Methodology development

Computational Developments

!!! Under Construction !!!

The main mission of the group to apply state-of-the-art computational methods for materials science at atomic scale. Particularly, accurate description of the electronic structure and states of solids, nanostructures, and point defects in them is our main goal. As we have faced several problems we initiated new developments or implementation of stand-alone and other codes to calculate the electronic structure of strongly correlated materials, excitation and impact ionization rates in them, Auger recombination rates for defects in solids, spin-dependent parameters such as hyperfine tensor or zero-field tensor associated with electron spin - electron spin interaction. Some of these codes were also integrated by ab-initio plane wave packages such as Quantum Espresso and VASP with the help of core developers (Dario Rocca at Giulia Galli group and Martijn Marsman at Georg Kresse group).

Impact ionization and Auger recombination rates

Impact ionization rates may be well-estimated by the imaginary part of the self-energy that can be calculated within the so-called GW approximation <bib id="Kotani paper"/>. With little modification of the official VASP implementation of GW code we applied this method for a strongly correlated material <bib id="PhysRevB.90.165142"/>. Another treatment is to calculate this integral

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where W is the screened Coulomb-interaction appearing in the GW formalism. We developed a stand-alone code (in real-space) to calculate these rates, and also integrated to the Quantum Espresso code in the frequency space with full treatment of W <bib id="PhysRevB.87.155402"/>. We applied this theory to calculate the impact ionization rates in silicon nanoparticles <bib id="Physical Review Letters 110 046804 (2013)."/>, or the Auger recombination rates in nitrogen-vacancy center (NV^—) in diamond <bib id="Physical Review Letters 110 167402 (2013)."/>.

Hyperfine tensor

Hyperfine interaction occurs between the spin of the electron and the spin of a nucleus. For defects the spin of the electron means the spin of the electron of the defect. For example NV^— has spin of S = 1.

We implemented the spin density contribution of the core electrons in the Fermi contact term into the hyperfine tensor calculation in collaboration with Martijn Marsman in Wien. We showed that this effect can be enormously large for some point defects, in contrast to previous expectations. Moreover, we find that the combination of HSE06 hybrid density functional together with the contribution of the core spin polarization provides accurate results on prominent point defects in various semiconductors <bib id="PhysRevB.88.075202"/>.

Zero-field tensor

The electron spin-sublevels of a high-spin defect (S>1/2) may split even in the absence of external magnetic field. This is called zero-field splitting. The zero-field splitting may be caused by the electron spin - electron spin interaction. We developed a theory how to calculate this zero-field tensor within the projector-augmentation-wave (PAW) framework <bib id="Journal of Physics: Condensed Matter 26 015305 (2014)"/> that has been recently implemented into VASP code.

Hybrid density functional with orbital-dependent screening

Hybrid functionals serve as a powerful practical tool in different fields of computational physics and quantum chemistry. On the other hand, their applicability for the case of correlated d and f orbitals is still questionable and needs more considerations. We formulated the on-site occupation dependent exchange correlation energy and effective potential of hybrid functionals for localized states and connect them to the on-site correction term of the DFT + U method. The resultant formula indicates that the screening of the onsite electron repulsion is governed by the ratio of the exact exchange in hybrid functionals. Our derivation provides a theoretical justification for adding a DFT + U-like on-site potential in hybrid-DFT calculations to resolve issues caused by overscreening of localized states <bib id="Physical Review B 87 205201 (2013); Physical Review B 90 035146 (2014)"/>.

Bibliography