Unraveling effects of disorder on the electronic structure of SiO(2) from first principles

We present a first-principles systematic study of the electronic structure of SiO(2) including the crystalline polymorphs alpha quartz and beta cristobalite, and different types of disorder leading to the amorphous phase. We start from calculations within density functional theory and proceed to more sophisticated quasiparticle calculations according to the GW scheme. Our results show that different origins of disorder have also different impact on atomic and electronic-density fluctuations, which affect the electronic structure and, in particular, the size of the mobility gap in each case.

Arvopaperistaminen ja luottojohdannaiset luottoriskin ja omien varojen hallinnan välineenä

Tässä tutkimuksessa selvitetään, voisiko Pohjola Pankki tehostaa omien varojen käyttöään ja luoda taloudellista lisäarvoa osakkeenomistajilleen hyödyntämällä arvopaperistamista tai luottojohdannaisia rahoitustaseen yritys-lainasaamisiin sisältyvän luottoriskin hallintaan. Työssä pohditaan myös ko. instrumenttien soveltuvuutta ja transaktion toteutusprosessia Pohjola Pankin näkökulmasta. Tutkimus on empiirinen tapaustutkimus, jossa on hyödynnetty kvantitatiivista tutkimusmetodia. Tulosten valossa Pohjola Pankki pystyy alentamaan omien varojen määrä luomaan lisäarvoa ja parantamaan riskipainotettua kannattavuuttaan arvopaperistamalla yrityslainasaataviaan tai ostamalla luottojohdannaisen. Huomioiden kohdeyrityksen luottoportfolion rakenteen ja transaktion toteuttamiseen vaadittavat toimenpiteet, soveltuu luottojohdannainen arvopaperistamista paremmin luottoriskin johtamiseen kohdeyrityksessä – edellyttäen, että suojaus-kustannukset eivät nouse liian korkeiksi ja vapautuvat omat varat saadaan hyödynnettyä mahdollisimman tehokkaasti.

Optical Excitations and Field Enhancement in Short Graphene Nanoribbons

The optical excitations of elongated graphene nanoflakes of finite length are investigated theoretically through quantum chemistry semiempirical approaches. The spectra and the resulting dipole fields are analyzed, accounting in full atomistic details for quantum confinement effects, which are crucial in the nanoscale regime. We find that the optical spectra of these nanostructures are dominated at low energy by excitations with strong intensity, comprised of characteristic coherent combinations of a few single-particle transitions with comparable weight. They give rise to stationary collective oscillations of the photoexcited carrier density extending throughout the flake and to a strong dipole and field enhancement. This behavior is robust with respect to width and length variations, thus ensuring tunability in a large frequency range. The implications for nanoantennas and other nanoplasmonic applications are discussed for realistic geometries.

Towards higher precision and operational use of optical homodyne tomograms

We present the results of an operational use of experimentally measured optical tomograms to determine state characteristics (purity) avoiding any reconstruction of quasiprobabilities. We also develop a natural way how to estimate the errors (including both statistical and systematic ones) by an analysis of the experimental data themselves. Precision of the experiment can be increased by postselecting the data with minimal (systematic) errors. We demonstrate those techniques by considering coherent and photon-added coherent states measured via the time-domain improved homodyne detection. The operational use and precision of the data allowed us to check purity-dependent uncertainty relations and uncertainty relations for Shannon and Renyi entropies.

Electronics and Optics of Graphene Nanoflakes: Edge Functionalization and Structural Distortions

The effects of edge covalent functionalization on the structural, electronic, and optical properties of elongated armchair graphene nanoflakes (AGNFs) are analyzed in detail for a wide range of terminations, within the framework of Hartree-Fock-based semiempirical methods. The chemical features of the functional groups, their distribution, and the resulting system symmetry are identified as the key factors that determine the modification of strutural and optoelectronic features. While the electronic gap is always reduced in the presence of substituents, functionalization-induced distortions contribute to the observed lowering by about 35-55% This effect is paired with a red shift of the first optical peak, corresponding to about 75% of the total optical gap reduction. Further, the functionalization pattern and the specific features of the edge-substituent bond are found to influence the strength and the character of the low-energy excitations. All of these effects are discussed for flakes of different widths, representing the three families of AGNFs.

SG Pseudo-Differential Operators and Weak Hyperbolicity

2002 Mathematics Subject Classification: 35S05, 47G30, 58J42.