The Physics of Disorder. David Drabold, Distinguished Professor of Physics
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Take away computation from the world and everything is wrapped in blind ignorance —Isidore, Bishop of Seville (560-636)
I have been privileged to have a wonderful group of collaborators, mentors and graduate students. I have also been fortunate in earlier years to have sustained support from the US National Science Foundation and the Army Research Office. Current research is funded by the United States Department of Energy, Pacific Northwest National Laboratory, and the Office of Naval Research.
We have focused our scientific energies in two primary directions:
Current interests include structure, charge and heat transport in advanced materials such as metal-carbon composites, sintered thermoelectric materials, amorphous graphite materials and coal, and also studies of amorphous solid surfaces, bulk metallic glasses, CBRAM materials.
A recent video on computing the space-projected conductivity:
In the first set of images below, we take a random walk through our research with images associated with the structure, electronic, transport and vibrational properties of materials. Click on each for details.
Next: two generalized Wannier functions in silicon computed order-N by projection methods (crystal left, amorphous middle). The electron-phonon coupling is amplified for localized electrons — on the right, we see the LUMO level in a-Si with large phonon-driven charge fluctuations. See also here. Somewhat surprisingly, topological disorder in Si does not affect the spatial non-locality of quantum mechanics as measured by the decay of the single-particle density matrix. Also, localized Kohn-Sham eigenvalues and states are strongly modulated by thermal disorder. This electron-phonon effect has serious consequences for for transport in disordered semiconducting materials.
Charge fluctuations in a localized state.
Phonons in amorphous silica: note the non-locality and the mixing: E=500/cm. The message here is that local pictures of normal modes in glasses are grossly oversimplified. There is extensive mode mixing and non-locality, the details of which are very energy-dependent.
The first (1989-1990) ab initio simulation of amorphous graphene [12 minutes into the show], and charming (?) primitive animation of molecular scattering events and dynamics of molecules. These simulations were based on FIREBALL, a real-space “ab initio tight binding” code pioneered by O. F. Sankey. More for molecules: silicon, carbon.A phonon in silica with energy about 500 wavenumbers.
Finally, our approach to the Bragg problem for amorphous systems: Force Enhanced Atomic Refinement, here the example of amorphous silicon. The videos show the convergence of the radial distribution function (left) and topology (right) using FEAR (in the latter beige is experimentally realistic coordination). This is a general method that is far more efficient than melt quench methods and properly builds in a priori experimental information in the process of model construction while including ab initio chemical information in an unbiased way. See also this. The method is general and results are published for silver-doped chalcogenides, many forms of carbon, and bulk metallic glasses.