Theory of correlated insulating behaviour and spin-triplet superconductivity in twisted double bilayer graphene

Two monolayers of graphene twisted by a small `magic' angle exhibit nearly flat bands leading to correlated electronic states and superconductivity, whose precise nature including possible broken symmetries, remain under debate. Here we theoretically study a related but different system with reduced symmetry - twisted {\em double} bilayer graphene (TDBLG), consisting of {\em two} Bernal stacked bilayer graphene sheets, twisted with respect to one another. Unlike the monolayer case, we show that isolated flat bands only appear on application of a vertical displacement field $D$. We construct a phase diagram as a function of twist angle and $D$, incorporating interactions via a Hartree-Fock approximation. At half filling, ferromagnetic insulators are stabilized, typically with valley Chern number $C_v=2$. Ferromagnetic fluctuations in the metallic state are argued to lead to spin triplet superconductivity from pairing between electrons in opposite valleys. Response of these states to a magnetic field applied either perpendicular or parallel to the graphene sheets is obtained, and found to compare favorably with a recent experiment. We highlight a novel orbital effect arising from in-plane fields that can exceed the Zeeman effect and plays an important role in interpreting experiments.Comment: main 15 pages, appendix 11 page

Dynamics of Frenkel excitons in pigment-protein complexes and hybrid systems

A Quantum Master Equation approach is applied to describe the exciton dynamics of the FMO complex, focusing on the effects of vibrations. Further, an excitonic model is applied to the description of the LH2 antenna complex to unravel the origin of the unusual B800 absorption band splitting and to connect this to observed exciton relaxation rates. In the second part a DFTB approach yielding a discrete representation of the electronic quantum mechanical charge density in terms of atom-centered Mulliken charges is used for the description of molecules nearby a metal nanosphere.In der vorliegenden Arbeit wurde der Zugang der Quanten-Master-Gleichung genutzt, um die Dynamik im FMO-Komplex zu untersuchen. Weiterhin wurde ein exzitonisches Modell für den LH2-Antennenkomplex der Bakterienform Alc. vinosum, entwickelt, um eine Erklärung der ungewöhnlichen Form der B800 Absorptionsbande und deren Beziehung zu den beobachteten Exziton-Relaxationsraten zu finden. Im zweiten Teil der Arbeit wurde ein DFTB-Zugang, der eine diskrete Darstellung quantenmechanischer elektronischer Ladungsdichten erlaubt, zur Beschreibung von Hybridsystemen genutzt