Przemysław Piekarz

Using nuclear inelastic scattering on Dy 161 , we determined the Dy-partial phonon density of states of epitaxial DySi 2 in bulk and self-organized nanoislands and nanowires (NWs) on vicinal Si(001) surface. Compared to the bulk, the nanoislands exhibit softening of the crystal lattice, which is further enhanced in the NWs and significantly affects their thermodynamic and elastic properties. Our calculations reveal that the origin of this effect is the vibrations of Dy atoms located at the surfaces of the nanoislands and the NWs as well as forming a ( 2 × 4 ) reconstruction on the Si(001) surface between the NWs. The results demonstrate that, opposite to the widely accepted anisotropic lattice mismatch model, the NWs form with their length along the [0001] direction of the strained hexagonal unit cell with lattice parameters close to the Si(001) surface.

The chromium crystal doped with ¹¹⁹Sn isotope was studied using the nuclear resonance inelastic x-ray scattering and first principles calculations. The Sn partial phonon density of states (PDOS) was obtained for three temperatures that correspond to different magnetic states of Cr. At all temperatures, the energy spectrum consists of a broad band around 18 meV and a narrow peak at 43 meV. The additional peak around 39 meV is observed only in the magnetically ordered phases, indicating the influence of magnetic order in chromium on lattice dynamics. The partial PDOS calculated with the antiferromagnetic order on Cr atoms show a very good agreement with the experimental data. It is revealed that the high-energy peak is lying above the phonon spectra of the pure bcc-Cr crystal. These are the local modes with the increased energies due to a strongly reduced distance between Sn and the nearest-neighbor Cr atoms.

Electron-correlation-driven phonon soft modes have been recently reported in the antiferromagnetic kagome FeGe compound and associated with the observed charge density wave (CDW). In this paper, we present a systematic investigation of the CDW origin in the context of the ab initio lattice dynamics study. Performing the group theory analysis of the aforementioned soft modes, we found that the stable structure has the Immm symmetry and can be achieved by shifts of Ge atoms. The occurrence of two soft modes induces the first-order phase transition, which leads to the CDW order. Additionally, we show that the final structure realizes a distorted honeycomb Ge lattice, as well as a nonflat kagome-like Fe net. For completeness, we present the electronic properties calculations. From the theoretical STM topography simulation, we indicate that the observed CDW occurs in the deformed honeycomb Ge sublattice.

In recent years, nanostructures with hexagonal polytypes of gold have been synthesised, opening new possibilities in nanoscience and nanotechnology. As bulk gold crystallizes in the fcc phase, surface effects can play an important role in stabilizing hexagonal gold nanostructures. Here, we investigate several heterostructures with Ge substrates, including the fcc and hcp phases of gold that have been observed experimentally. We determine and discuss their interfacial energies and optimized atomic arrangements, comparing the theory results with available experimental data. Our DFT calculations for the Au-fcc(011)/Ge(001) junction show how the presence of defects in the interface layer can help to stabilize the atomic pattern, consistent with microscopic images. Although the Au-hcp/Ge interface is characterized by a similar interface energy, it reveals large atomic displacements due to significant mismatch. Finally, analyzing the electronic properties, we demonstrate that Au/Ge systems have metallic character, but covalent-like bonding states between interfacial Ge and Au atoms are also present.