Relativity and group IV the periodic table

 If we look down group IV of the periodic table (C, Si, Ge, Sn, Pb) carbon (diamond) is an insulator, silicon and germanium are semiconductors, tin has both semiconducting and metallic forms and lead is a metal. Why?

Charge transport in organic solar cells (Ben Powell & Paul Burn)

There are two key elements that control the efficiency of a solar cell. How many electrons can we generate for a given illumination level (sunlight) and how much useful work can we get out of those electrons. In this project we study the transport of electrons through organic solar cells using a mixture of mathematical and computational modelling. We will seek to answer questions such as do the electrons hop or tunnel through the active layers, what is the energy costs associated with such transport, and are there molecular designs that can optimise the processes.

Relativistic effects in solar cells and organic LEDs (Ben Powell & Paul Burn)

Purely organic molecules usually absorb or emit (fluorescence) light only via their singlet states because transitions to the triplet state are ‘forbidden’.  However, in organometallic complexes with heavy metals such as iridium or platinum strong relativistic effects (such as spin-orbit coupling) mean that transitions to and from triplet states are allowed. However, at this stage it is not possible to predict or even explain why some complexes perform very well, e.g., are highly luminescent while others with only slight structural changes are not. We have been working on a method that will allow this to be done for the first time. The projects in this area will compare the solutions of the Schrodinger and Dirac equations to study the role of relativistic quantum-mechanical effects in the materials to expla

Gauge fluctuations in organic superconductors (Ben Powell; PhD only)

This project will investigate a simple question: why does superconductivity disappear as we raise the temperature? To understand this question one needs to understand that two ingredients are required for superconductivity (i) the electrons must pair up into “Cooper pairs” to overcome the Pauli exclusion principle and (ii) long-range phase coherence must exist between the Copper pairs, i.e. the wavefunction must have the same phase in macroscopically separated parts of the sample.

Dimensionless ratios in condensed matter physics (Ben Powell)

This project builds on the work of a previous Honours student who published his results in the high-profile journal Nature Physics.

Deep physical insights have come from studying ratios of measured quantities that take universal values, which depend on only fundamental constants. Examples of universal ratios include the Wiedemann-Franz, Sommerfeld-Wilson, Kadowaki-Woods and Korringa ratios. Materials that violate these laws have provided particular insight into, non-Fermi liquids, strong electronic correlations, and magnetic fluctuations.

Quantum Chemistry of Molecules for Organic Solar Cells (Ben Powell)

Everyone knows that climate change and the cost/availability of fossil fuel are going to change the way that mankind produces and uses energy. Organic solar cells are seen as a lead "next generation" solar cell technology and will undoubtedly have a significant role in the future energy mix. In the Centre for Organic Photonics and Electronics (COPE) we design and synthesise new materials for photon harvesting in organic solar cells. We also fabricate and test prototype devices in state-of-the-art clean room facilities.

The renormalisation group for quantum chemistry (Ben Powell)

Wilson’s renormalisation group is often regarded as the most important idea in physics from the second half of the twentieth century. It has had profound influence in fields as disparate as condensed matter physics and high-energy particle physics. One of the key ideas of the renormalisation group is that rather than study the full Hamiltonian of the system of interest, which is usually intractable, one should study an effective low energy Hamiltonian. An effective Hamiltonian is derived by ‘integrating out’ the high-energy degrees of freedom of the system.

Can density functional theory describe the Mott insulator? (Ben Powell)

Density functional theory (DFT) is one of the most computationally tractable methods of solving the Schrödinger equation for atoms, molecules, and solids. It is such a valuable tool in theoretical chemistry and solid state physics that Walter Kohn and John Pople shared Nobel Prize for developing it. Yet there are many situations where DFT does not work well [1]. One of the most important of these situations is when the interactions and hence the correlations between electrons are strong.

Is a superconductor nearly a spin liquid insulator? (Ben Powell)

Imagine that we are having a dinner party. We have decided to seat our guests around a table so that each person has a member of the opposite sex on either side of them. If I have a square table and four guests this is not difficult. However, if we have a triangular table and three guests it is impossible. In an antiferromagnet the electrons want to sit with their spins pointing in the opposite direction to their neighbours. Again this is straightforward to arrange on a square lattice but impossible to achieve on the triangular lattice. This effect is known as frustration.

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