Electronic structure of pristine and Ni-substituted LaFeO3 from near edge x-ray absorption fine structure experiments and first-principles simulations

We present a joint theoretical and experimental study of the oxygen K-edge spectra for LaFeO3 and homovalent Ni-substituted LaFeO3 (LaFe0.75Ni0.25O3), using first-principles simulations based on density-functional theory with extended Hubbard functionals and x-ray absorption near edge structure (XANES) measurements. Ground-state and excited-state XANES calculations employ Hubbard onsite U and intersite V parameters determined from first principles and the Lanczos recursive method to obtain absorption cross sections, which allows for a reliable description of XANES spectra in transition-metal compounds in a very broad energy range, with an accuracy comparable to that of hybrid functionals but at a substantially lower cost. We show that standard gradient-corrected exchange-correlation functionals fail in capturing accurately the electronic properties of both materials. In particular, for LaFe0.75Ni0.25O3 they do not reproduce its semiconducting behavior and provide a poor description of the pre-edge features at the O K edge. The inclusion of Hubbard interactions leads to a drastic improvement, accounting for the semiconducting ground state of LaFe0.75Ni0.25O3 and for good agreement between calculated and measured XANES spectra. We show that the partial substitution of Ni for Fe affects the conduction-band bottom by generating a strongly hybridized O(2p)-Ni(3d) minority-spin empty electronic state. The present work, based on a consistent correction of self-interaction errors, outlines the crucial role of extended Hubbard functionals to describe the electronic structure of complex transition-metal oxides such as LaFeO3 and LaFe0.75Ni0.25O3 and paves the way to future studies on similar systems.

Crystalline and correlated phases in two-dimensional transition metal dichalcogenides

Diego José Pasquier

This thesis is dedicated to the study of various aspects of the electronic structure of two-dimensional transition metal dichalcogenides (TMDs) of chemical composition MX

_2

(where M is a transition metal atom and X= S, Se, Te), using a combination of \textit{ab inito} density-functional methods. We first address the relative stability of the

1T

and

1H

phases of two-dimensional TMDs as a function of the column of the transition metal atom in the periodic table. Using a Wannier-function approach, we calculate crystal field and ligand field parameters for a broad range of members of this family of materials. Taking TaS

_2

as an example, we show how the splitting of the

d

electron states arises from an interplay of electrostatic effects and hybridization with the ligands'

s

p

and

d

states. We show that the ligand field alone cannot explain the stabilization of the

1H

polymorph for

d^1

and

d^2

TMDs, and that band structure effects are dominant. We present trends of the calculated parameters across the periodic table, and argue that these allow developing simple chemical intuition. Secondly, we study the occurrence of charge density wave phases and periodic lattice distortion in metallic

1T

transition metal dichalcogenides. The phonon dispersion and fermiology of representative examples with different

d

electron counts are studied as a function of doping. Two qualitatively different behaviours are found as a function of the filling of the

t_{2g}

subshell. We argue that away from half-filling, weak-coupling nesting arguments are a useful starting point for understanding, whereas closer to half-filling a strong-coupling real-space picture is more correct. Using Wannier functions, it is shown that strong metal-metal bonds are formed and that simple bond-counting arguments apply. Thirdly, the recently synthesized

1T

phase of NbSe

_2

, in monolayer form, is investigated from first principles. We find that

1T

-NbSe

_2

is unstable towards the formation of an incommensurate charge density wave phase, whose periodicity can be understood from the Fermi surface topology. We investigate different scenarios for the experimentally observed superlattice and insulating behaviour, and conclude that the star-of-David phase is the most stable commensurate charge density wave phase. We study the electronic properties of the star-of-David phase at various levels of theory and confirm its Mott insulating character, as speculated and in analogy with TaS

_2

. The Heisenberg exchange couplings are found to be ferromagnetic, which suggests a parallel with the so-called flat-band ferromagnetism in certain multiband Hubbard models. Finally, we address the possibility of the occurrence of the excitonic insulator phase in single-layer TiSe

_2

. The relative role of electron-electron and electron-phonon interactions in driving the charge density wave in layered and two-dimensional TiSe

_2

has been disputed and is still unresolved. We calculate the electronic structure and finite-momentum exciton spectrum from hybrid density functional theory. We find that in a certain range of parameters, excitonic effects are strong and the material is close to a pure excitonic insulator instability. A possible necessary condition for the physical realization of a pure excitonic insulator is proposed.

EPFL2019

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

This thesis presents the first experimental results of a scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) investigation of rubrene at the supramolecular, molecular and submolecular level. Based on its semiconducting and fluorescent properties, this molecule is of particular interest in view of the emerging fields of molecular electronics and optoelectronics which could one day replace the conventional technology relying on semiconductors such as silicon and gallium arsenide. The goal is the substitution of these inorganic materials by cheap and flexible layers of semiconducting organic molecules for a new class of diodes and transistors, as well as the realization of electronic switches based on individual molecules. One fundamental approach is to take advantage of the molecular self-assembly behavior which results in the creation of well-ordered supramolecular structures. The investigations of the self-assembly of rubrene adsorbed on metal surfaces (Au(111), Au(100), Ag(111), and Ag(100)), which were carried out within the framework of this thesis, show a surprising diversity of supramolecular structures. Amongst other shapes, the molecules organize themselves into geometries of perfect hexagonal and pentagonal symmetry and create multifaceted patterns on the surface. A fascinating peculiarity consists in the spontaneous construction of nested structures which are built up by a hierarchical self-assembly of individual molecules into pentagonal supermolecules which form in a second step perfect supramolecular decagons. The geometric shape of rubrene is characterized by a structural asymmetry leading to the existence of two mirror imaged versions of the molecule which are not superimposable to each other, such as for instance our left and right hand or the helical DNA. The aspect of chirality is crucial for basic processes in living systems and calls for a fundamental understanding of the interaction mechanisms occurring between chiral molecules. The experiments on rubrene reveal that the intermolecular bonding differentiates between the two chiral types of the molecule (chiral recognition), yielding the self-organization into homochiral structures. These assemblies exhibit a geometry which is again chiral, demonstrating a propagation of chirality throughout the three stages of the supramolecular hierarchy. The semiconducting behavior of rubrene is furthermore probed by STS measurements detecting the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The experimental data uncover that different adsorption conformations exhibit characteristic HOMO energies and reveal adsorption conformations of rubrene which preserve the intrinsic electronic structure of the free molecule. Furthermore, a switching of the molecular conformation and the electronic structure of one rubrene conformer is induced with the STM.

EPFL2006

Theoretical investigations of magnetic properties of MRI contrast agents and carbon nanostructures

Oleg Yazyev

The phenomenon of magnetism is one of the key components of today's technological progress. Magnetic interactions and magnetic materials are essential for the scientific disciplines of physics, chemistry and biology, making this subject truly multidisciplinary. This thesis is devoted to magnetic properties of two classes of substances. The first class represents the complexes of the ions of paramagnetic metals, primarily of the gadolinium(III) ion. These molecular compounds have an important application as magnetic resonance imaging (MRI) contrast agents for medical diagnostics. The design of more efficient MRI contrast agents requires a detailed knowledge of their magnetic properties. The other part of the thesis considers the broad class of recently discovered carbon nanostructures and materials. Their extraordinary physical properties foretell future applications of these materials in electronics, medicine and other fields. For instance, carbon nanotubes loaded with gadolinium(III) ion clusters are highly efficient MRI contrast agents. By using accurate density functional theory calculations in combination with classical molecular dynamics simulations, we determine hyperfine and quadrupole coupling constants on the nuclei of a first coordination sphere water molecule in gadolinium(III) aqua complexes. These parameters play a crucial role in the description of the key function, relaxivity, of MRI contrast agents. We found that the spin-polarization effect induced by the paramagnetic gadolinium(III) ion results in a Fermi contact hyperfine coupling of both the 1H and 17O nuclear spins and affects the dipole hyperfine coupling of the 17O nuclear spin of the inner coordination sphere water molecule. The 17O quadrupole coupling parameters of a coordinated water molecule are found to be very similar to that of neat water. We also apply the methodology of first principles molecular dynamics in order to perform realistic simulations of paramagnetic metal ions in water solution. This allows us to assess structure, dynamics and hyperfine interactions on the water molecules in the inner and outer coordination spheres of two metal ions: chromium(III) and gadolinium(III). In order to perform such calculations, we develop a novel approach for the evaluation of hyperfine coupling constants in pseudopotential electronic structure techniques. Our method takes into account the contribution of core electrons. In the second part of the thesis, we consider magnetic properties of a broad class of carbon nanostructures derived from two-dimensional graphene. We find that in metallic carbon nanotubes, an isotropic Knight shift, a hyperfine contribution to the nuclear magnetic resonance chemical shift, shows a regular dependence on the nanotube diameter. By using a more general approach, we reveal systematic dependences of magnetic interactions between arbitrarily distributed spin-polarized conduction electrons and nuclear spins in the carbon nanostructures derived from graphene. This knowledge is important for interpreting the results of magnetic resonance experiments and for evaluating the performance of carbon nanostructures as materials for alternative approaches in electronics, spintronics and quantum information processing based on electron and nuclear spins. In addition, we study magnetism in graphene induced by single-atom defects. The predicted itinerant magnetism due to the defect-induced states in graphenic materials may account for the experimental observations of ferromagnetism in irradiated graphite which has potential applications in technology. Finally, our first principles molecular dynamics study reveals the mechanisms of the irradiation-induced defect formation. We show that certain defect structures in layered carbon materials can be created selectively by irradiation at predefined conditions.

EPFL2007