Electronic Properties of Topological Materials: Optical Excitations in Moebius Conjugated Polymers

Electronic structures and optical excitations in Moebius conjugated polymers are studied theoretically. Periodic and Moebius boundary conditions are applied to the tight binding model of poly(para-phenylene), taking exciton effects into account. We discuss that oligomers with a few structural units are more effective than polymers for observations of effects of discrete wave numbers that are shifted by the change in boundary condition. Next, calculations of optical absorption spectra are reported. Certain components of optical absorption for an electric field perpendicular to the polymer axis mix with absorption spectra for an electric field parallel to the polymer axis. Therefore, the polarization dependences of an electric field of light enable us to detect whether conjugated polymers have the Moebius boundary.Comment: 10 pages, 6 figures, to be published in J. Phys. Soc. Jpn., Vol. 74 No. 2 (February, 2005), Letter sectio

Conformational effects on the Circular Dichroism of Human Carbonic Anhydrase II: a multilevel computational study

Circular Dichroism (CD) spectroscopy is a powerful method for investigating conformational changes in proteins and therefore has numerous applications in structural and molecular biology. Here a computational investigation of the CD spectrum of the Human Carbonic Anhydrase II (HCAII), with main focus on the near-UV CD spectra of the wild-type enzyme and it seven tryptophan mutant forms, is presented and compared to experimental studies. Multilevel computational methods (Molecular Dynamics, Semiempirical Quantum Mechanics, Time-Dependent Density Functional Theory) were applied in order to gain insight into the mechanisms of interaction between the aromatic chromophores within the protein environment and understand how the conformational flexibility of the protein influences these mechanisms. The analysis suggests that combining CD semi empirical calculations, crystal structures and molecular dynamics (MD) could help in achieving a better agreement between the computed and experimental protein spectra and provide some unique insight into the dynamic nature of the mechanisms of chromophore interactions

Attosecond Coherent Control of Symmetry Breaking and Restoration in Atoms and Molecules

Symmetry is a fundamental phenomenon in science, and symmetry breaking is often the origin of subsequent processes which are important in chemistry, physics and biology. As is well-known, a laser pulse can break the electronic symmetry in atoms and molecules by creating a superposition of electronic eigenstates with different irreducible representations, which typically initiates attosecond ultrafast charge migration. In the first part of this dissertation, an original theory of coherent laser control is proposed to induce the symmetry restoration of the electronic structure in atoms and molecules after symmetry breaking, with application to the oriented benzene molecule and to the ^{87}Rb atom. Four different strategies are proposed and corresponding sufficient conditions for symmetry restoration are derived analytically. The numerical and analytical results agree perfectly with each other. Meanwhile, the theoretical predictions for the ^{87}Rb atom have been confirmed by experimental partners in Japan, by means of high contrast Ramsey interferometry with a precision of about three attoseconds. The second part is devoted to the electronic flux during charge migration in oriented benzene molecule. Two different patterns of adiabatic attosecond charge migration are investigated by laser induced preparation of two different non-aromatic superposition states. From the knowledge of the time-dependent many-body wave functions as a linear combination of many-electron wave functions obtained from conventional quantum chemistry calculations, we derive expressions for the time-evolution of the one-electron density and the electronic flux. This allows to specify the number of electrons flowing during a given charge migration process, together with the mechanism of charge migration. In conclusion, this dissertation shows, for the first time, that the symmetry of electronic structure in atoms and molecules can not only be broken but also be restored by means of simple laser pulses. The coherent control strategies require strict control over the time-dependent phases of electronic wave functions. In practice, the precision required is few attoseconds - much shorter than the timescale of charge migration in such systems. The analysis of charge migration indicates that similar superposition states may lead to quantitative differences in the number of electrons flowing.Symmetrie ist ein fundamentales Phänomen in der Naturwissenschaft, und Symmetriebrechung kann bedeutende Folgeprozesse in der Chemie, Physik und Biologie auslösen. Insbesondere kann die elektronische Symmetrie von Atomen und Molekülen bekanntlich durch einen Laserpuls gebrochen werden, und zwar durch die Erzeugung einer Superposition von elektronischen Zuständen mit verschiedenen irreduziblen Darstellungen - dies bewirkt dann typischerweise ultraschnelle Ladungsmigration auf der Attosekundenzeit-skala. Der erste Teil dieser Dissertation entwickelt eine grundlegende Theorie der kohärenten Laserpuls-Kontrolle mit dem Ziel der Wiederherstellung der elektronischen Symmetrie in Atomen und Molekülen nach Symmetriebrechung, mit Anwendungen auf das orientierte Benzolmolekül sowie auf das ^{87}Rb Atom. Es werden insgesamt vier Strategien zur Wiederherstellung der Symmetrie vorgestellt, wobei hinreichend zielführende Bedingungen analytisch hergeleitet werden. Die analytischen Ergebnisse stimmen exzellent mit numerischen Quantendynamiksimulationen überein. Die theoretischen Vorhersagen für das ^{87}Rb Atom wurden inzwischen mit Hilfe der hoch-kontrastreichen Ramsey Interferometrie von experimentellen Partnern in Japan mit einer Genauigkeit von drei Attosekunden bestätigt. Der zweite Teil untersucht den Elektronenfluss während der Ladungsmigration in orientierten Benzolmolekülen. Dabei werden für zwei unterschiedliche nicht-aromatische elektronische Superpositionszustände verschiedene Typen der adiabatischen Ladungsmigration auf der Attosekundenzeitskala aufgezeigt. Aus der Kenntnis der zeitabhängigen Viel-Elektronen-Wellenfunktion als Linearkombination elektronischer Eigenfunktionen, die mit Hilfe konventioneller Verfahren der Quantenchemie berechnet werden, werden Ausdrücke für die Zeit-Evolution der Ein-Elektronen-Dichte und des Elektronenflusses hergeleitet. Daraus ergibt sich die jeweilige Zahl der Elektronen, die zur Ladungsmigration beitragen, sowie der Mechanismus der Ladungsmigration. Zusammenfassend zeigt diese Dissertation erstmals, dass die Symmetrie der Elektronenstruktur von Atomen und Molekülen mit Hilfe von einfachen Laserpulsen nicht nur gebrochen, sondern auch wiederhergestellt werden kann. Die Strategien der kohärenten Laserkontrolle verlangen dazu die strenge Kontrolle der zeitabhängigen Phasen der elektronischen Wellenfunktionen. Praktische Anwendungen erfordern dafür eine zeitliche Genauigkeit von wenigen Attosekunden - also noch viel genauer als die ohnehin schon ultrakurze Zeitskala der Ladungsmigration. Untersuchungen der Ladungsmigration zeigen, dass vergleichbar ähnliche Superpositionen elektronischer Wellenfunktionen zu quantitativ verschiedenen Elektronenflusszahlen führen können

Tunable Hybridization Between Electronic States of Graphene and Physisorbed Hexacene

Non-covalent functionalization via physisorption of organic molecules provides a scalable approach for modifying the electronic structure of graphene while preserving its excellent carrier mobilities. Here we investigated the physisorption of long-chain acenes, namely, hexacene and its fluorinated derivative perfluorohexacene, on bilayer graphene for tunable graphene devices using first principles methods. We find that the adsorption of these molecules leads to the formation of localized states in the electronic structure of graphene close to its Fermi level, which could be readily tuned by an external electric field. The electric field not only creates a variable band gap as large as 250 meV in bilayer graphene, but also strongly influences the charge redistribution within the molecule-graphene system. This charge redistribution is found to be weak enough not to induce strong surface doping, but strong enough to help preserve the electronic states near the Dirac point of graphene.Comment: 17 pages, 7 figures, supporting informatio