No description available.

No description available.

No description available.

No description available.

No description available.

In this thesis, the recent advances in studies on rare-earth metal intercalated fullerene solids with emphasis on their structural, electronic, and magnetic properties. The investigations on the rare-earth based fullerides have been concentrated on their structural, electronic, and magnetic properties. Intercalation of C(_60) with rare-earth metals results in interesting compounds not only for the appearance of superconductivity but also for the magnetic properties and mixed valence phenomena related to the localised 4f electrons. Of particular interest, I discuss the results obtained from various experiments on rare-earth based mixed valence fullerides, of which displays a remarkable sensitivity of rare-earth valency to external stimuli, such as temperature and pressure. Among the family of rare-earth fullerides, Sm(_2.75)C(_60) was the first known molecular-based material to show valence fluctuation associated with the highly-correlated narrow-band behaviour of the 4f electrons in Sm ions. Improvement in the synthetic technique to produce single-phase rare-earth doped fullerides have opened the way to carry out detailed and systematic study of the structural properties of the RE(_2.75)C(_60) (RE = Sm, Eu, and Yb) as a function of temperature and pressure, which were carried out using the synchrotron X-ray powder diffraction technique. The obtained results have lead us to find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions. In addition, we have observed that by taking precise control on the nature of dopants, the tuning of the onset temperature and pressure of this valence transition were possible. Direct measurements on the valence states of the rare-earth ions in the fulleride salts as a function of temperature were carried out using X-ray absorption spectroscopy using the alkaline-earth and rare-earth mixed compound, (Sm(_2/3)Ca9_1/3))(_2.75) C(_60). The obtained spectra have provided clear evidence to confirm the electronic nature of the low-temperature first-order valence transition.

In this thesis, we model theoretically spectra measured for bilayer graphene obtained using the angle-resolved photoemission spectroscopy, magneto-optical absorption spectroscopy and electronic Raman spectroscopy. The theories are based on the tight-binding description of the IT bands in the material. In particular, we concentrate on the comparison of the four-band model and its effective low-energy approximation neglecting the split high-energy bands, in the description of specific spectra. We demonstrate that both for monolayer and bilayer graphene, the observed anisotropy of angle-resolved photoelectron spectroscopy spectra reflects the electronic chirality in the system. However, for bilayer graphene, the influence of the nonchiral dirner states not captured within the effective approximation is significant and should not be neglected. We also show that the anisotropy of the constant-energy maps may be used to extract information about the magnitude and sign of interlayer coupling parameters and about symmetry breaking inflicted on a bilayer by the underlying substrate. We then determine selection rules and optical strengths of the inter-Landau-level excitations among any of the IT bands and including the physically most relevant symmetry-breaking parameters. We then present a self-consistent calculation of the interlayer asymmetry caused by an applied electric field in magnetic fields. We show how this asymmetry influences the Landau level spectrum in bilayer graphene and the observable inter-Landau level transitions when they are studied as a function of high magnetic field at fixed filling factor as measured experimentally. We also analyse the magneto-ptical spectra of bilayer flakes in the photon-energy range corresponding to transitions between degenerate and split bands of bilayers. Finally, we investigate the contribution of the low- energy electronic excitations toward the Raman spectrum of bilayer graphene for the incoming photon energy Ω » leV Using the four-band model, we derive an effective scattering amplitude that can be incorporated into the two-band approximation and show that this amplitude is different from the contact interaction amplitude obtained within the two-band model alone. We then calculate the spectral density of the inelastic light scattering accompanied by the excita- tion of electron-hole pairs in bilayer graphene. In the absence of a magnetic field this contribution is constant and in doped structures has a threshold at twice the Fermi energy. In an external magnetic field, the dominant Raman-active modes are the n_ ->n+ inter-Landau-level transitions with crossed polarization of in/out photons. We estimate the quantum efficiency of a single n_ ->n+ transition in the magnetic field of 10 T as In_-'f1+ - 1012.

We consider two parallel separately controlled graphene monolayers in which external gates induce a finite density of electrons in one layer and holes in another layer. In this thesis the theory of the excitonic insulator in such systems is developed. We analyze the symmetry of the excitonic state in the system, classify all possible phases, and build a phase diagram that takes into account the effect of the symmetry breaking due to the external electric and magnetic fields. The large-N approximation is used, where N = 8 is a number of electron's species in the system. Taking into account leading and sub-leading orders in 1/N expansion of the dynamically screened Coulomb interaction in the system, for the most energetically stable phase in absence of magnetic field we find a gap in the single-particle excitation spectrum at zero temperature. The result for the gap is found up to the first sub-leading order in 1/N. We determined the leading 1/N contribution to the exponential pre-factor in the expression for the gap Δ=Cexp[-2N]EF . Also we consider the simplified interaction, which reflects two most important features of the dynamically screened interaction: 1 )the interaction between charge carriers on Fermi surface, which is the most important for the excitonic condensation, and 2)the undamped plasmon. In such a case, in contrast to the suggested by other authors Ref.[59] enhancement of transition temperature Tc by plasmon pole in the dynamically screened interaction, we find no enhancement of Tc and arrive at Tc≈10-7EF, which is comparable to the result Tc≈10-7EF which was obtained using a statically screened interaction [42, 43]. Moreover, we found the presence of Goldstone modes with linear spectrum, therefore fluctuations of the order parameter suppress the transition into the excitonic insulator state even further.

No description available.

Le dioxyde de carbone occupe une place particulière dans l'amplification anthropogénique de l'effet de serre. En particulier, son utilisation dans la synthèse de carbonates organiques à partir d'alcools constituerait une alternative à l'emploi actuel du phosgène ou du monoxyde de carbone. L'insertion de CO2 dans Sn-OCH3 liaison de dialkyldimethoxystannanes est reconnue comme la première étape pour la formation du carbonate de diméthyle (DMC) à partir du méthanol et CO2. L'identification des étapes ultérieures est cruciale pour améliorer l'activité et est encore l’objet de débats car les espèces étain ont la propension à l’oligomérisation. Nous avons utilisé des calculs basés sur la Théorie de la Fonctionnelle de la Densité pour donner un aperçu sur le mécanisme de réaction. L'insertion de CO2 est promue par l’interaction acide-base concertée de Lewis de CO2 avec l'étain et l'atome d'oxygène du ligand méthoxy. Le chemin principal de la réaction pour la formation de DMC est proposé selon un réarrangement intramoléculaire des espèces monomériques, Me2Sn[OC(O)OCH3]2. Le processus conduit à un transfert d'un groupe méthyle d'un carbonate de méthyle à l'autre via un anneau de 4 atomes ou 6 atomes formant un fragment Sn-CO3. Dans une deuxième étape, le méthanol réagit avec un intermédiaire et conduit à la formation de DMC et un complexe de trimère qui peut permettre la régénération des réactifs. Mais un complexe décanucléaire d’étain peut être aussi produit par une réaction latérale. Enfin, un cycle catalytique pour produire le DMC a pu être élaboré. Les calculs DFT sont en accord avec les données expérimentales et permettent une analyse comparative des chemins de réaction / Carbon dioxide occupies a special place in the amplification of the anthropogenic greenhouse effect. In particular, its use in the synthesis of organic carbonates from alcohols, constitute an alternative to the current application of phosgene or carbon monoxide. The insertion of CO2 into the Sn-OCH3 bond of dialkyldimethoxystannanes is recognized as the first step to dimethyl carbonate (DMC) formation from methanol and carbon dioxide. The identification of the subsequent steps is crucial for activity improvements and is still under debate as the tin species have the propensity for oligomerization. We have used density functional theory calculations to provide insight into the reaction mechanism. The CO2 insertion into the Sn-OCH3 bond is promoted by the concerted Lewis acid-base interaction of CO2 with tin and the oxygen atom of the methoxy ligand. The major reaction pathway to DMC is proposed to occur via an intramolecular rearrangement of the monomeric species, Me2Sn[OC(O)OCH3]2. The process results in the transfer of a methyl group from one methyl carbonate to the other via a 4-membered or 6-membered ring forming the Sn-CO3 moiety. In a second stage, methanol reacts with one intermediate and leads to DMC formation and a trimer compound which may allow the regeneration of the reactant. Also a decanuclear tin complex is produced due to the lateral reaction. Finally a catalytic cycle for DMC production in methanol can be elaborated. DFT calculations are in agreement with the experimental data and allow a comparative analysis of reaction channels