Current Research Projects of the Defect Chemistry and Spectroscopy group

Field-Controlled Evaporation Mechanisms for Surface Atoms
Extremely strong (~10 V/nm) electric fields rupturing atomic bonds is a relatively well-studied concept in the field of molecular chemistry. When extended to crystalline systems, i.e. material surfaces, this concept is known as field evaporation and its exact mechanisms become more challenging to predict. Field evaporation is the central phenomenon that enables atom probe tomography (APT), and obtaining atomically-accurate APT reconstructions will be impossible without an atomically-accurate understanding of how ions initially form and depart from the surface. By performing first-principles calculations on faceted surfaces under extreme fields, we search for such an understanding. more
In order to prepare raw data from scanning transmission electron microscopy for analysis, pattern detection algorithms are developed that allow to identify automatically higher-order feature such as crystalline grains, lattice defects, etc. from atomically resolved measurements. more
Generalized dipole correction for charged slabs
It is very challenging to simulate within DFT extreme electric fields (a few 1010 V/m) at a surface, e.g. for studying field evaporation, the key mechanism in atom probe tomography (APT). We have developed a straight-forward scheme to incorporate an ideal plate counter-electrode in a nominally charged repeated-slab calculation by means of a generalized dipole correction of the standard electrostatic potential obtained from fully periodic FFT. more
We simulate the ionization contrast in field ion microscopy arising from the electronic structure of the imaged surface. For this DFT calculations of the electrified surface are combined with the Tersoff-Hamann approximation to electron tunneling. The approach allows to explain the chemical contrast observed for NiRe alloys. more
Hubbard U corrections for Molecular Orbitals
We have developed a MO projector scheme to apply Hubbard-U corrections within density-functional theory to molecular orbitals (MOs). more
Optimized key algorithms for our plane-wave DFT code S/PHI/nX
Modern CPUs provide a number of features to increase the computational power. Unfortunately, this increased computing power is often not used in practice, because the CPU can process the data more quickly than the memory can deliver it. To make full use of this enhanced computing power, the existing algorithms need to be revised to better exploit the computing power by exposing hidden parallelism to the CPU and improving the data locality in the data access patterns. We identified key routines in our plane-wave DFT code that offer such tuning opportunities and demonstrate a significant speed-up over standard approaches. more
We have extended the sxdefectalign correction scheme to account for charged defects located at surfaces or interfaces. The scheme allows to extrapolate the formation energy of the defect from very small supercells, even if artificial fields in the calculation are sizeable. more
Go to Editor View