As reported in my last post, I recently travelled to Vienna to see my research collaborators, but I didn’t say much, if anything, about what we would be working on. Basically it’s an ongoing project on developing nuclear clocks by doping suitable crystals with Th-229, which has an unusually low nuclear ground state energy. The main purpose of this trip was to meet with everyone in person and try to agree a route to get the work written up. This was our plan last year, but everyone involved got inundated with other duties, and it never happened.
There are three groups involved in this project: the experimental physics group who are growing and characterising the crystals, a group with DFT expertise who have applied electronic structure calculation methods to the material, and myself (I have modelled the doped materials using interatomic potentials). The first two groups are located in two different sections of the Technical University of Vienna.
The chosen host crystal in this project is calcium fluoride, CaF2. It appears to be relatively easy to dope, and the crystal transparency is not affected by the doping. This latter point is important for device operation, and implies that the band gap is unaffected by the doping. Before crystal growing was attempted, I carried out calculations on Th-doped CaF2 to find the optimal location of the dopant ion, and the form of charge compensation (since Th has a 4+ charge). Assuming substitution at the Ca2+ site, the lowest energy charge compensation scheme was found to be 2 F interstitials. There are a number of possible configurations of dopant Th and the 2 interstitials, and the lowest energy configuration was identified as having the interstitials in a right angle with respect to the Th.
We invited the DFT group to be involved, in order to see if DFT calculations confirmed the results from my atomistic calculations, but also to model the band gap to see if it was affected by the doping (noting that the experimental results suggested that this was not the case). The result was very encouraging; DFT made the same predictions about the charge compensation and location of charge compensating interstitials, and confirmed that the Th states would not affect the band gap.
As I said at the beginning of this post, the objective of this week’s meeting was to try again to agree a working strategy to write the paper, and I am more confident that it will happen this time! Watch this space and my Twitter feed for updates. Now I’m heading back home, and thinking about the next project. I have some new calculations on LiNbO3 to do, and a talk to write, which will keep me busy.