PhD Defense: Sakse Emil Dalum
The CISS Effect
In recent years, a wide range of experiments have been done that show spin selectivity of electrons after they have passed through a layer of chiral molecules. This effect has been termed chiral-induced spin selectivity (CISS) and is the subject of this thesis. Several attempts have been made to explain the effect theoretically, but no consensus on the origin or mechanism has emerged. In this thesis, we derive a mathematical framework that aims to provide a foundation for a correct treatment of the effect. From simple arguments involving time reversal symmetry, the so-called no-polarisation theorem emerges. In essence, it says that the system must start in a non-thermal distribution, i.e. a distribution where states with the same energy are unequally populated. Some of the experiments involve transient effects, and we show that the same formalism that is used in steady state can be generalised in such a way that it can be used to make statements about the transient behaviour. In experiments involving magnetic leads, another phenomenon emerges. In this case, the equilibrium distribution of a magnetic lead coupled to a non-magnetic lead through a layer of chiral molecules changes non-trivially, when the magnetisation is flipped. Therefore, when a bias is applied, the system behaves differently for different orientations of the magnetic field. The theory is investigated further by studying models of organic molecules. Here it is found that the polarisation can only reach a sizeable value, if the molecular spectrum contains levels that are close in energy, compared to the size of the coupling to the leads. It is shown that accidental degeneracies appear in the spectrum of polyacetylene, when the bonds between neighbouring atoms are twisted in such a way that it forms a helix. These degeneracies are shown to be a general feature of twisted chains of p-like orbitals. Similar features are shown to be present in a model of helicene.