Research in the Soos Group

We study electronic processes in organic molecular solids and conjugated polymers or molecules. Our approach is theoretical and aimed at explaining other people's spectroscopic or transport data. We are seeking a unified, semi-quantitative description of spin, optical, charge-transfer and charge carrying excitations in the organic solid state, starting with the known ground-state structure. We are particularly interested in new or puzzling observations for which an initial interpretation is needed. Hence we collaborate with experimental groups, both at Princeton and elsewhere.

The problems we address require a combination of solid-state physics and chemistry. We want to combine molecular aspects with extended systems. The evolution of atomic or molecular clusters into solids is a related general problem for both theory and experiment.

Hückel theory for conjugated molecules or tight-binding theory for metals or semiconductors are familiar quantum cell models. Such models are restricted to frontier orbitals and contain empirical parameters. More elaborate versions incorporate electron-electron and/or electron-vibration interactions. Some examples are Hubbard, Pariser-Parr-Pople, Heisenberg, Ising, Holstein, Peierls and Su-Schrieffer-Heeger model.

Some general challenges and themes:

1. Molecular crystals, free-radical solids and conjugated polymers have narrow electronic bands (relatively small overlap) that limit normal applications of band theory. They illustrate medium or strong correlations among electrons and require new theoretical methods that we try develop and apply. Diagrammatic valence bond theory is a good example.

2. The powerful quantum chemical or density functional approaches to electronic structure deal mainly with the ground state and its potential surface. Excited states are more difficult in general, but are crucial for new applications such as light emitting diodes, thin film transistors, nonlinear optical materials, organic conductors and superconductors, solar cells and many other potential devices. Moreover, extended systems pose additional complications or opportunities.

3. There has been remarkable progress in controlling and characterizing structure and interfaces on the atomic scale. Such control opens the way to deeper understanding of electronic processes as well as to applications, since decisive tests can be made. At the same time, more quantitative molecular computations can provide some solid-state parameters.

4. There are many questions to answer before the electronic properties of organic materials can be understood or exploited. There is great interest in organic thin films, either molecular or polymeric, and both are likely to have applications in the emerging field of organic electronics. Such materials research combines chemistry and physics, molecular and extended properties in novel ways.

Back to home page

last updated: 27 Nov 2001