Novel electronic structures in elements under high pressure

An electride structure in an element at high pressure.

We used ’Ab initio random structure searching’ to study novel electronic structures in elements under extreme pressure. At multi-TPa pressures, certain elements are found to adopt an electride structure where valence electrons pile up in the interstitial regions. This signals a  breakdown of the nearly free electron model and necessitates an alternative theoretical description.

The work was carried out under the supervision of Professor Richard Needs and Professor Chris Pickard. Calculations were carried out on the Cambridge High Performance Computing Cluster Darwin.



The competition between gravity and flow focusing in two-layered porous media

Geological storage options for carbon dioxide.

The gravitationally driven flow of a dense fluid within a two-layered porous media is examined experimentally and theoretically. We find that in systems with two horizontal layers of differing permeability a competition between gravity driven flow and flow focusing along high-permeability routes can lead to two distinct flow regimes. When the lower layer is more permeable than the upper layer, gravity acts along high- permeability pathways and the flow is enhanced in the lower layer. Alternatively, when the upper layer is more permeable than the lower layer, we find that for a sufficiently small input flux the flow is confined to the lower layer. However, above a critical flux fluid preferentially spreads horizontally within the upper layer before ultimately draining back down into the lower layer. This later regime, in which the fluid overrides the low-permeability lower layer, is important because it enhances the mixing of the two fluids. We show that the critical flux which separates these two regimes can be characterized by a simple power law. Finally, we briefly discuss the relevance of this work to the geological sequestration of carbon dioxide and other industrial and natural flows in porous media.

The paper is available here. The research was carried while I was at the Department of Applied Mathematics and Theoretical Physics in Cambridge.