quantities are coupled together by the electron-phonon and phonon-spin fluctuation interactions. The phonon-spin fluctuation interaction is derived by making use of the spin-orbital coupling. For a strongly correlated system, the spin and charge density excitations have to be considered self-consistently. They are intimately related. The effects of antiparamagnons on phonons are also investigated. Antiparamagnons can have dramatic effects on phononic properties. It is found that new modes are formed in the presence of antiferromagnetic spin fluctuations. The de Haas-van Alphen effect in marginal and nearly antiferromagnetic Fermi liquids is studied. It is found that the de Haas-van Alphen frequency is unaffected by the anomalous response functions of the marginal and nearly antiferromagnetic Fermi liquids due to the absence of real parts of self-energies on the imaginary frequency axis. / Graduation date: 1993

Ce mémoire de « Habilitation à Diriger des Recherches » est divisé en deux parties. <br /><br /> La partie I est composée de trois chapitres. Les deux premiers traitent des interactions entre porteurs et vibrations du réseau. Le troisième est consacré aux états électroniques non-liés des boîtes quantiques. <br />Le premier chapitre de la première partie (I-1) présente une description générale de l'interaction électron-phonon dans les semiconducteurs, du massif aux boîtes quantiques. L'étude des interactions entre porteurs et vibrations du réseau a grandement bénéficié des importantes avancées dans les domaines de l'élaboration de matériaux nanostructurés et des techniques d'analyse spectroscopiques. L'accès à la structure fine des transitions optiques (par exemple, par des techniques de sonde locale à haute résolution spectrale et/ou temporelle) a permis la mise en évidence de différents effets liés aux couplages électron-phonons. Ainsi, les ailes de phonons acoustiques d'une paire radiative confinée dans une boîte quantique ont été observées dans différents types de boîtes quantiques. De même, l'existence d'un fort couplage entre les porteurs confinés et les vibrations optiques est désormais bien établie. Finalement, les processus de diffusion des porteurs par les phonons, omniprésents en physique des semiconducteurs massifs, est encore aujourd'hui au centre de nombreuses études expérimentales et théoriques dans les nanostructures. Le chapitre 1 présente donc une revue de ces différents aspects du couplage entre porteurs et phonons : leur description et leurs conséquences optiques. Il faut noter que le matériel de ce chapitre ne correspond pas à une contribution personnelle de « recherche » dans le domaine, mais plutôt à un souci pédagogique de présentation, d'une manière unifiée, des différentes facettes de l'interaction électron-phonon dans les boîtes quantiques auto-organisées <br />Le deuxième chapitre de la première partie présente une discussion détaillée des états polarons (première partie du chapitre : I-2A) et de la relaxation en énergie (deuxième partie du chapitre : I-2B) dans les boîtes quantiques auto-organisées de semiconducteurs. Ces états correspondent aux véritables excitations élémentaires des boîtes quantiques auto-assemblées. En effet, la description d'ordre zéro en termes d'états découplés électrons-phonons devient inappropriée en présence d'un fort couplage entre les porteurs confinés et les vibrations du réseau. On n'a pas fini d'explorer toutes les conséquences de ce couplage fort, encore aujourd'hui au centre de nombreuses études théoriques et expérimentales. Parmi les champs actuels d'investigations dans ce domaine, il faut citer :<br />(i) l'étude de la dynamique (linéaire) de relaxation de l'énergie des états excités d'une boîte quantique, mettant en jeu des processus intrinsèquement liés aux états mixtes de polarons (comme la désintégration par couplage anharmonique ; voir deuxième partie du chapitre) ;<br />(ii) les mises en évidence de couplage polaron pour les transitions optiques des boîtes quantiques chargées avec un trou (domaine FIR) ou intrinsèques (en optique interbande) ;<br />(iii) les études récentes de la dynamique non-linéaire (sous excitation FIR intense) des transitions polarons des boîtes;<br />(iv) l'observation d'un effet polaron pour les boîtes doublement chargées.<br /><br />Le chapitre trois de la première partie (I-3) est un article de revue sur les états non-liés des boîtes quantiques de semiconducteurs. La plupart des études sur les boîtes quantiques sont focalisées sur la partie basse énergie du spectre (états liés). Le spectre continu joue toutefois un rôle de premier plan dans beaucoup de situations : par exemple, ce sont les états finals pour les processus d'ionisation des boîtes dopées, ou les états initials pour les processus de capture (chargement des boîtes). Ces processus d'ionisation et (re)-capture se trouvent au sein même du fonctionnement de différents dispositifs, comme les détecteurs et lasers à base de boîtes quantiques. De même, ils jouent un rôle décisif dans les transitions optiques inter-bandes, à cause des transitions « croisées », c'est-à-dire, faisant intervenir des états liés d'une bande avec des états du continuum de l'autre. Pourtant, peu de travaux ont été consacrés à l'analyse de ces états. Une bonne partie du travail discuté dans ce chapitre est le résultat d'une longue étude, initiée au LPA il y a maintenant une petite dizaine d'années avec les premières études des mécanismes de capture assistée par les phonons et de relaxation Auger intra-boîte, poursuivie par le travail d'A. Vasanelli sur le fond spectral d'absorption inter-bandes, et consolidée par les récentes simulations numériques de N. D. Phuong et N. Regnault de la magnéto-absorption de différentes structures à boîtes quantiques pour la photo détection IR. Comme souligné à la fin de ce chapitre, les états non-liés jouent un rôle important dans bien d'autres situations physiques, comme pour les états « liés » excités à plusieurs particules. Ces voies de recherche, plus récentes, sont encore à leurs commencements.

Transport properties of the two-dimensional electron gas (2DEG) in high magnetic fields are used to investigate scattering processes affecting the resistivity of GaAs-GaAlAs and GaInAs-InP heterojunctions and quantum wells: especially coupling of electrons to acoustic and optic phonons; and transitions between electric subbands. The experiments fall into two groups: A systematic study of magnetophonon resonance (MPR) between 30K and 300K. Resonance positions indicate a coupling substantially below the LO phonon energy, expected from 3D measurements. GaAs-GaAlAs hetero junctions show amplitudes varying smoothly with electron density (n_s) and closely related to the 4K mobility. On rotation in magnetic field they decrease rapidly as the resonance position returns to the LO value. In modulation doped structures the damping factor is determined by remote impurity scattering. As n_s is increased in GaInAs-InP the coupling frequency decreases dramatically from the GaAs-like LO at 272cm^-1 to the InAs-like TO at 226cm^-1. At higher electric fields the 'normal' MPR maxima invert, starting at low magnetic fields, to form 'hot electron' MPR minima, with maximum amplitude at ~60K. This is the first direct observation of HEMPR in 2D and is explained in a diffusion picture. At lower electric fields, additional resonances are identified with resonant cooling by inter-subband scattering. Comparisons are made with calculations and explanations sought including consideration of interface phonons; coupled plasmon-phonon modes; and shifts of the resonance positions due to the shape of the density of states. Low temperature magnetoresistance measurements in GaAs-GaAlAs heterojunctions with more than one occupied electric subband. Shubnikov-de Haas oscillations in perpendicular magnetic fields contain non-additive terms at electron temperatures > 2K where acoustic phonon mediated inter-subband scattering is comparable to intra-subband scattering. Subband separations and greatly enhanced g-factors [largest for electrons in the upper subband ] are deduced from the oscillations. Damping of the oscillations in field, gives values for quantum lifetimes (τ_s), much smaller than τ_tʼ, deduced from mobility. With two subbands occupied τ_s is always largest for the upper subband, while relative sizes of τ_t depend on sample quality. Study of electron energy loss rates, from thermal damping of the oscillations, shows enhancement in the region kT_e ~ ħω_cʼ, which is evidence for cyclotron phonon emission. Depopulating subbands in parallel fields causes the resistance to drop, by up to 60%, due to suppression of inter-subband scattering. Systematic studies show this scattering rate is independent of n_s.

Thesis (Ph.D.)--Ohio University, August, 2008. / Title from PDF t.p. Includes bibliographical references.

The Fröhlich coupling describes the interaction between electrons and infrared-active vibrations at long wavelength in polar semiconductors and insulators, and may result in the formation of polaronic quasiparticles. Polarons are electrons dressed by a phonon cloud, which can strongly affect the electronic properties of the crystal. Despite their ubiquitous role in a broad range of technologies, first-principles investigations of the electron-phonon interaction in polar materials are scarce. In this thesis we develop a general formalism for calculating the electron-phonon matrix element in polar semiconductors and insulators from first principles, which represents a generalization of the Fröhlich model and can be used to compute the polar electron-phonon coupling as a straightforward post-processing operation. We apply this procedure to explore an important material for photovoltaics, the hybrid lead halide perovskite CH₃NH₃PbI₃. In this case we show that the temperature dependence of emission line broadening is dominated by Fröhlich coupling. Our method is formulated in conjunction with an ab initio interpolation technique based on maximally localized Wannier functions, which allows to describe all forms of electron-phonon coupling on the same footing. We demonstrate the validity of this approach on the prototypical examples GaN and SrTiO₃. Focusing on anatase TiO₂, a transition metal oxide of wide technological interest, we establish quantitatively the effect of including the ab initio Fröhlich coupling in the calculation of electron lifetimes. The rest of the thesis is devoted to exploring the quasiparticle properties in doped oxides. In particular, we investigate angle-resolved photoemission spectra from first principles in doped anatase TiO₂ by proposing a novel framework that combines our ab initio matrix elements, including the dynamical screening arising from the added carriers, and the cumulant expansion approach. We compare our results with experimental data, and show that the transition from a polaronic to a Fermi liquid regime with increasing doping concentration originates from nonadiabatic polar electron-phonon coupling. We further validate this mechanism by calculating angle-resolved photoemission spectra in the ferromagnetic semiconductor EuO.

Investigamos os efeitos de tamanho e do potencial de confinamento finito V0 nas taxas de espalhamento de absorção e de emissão de elétrons interagindo com os fônons longitudinais ópticos (fônons LO) de um fio quântico cilíndrico de GaAs à temperatura ambiente. Calculamos as taxas de espalhamento inter e intra-sub-banda e a taxa de espalhamento total para uma temperatura de 300 K, pois nesta temperatura o mecanismo de espalhamento dominante em semicondutores do tipo III-V é aquele devido aos fônons LO. Qualitativamente a taxa de emissão intra-sub-banda neste sistema tem o mesmo comportamento da sua correspondente em estruturas 2D. Para a absorção encontramos uma mudança suave de comportamento da taxa de absorção intra-sub-banda quando o raio do fio é da ordem do diâmetro do polaron (ou seja, da ordem de 80 ANGSTROM). Para raios pequenos ela tem um comportamento similar ao do bulk, mas para raios maiores ela cresce até atingir um máximo e depois cai monotonicamente à medida que aumentamos a energia do portador. Vimos que, o tamanho do fio e o potencial de confinamento têm grande influência na taxa de espalhamento total. / We investigated the size effects and the effects of the finite confining potential V0 on the absorption and emission scattering rates of electron interacting with longitudinal optical (LO) phonons for a cylindrical GaAs quantum wire. We calculated the inter and intrasubband total scattering rate for a temperature of 300K, because in this temperature the dominant mechanism of scattering in semiconductors III-V is that due LO phonons. Qualitatively the intrasubband emission scattering rate in this system has the same behavior of the correspondent in 2D structures. For absorption we found a smooth change in the intrasubband absorption scattering rate behavior when the radius the wire is near the polaron diameter (ie, about 80 ANGSTROM). For small radius the scattering rate has a similar behavior as that of the bulk, but for large radius it increases until reach a maximum and after ir drops monotonicaly with increase of carrier energy. We found that the size effect and the confining potential have a large influence in the total scattering rate

In this thesis I present contributions to the development, analysis and application of tensor network state methods for numerically exact quantum dynamics in one and two-dimensional lattice systems. The setting of numerically exact quantum dynamics is introduced in Chapter 2. This includes a discussion of exact diagonalization approaches and massively parallel implementations thereof as well as a brief introduction of tensor network states. In Chapter 3, I perform a detailed analysis of the performance of n-ary tree tensor network states for simulating the dynamics of two-dimensional lattices. This constitutes the first application of this class of tensor network to dynamics in two spatial dimensions, a long-standing challenge, and the method is found to perform on par with existing state-of-the-art approaches. Chapter 4 showcases the efficacy of a novel tensor network format I developed, tailored to electron-phonon coupled problems in their single-electron sector, through an application to the Holstein model. The applicability of the approach to a broad range of parameters of the model allows to reveal the strong influence of the spread of the electron distribution on the initial state of the phonons at the site where the electron is introduced, for which a simple physical picture is offered. I depart from method development in Chapter 5 and analyse the prospects of using tensor network states evolved using the time-dependent variational principle as an approximate approach to determine asymptotic transport properties with a finite, moderate computational effort. The method is shown to not yield the correct asymptotics in a clean, non-integrable system and can thus not be expected to work in generic systems, outside of finely tuned parameter regimes of certain models. Chapters 6 and 7 are concerned with studies of spin transport in long-range interacting systems using tensor network state methods. For the clean case, discussed in Chapter 6, we find that for sufficiently short-ranged interactions, the spreading of the bulk of the excitation is diffusive and thus dominated by the local part of the interaction, while the tail of the excitation decays with a powerlaw that is twice as large as the powerlaw of the interaction. Similarly, in the disordered case, analysed in Chapter 7, we find subdiffusive transport of spin and sub-linear growth of entanglement entropy. This behaviour is in agreement with the behaviour of systems with local interactions at intermediate disorder strength, but provides evidence against the phenomelogical Griffith picture of rare, strongly disordered insulating regions. We generalize the latter to long-ranged interactions and show that it predicts to diffusion, in contrast to the local case where it results in subdiffusive behaviour.

<p> A unified approach, employing effective phonon frequency distributions, is used to investigate effects of phonon anisotropy in the hcp metals.</p> <p> Phonon information is included by means of empirical force constant models, and pseudopotentials are used to describe the electron-ion interaction.</p> <p> Zinc and thallium superconducting gaps are determined as a function of position on the Fermi surface. The gap anisotropy is used to calculate thermodynamic properties.</p> <p> The normal state electron-phonon mass enhancement and the imaginary part of the electron self-energy are calculated as a function of temperature and Fermi surface position. Anisotropic transport scattering times are defined, calculated and used to obtain the polycrystalline and single crystal resistivities. Comparison is made with resistivities obtained by the variational approach.</p> / Thesis / Doctor of Philosophy (PhD)

In this dissertation, we employ a number of computational methods, including Ab Initio, Density Functional Theory, and Molecular Dynamics simulations to investigate key microscopic parameters that govern charge-transport and charge-generation in single-component and bimolecular charge-transfer organic semiconductors. First, electronic (transfer integrals, bandwidths, effective masses) and electron-phonon couplings of single-component organic semiconductors are discussed. In particular, we evaluate microscopic charge-transport parameters in a series of nonlinear acenes with extended pi-conjugated cores. Our studies suggest that high charge-carrier mobilities are expected in these materials, since large electronic couplings are obtained and the formation of self-localized polarons due to local and nonlocal electron-phonon couplings is unlikely. Next, we evaluate charge detrapping due to interaction with intra-molecular crystal vibrations in order to explain changes in experimentally measured electric conductivity generated by pulse excitations in the IR region of a photoresistor based on pentacene/C60 thin film. Here, we directly relate the nonlocal electron-phonon coupling constants with variations in photoconductivity. In terms of charge-generation from an excited manifold, we evaluate the modulation of the state couplings between singlet and triplet excited states due to crystal vibrations, in order to understand the effect of lattice vibrations on singlet fission in tetracene crystal. We find that the state coupling between localized singlet and correlated triplet states is much more strongly affected by the dynamical disorder due to lattice vibrations than the coupling between the charge-transfer singlet and triplet states. Next, the impact of Hartree-Fock exchange in the description of transport properties in crystalline organic semiconductors is discussed. Depending on the nature of the electronic coupling, transfer integrals and bandwidths can show a significant increase as a function of the amount of the Hartree-Fock exchange included in the functional. Similar trend is observed for lattice relaxation energy. It is also shown that the ratio between electronic coupling and lattice relaxation energy is practically independent of the amount of the Hartree-Fock exchange, making this quantity a good candidate for incorporation into tight-binding transport models. We also demonstrate that it is possible to find an amount of the Hartree-Fock exchange that recovers (quasi-particle) band structure obtained from a highly accurate G0W0 approach. Finally, a microscopic understanding of a phase transition in charge-carrier mobility from temperature independent to thermally activated in stilbene-tetrafluoro-tetracyanoquinodimethane crystal is provided.

Quantenkaskadenlaser (QCLs) wurden vor gerade zwanzig Jahren erfunden und haben seitdem stetig im weltweiten Markt der optoelektronischen Bauelemente für den Infrarot an Bedeutung gewonnen. Anwendungsbeispiele für aktuelle und potenzielle Einsatzgebiete von QCLs sind photoakustische Spektroskopie, Umweltüberwachung, Simulation von heißen Körpern, und optische Freiraumdatenübertragung. Rekord optische Leistungen von 14 W und Leistungseffizienzen zwischen 15-35 % wurden bei mittelinfraroten QCLs für Betriebstemperaturen zwischen 80-300 K erreicht. Die weitere Verbesserung dieser Eigenschaften hängt nicht nur von Aspekten wie Wärmemanagement und Chip-Packaging ab, sondern auch von Verbesserungen im Laserdesign zwecks der Reduzierung des Ladungsträgerleckstroms. Dennoch sind die verschiedenen Mechanismen und Komponenten des Leckstroms in Quantenkaskadenlasern leider noch nicht gründlich untersucht worden. Die vorliegende Arbeit liefert a realistische Beschreibung der Ladungsträgertransports in QCLs. Wir beschreiben u.a. Leckströme vom Quantentopf- in höhere Zustände und diskutieren elastische und inelastische Streumechanismen von Ladungsträgern bei mittelinfraroten Quantenkaskadenlasern. Wir illustrieren außerdem die Notwendigkeit zur Berücksichtigung der Elektronentemperatur für eine vollständigere Analyse der Ladungsträgertransporteigenschaften von Quantenkaskadenlasern. Methoden zur experimentellen Ermittlung des temperaturabhängigen Leckstroms in Quantenkaskadenlasern werden präsentiert. Unser Ansatz liefert eine Methode zur effektiven Analyse von der QCL-Leistung und Vereinfacht die Optimierung von QCL aktive Regionen. / Two decades after their invention in 1994, quantum-cascade lasers (QCLs) become increasingly important in the global infrared optoelectronics market. Photoacoustic spectroscopy, environment monitoring, hot object simulation, and free-space communication systems are selected examples of the current and potential applications of QCLs. Record optical powers as large as 14 W and power-conversion efficiencies ranging between 15-35 % have been reported for MIR QCLs for temperatures 80-300 K. Further improvement of these characteristics depends not only of aspects as heat management and chip-packaging, but also on improving the active-region design to reduce several leakage channels of charge carriers. However, mechanisms through which leakage of charge carriers affects QCLs performance have not been thoroughly researched. A better understanding of the several (non-radiative) scattering mechanisms involved in carrier transport in QCLs is needed to design new structures and optimize their performance. This work provides a realistic description of charge carriers transport in QCLs. We discuss in particular carrier leakage from QCL quantum-well confined states into higher and lower states. The two main mechanisms for non-radiative intersubband scattering in MIR QCLs are electron-longitudinal-optical-phonon scattering and interface roughness-induced scattering. We present methods for the experimental determination of the leakage current in QCLs at and above laser threshold, which allowed us to estimate the sheet distributions of conduction band states and better understand the impact of temperature activated leakage on QCLs characteristics. We found that even at temperatures low enough to neglect ELO scattering, carriers leakage due to IFR becomes significant for devices operating at high electron temperatures. Altogether, this approach offers a straightforward method to analyze and troubleshoot new QCL active region designs and optimize their performance.