TYC Lunchtime Get-Together

TYC@UCL Lunchtime Get-Togethers consist of two short presentations from either students, visitors or academics taking place over lunch and drinks. They are a great opportunity to get to meet other students, catch up with friends and colleagues, and listen to members and visitors talk about their research.

Food and drink will be provided so please just bring yourselves!

Molecular to Atomic Transition of Hydrogen at 1000K using Density Functional Theory

Adam Martins

There is much uncertainty in high pressure hydrogen as results from simulation studies disagree and the pressure and temperature regime is difficult to achieve in the laboratory. As giant gaseous planets predominantly contain hydrogen it plays a major role in determining intrinsic properties of Jovian interiors. Internal properties such as the molecular to atomic transition will greatly influence the structure and dynamics of Jupiter, giving rise to more layers and possibly hindering convection, having consequences throughout the interior. We have investigated a first-order phase transition of hydrogen at the 1000 K isotherm based on first-principles molecular dynamics using density functional theory with the projector augmented plane wave method (PAW) and the generalised gradient approximation (GGA). Cell sizes of 512, 1024 and 2048 atoms are used and densities chosen range from 600 kgm$^{-3}$ to 1100 kgm$^{-3}$ giving pressures of 100 GPa to 280 GPa. We find a sudden change in structure from molecular to atomic hydrogen as the pressure and density increase. Electronic energy gaps are seen to gradually close becoming metal like at the transition from molecular to atomic. Also investigated is the self diffusion coefficient and hence internal dynamics, allowing insight into the degree of change of diffusion through the interior. In addition we find the DC conductivity calculated by means of the Kubo-Greenwood formula and it's dependence on k-point sampling, showing the likely location of magnetic field generation and its unique properties. 

Representing electronic excitation effects in classical molecular dynamics

Szymon Daraszewicz

Classical molecular dynamics (MD) is conventionally used to study radiation damage in matter. However, this method is not directly applicable to the most energetic radiation events, where the energy is deposited in the excited electrons or carrier pairs, since the electronic structure is not explicitly represented in MD. For instance, in ion irradiation the proportion of energy lost to the electrons rises with increasing ion velocity and for swift heavy ions, of typical energies of several MeV/u, the electronic energy loss dominates.

To represent the transient state of the electron-lattice non-equilibrium we use an integrated atomistic-continuum approach, where the MD part represents ions and a continuum model represents electrons (metals) or carrier pairs (semiconductors). We would like to demonstrate such a hybrid MD model and its applications to swift heavy ion irradiation of silicon and short-pulse laser illumination of gold.  A simpler method, which represents electrons as a frictional force at the level of a MD thermostat, also exists. This so-called electronic-friction would be presented in the context of very large-scale cascade simulations in alpha-iron.