Resonant Inelastic X-ray Scattering Studies of Low-Dimensional and Frustrated 3d Transition-Metal Oxides

3d transition-metal oxide materials with strong electron correlations and low dimensionality give rise to emerging exotic phases. Resonant inelastic X-ray scattering (RIXS) has developed as a powerful spectroscopic tool for probing the collective excitations in such correlated materials. The RIXS technique provides an ideal approach for assessing electronic instabilities in condensed matter. This thesis comprises the experimental RIXS response of the spin chain-ladder cuprates Sr14-xCax(Cu,Co)24O41 and frustrated honeycomb nickelate Na2Ni2TeO6. The spin and charge excitations of Sr14-xCaxCu24O41 (x=0 & 12.2) are studied by Cu L3-edge RIXS. With increasing Ca content, a crossover from collective two-triplon excitations (x=0) to a damped incoherent magnetic spectrum ~280 meV (x=12.2) in the ladders is uncovered from the RIXS spectra, dominated by spin-flip ÎS=1 scattering. With model calculations, the localized broad magnetic mode at x=12.2 is shown to be a consequence of enhanced elecrtronic localization. This is supported by polarization-dependent measurements, where the evaluated non-spin-flip ÎS=0 scattering is showing weight depletion below 1 eV. The RIXS spectral evolution suggests that a suppressed carrier mobility dominates the low-energy physics in Ca-rich phases. The low-energy excitations of Sr14Cu24O41 are also studied by O K-edge RIXS at the upper Hubbard band. The RIXS spectra show a sharp dispersing peak of similar energies to the ÎS=1 two-triplon excitations, and a broad non-dispersive mode at higher-energy. By comparing the experimental results to the existing theories of doped ladders, the observed spectral modes are attributed to interacting holon-spinon quasiparticles, with additional spectral contributions from the ÎS=0 multi-triplon bound states and continuum. These observations highlight the RIXS capability for resolving composite spin-charge quasiparticles and long-lived ÎS=0 magnetic excitations, which are difficult to detect by other experiments. Charge order and phonons for the two-leg ladders of Sr14(Cu,Co)24O41 are studied. At the ladder hole peak in O K-edge XAS signal, a concomitant elastic enhancement and phonon-softening is observed across all Co doping levels. The in-plane diffraction and phonon-softening response are found to enhance with increasing Co doping. Morevoer, the observed diffraction signal is further characterized by energy- and temperature-dependent measurements. Lastly, given the strong electron-phonon coupling (EPC) observed in O K-edge RIXS spectra, the EPC strength can be analyzed from the RIXS intensity decay of the phonon overtones . The observed in-plane resonant diffraction and inelastic phonon response strongly suggest an enhanced stripe order via magnetic impurities, reminiscent of the reports in CuO4 plaquettes. Electronic structure and EPC of the honeycomb nickelate Na2Ni2TeO6 are studied. Compared with current literature, the localized excitations (1-4 eV) in the Ni L3-edge RIXS spectra are dominated by the crystal-field splitting of a Ni2+ ion in octahedral coordination. At the O K-edge, the large EPC ~285 meV with 5 recognizable overtones (frequency of ~80 meV) are revealed in the RIXS spectra. The observed modes provide a systematic characterization of charge, orbital and lattice dynamics for a newly proposed frustrated antiferromagnet. This thesis has been carried out in a collaboration between the Paul Scherrer Institut and the Laboratory for Quantum Magnetism at EPFL.

Spectroscopic Studies of the Electronic Structure in Layered Cuprates and Ruthenates

Claudia Giuseppina Fatuzzo

In materials where electrons interact strongly, a number of exotic and exciting phenomena arise. The mechanisms at the base of many of these phenomena remain debated, as strongly correlated electron physics represents one of the biggest challenges for modern condensed matter physics. Transition metal oxides are a class of strongly correlated systems which exhibit a multitude of physical properties, among which unconventional superconductivity, incommensurate charge and spin ordering, partial anisotropic gapping, strangemetallicity, colossal magnetoresistance, multiferroicity, antiferromagneticMott insulation, etc.. These properties are often observed in close vicinity, when pressure, temperature or chemical composition are varied. As a consequence, it is challenging to disentangle not only their origin, but also their effects on the elementary excitations of the system. In this thesis, high quality single crystals of copper and ruthenium based transition metal oxides (cuprate and ruthenate compounds) were investigated with synchrotron-based spectroscopic techniques. Our angle resolved photoemission spectroscopy (ARPES) studies focused on the normal state of cuprates, at hole content varying from under- to over-doping. First, the low energy single particle excitations in overdoped uprates were verified to fulfill the mathematical conditions for Landau Fermi quasiparticles. In a second study, the evolution of the spectral gap was followed as function of doping and temperature, across the charge order, pseudogap and strangcuprates, ruthenates, strong correlations, RIXS, ARPES, XAS, pseudogap, strange metal, spin orbit coupling, crystal field.e metal phases of a cuprate compound, La1.6¡xNd0.4SrxCuO4. This systematic study allowed the identification of an optimal doping regime for the investigation of the pseudogap physics in cuprates. The orbital structure of single layer ruthenates was explored combining X-ray absorption and resonant inelastic X-ray spectroscopies (XAS and RIXS). Since spectroscopies at the L absorption edge of 4d materials are challenged by insufficient energy resolution, we performed our studies at the oxygen K edge, in the soft X-ray energy range. By exploiting the strong orbital hybridization between the oxygen 2p and the ruthenium 4d states, we obtained high resolution experimental access to the 4d electron properties. The results were interpreted through a simple model Hamiltonian, with analytic solutions that provide a consistent description of the low energy features observed.

EPFL2017

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

Investigation of Two-Dimensional Electron Gases with Angular Resolved Photoemission Spectroscopy

Isabella Gierz

In this thesis we study the electronic structure of different two-dimensional (2D) electron systems with angular resolved photoemission spectroscopy (ARPES). This technique is based on the photoelectric effect and directly probes the electronic structure of a system. By carefully analyzing the measured band structure with respect to peak position and line width we can determine the complex self-energy Σ that describes the renormalization of the electron's energy and the change in lifetime due to many-body interactions. The 2D electron systems investigated in this work are surface alloys on Ag(111), bismuth trimers on Si(111) and epitaxial graphene monolayers grown on SiC(0001). Surface alloys on Ag(111) are formed by depositing 1/3 of a monolayer of bismuth, lead or antimony (alloy atoms) on the clean silver surface. Although (Bi,Pb,Sb) and Ag atoms are immiscible in the bulk they form long-range ordered surface alloys, where every third Ag atom is replaced by an alloy atom. These systems as well as the Bi trimers on Si(111) show a spin splitting of the 2D band structure due to the Rashba-Bychkov (RB) effect. The RB model states that in a symmetry broken environment (such as the surface of a semi-infinite crystal) the spin-orbit interaction will lift the spin-degeneracy of the band structure. Such a spin-split band structure bares great potential for applications in the field of spintronics, e.g. in a Datta-Das spin field effect transistor. In the present work we investigate the origin of the observed giant spin splitting in surface alloys, especially the interplay between structural parameters and the atomic spin-orbit interaction. Furthermore, we will show that it is possible to transfer these concepts to a semiconducting substrate, which is better suited for spintronics applications. The third system under investigation — graphene — is an ideally two-dimensional crystal. It consists of a single layer of carbon atoms arranged in a honeycomb lattice, and its charge carriers are confined within a plane that is just one atom thick. These charge carriers behave like massless Dirac particles and possess extremely high carrier mobilities. This makes graphene a promising material system for high-speed electronic devices. In order to reach this ambitious goal one needs reliable methods for the large-scale production of high quality graphene films. Epitaxial growth on silicon carbide (0001) substrates is the method of choice in this case, as it offers the advantage of a precise thickness control and a semiconducting substrate at the same time. However, the presence of the substrate reduces the carrier mobility of graphene's charge carriers considerably. Therefore, it is necessary to decouple the graphene layer from the substrate after epitaxial growth. A second issue that needs to be addressed, are viable doping methods for graphene. As graphene's peculiar band structure results from a sensible interplay between electrons and crystal lattice it is not an option to replace single atoms of the graphene lattice by dopants as is common practice when doping silicon. In order to preserve its band structure, graphene is usually doped by adsorbing atoms or molecules on its surface. As graphene grown on SiC is n-doped due to charge transfer from the substrate, appropriate means for p-type doping are clearly required. In this thesis, we will present a new growth method for quasi free-standing graphene on SiC(0001) and viable means for p-type doping.

EPFL2011