Research fields

Modern computational methods enable theoretical studies of the electronic, structural, and  dynamical properties of materials starting from the basic laws of nature such as quantum mechanics and statistical physics. Quantitative studies based on the first principles calculations not only result in deeper understanding of  fundamental problems in the condensed matter physics, involving often complex interrellations between the electronic and structural properties, but also allows to predict new phenomena and design novel functional  materials and electronic devices. Our reasearch activity focuses on solid state properties of a broad range of materials including bulk crystals, nanostructures, surfaces, and disordered systems. The main fields of studies conducted in DMRC include inter alia the basic structural, electronic, and phonon properties of crystals, phase transitions, electron-phonon interaction, phonons in multilayes and surfaces, thermodynamic and elastic properties of minerals, lattice dynamics in strongly correlated systems, superconductors, molecular crystals, shape memory and disordered alloys.

Phonons ab initio. Lattice dynamics determines basic thermodynamic properties of crystals and plays a fundamental role is such phenomena like superconductivity and structural phase transitions. The phonon spectra can be studied within the direct method, which is implemented in the Phonon program written by Krzysztof Parlinski. This method is based on calculation of the interatomic forces in the supercell with the periodic boundary conditions. The Hellmann-Feynmann forces are obtained using one of the density functional theory codes like VASP, Wien2k, or SIESTA. The direct method has been used to calculate the phonon dispersion curves and phonon density of states in numerous crystals, multilayers, and surfaces.

Phase transitions. The structural transitions in crystals are often induced by the soft modes, which appear due to changes in external conditions (pressure or temperature). Such phonon modes with imaginary frequencies can be used to analyze the mechanism of phase transition and allows to predict the low-symmetry stable structure. The direct method has been used to study phase transitions induced by the soft modes in several compounds  (e.g. ZrO2, SnO2, CaTiO3). The structural transitions of the first order and the stability phase relations in the pressure-temperature phase diagram can be studied within the quasiharmonic approximation (MgSiO3). In systems with a strong electron-lattice coupling, the structural transition often invoke changes in the electronic structure  or magnetic order. Two examples of such studies are the Verwey transition in magnetite  (Fe3O4) and the magnetostructural transition in MnAs.
 
Structure of the Earth. The Earth's crust and mantle are build of rock-forming minerals, mainly the various forms of magnesium-silicon oxides. The physical characteristics of constituent minerals, which determine the basic thermodynamical and mechanical properties of the Earth's interior, are usually not accessible from direct measurements, and come from seismic studies and theoretical simulations. DMRC is involved intensively in the studies of structural, elastic, and thermodynamical properties of minerals (Mg2SiO4, Fe2SiO4). In recent years, DMRC participated in the geophysical european project  "Crust to Core (c2c)-the fate of subducted material".

Multilayers, thin films, and surfaces. In systems with lower dimensionality, the atomic interactions and crystal structure are largely modified, having a significant  effect on the dynamical properties. Due to geometrical constraints, the standard experimental techniques like inelastic neutron and x-ray scattering cannot be used directly for phonon studies. By implementing the ab initio direct method, we could calculate the phonon dispersion curves and density of states in multiple multilayers (FePt, Fe/FeSi), monolayers (Fe/W), and surfaces (MgO, Fe). In collaboration with the European Synchrotron Radiation Facility (ESRF) in Grenoble, the theoretical results were verified by the resonant nuclear inelastic scattering measurements.

Strong correlations. In strongly correlated materials, like transition-metal oxides or f-electron systems, local electron interactions radically modify the electronic structure through e.g. a metal-insulator transition. Such changes influence also lattice dynamics leading to new effects not observed in usual metals. Due to strong local electron interactions, the standard density functional theory fails to describe properly the electronic structure and leads often to significant discrepancies in the crystal structure and phonon energies. By including the Hubbard interaction within the LDA+U approach, we have calculated the phonon spectra for selected correlated systems obtaining better agreement with the experimental data (PuCoGa5).