Tunability of the dielectric response of epitaxially strained SrTiO3 from first principles

The effect of in-plane strain on the nonlinear dielectric properties of SrTiO3 epitaxial thin films is calculated using density-functional theory within the local-density approximation. Motivated by recent experiments, the structure, zone-center phonons, and dielectric properties with and without an external electric field are evaluated for several misfit strains within +-3% of the calculated cubic lattice parameter. In these calculations, the in-plane lattice parameters are fixed, and all remaining structural parameters are permitted to relax. The presence of an external bias is treated approximately by applying a force to each ion proportional to the electric field. After obtaining zero-field ground state structures for various strains, the zone-center phonon frequencies and Born effective charges are computed, yielding the zero-field dielectric response. The dielectric response at finite electric field bias is obtained by computing the field dependence of the structure and polarization using an approximate technique. The results are compared with recent experiments and a previous phenomenological theory. The tunability is found to be strongly dependent on the in-plane lattice parameter, showing markedly different behavior for tensile and compressive strains. Our results are expected to be of use for isolating the role of strain in the tunability of real ultrathin epitaxial films.Comment: 11 pages, with postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/ant_srti/index.htm

Towards a first principles description of phonons in Ni50_{50}Pt50_{50} disordered alloys: the role of relaxation

Using a combination of density-functional perturbation theory and the itinerant coherent potential approximation, we study the effects of atomic relaxation on the inelastic incoherent neutron scattering cross sections of disordered Ni$_{50}$Pt$_{50}$ alloys. We build on previous work, where empirical force constants were adjusted {\it ad hoc} to agree with experiment. After first relaxing all structural parameters within the local-density approximation for ordered NiPt compounds, density-functional perturbation theory is then used to compute phonon spectra, densities of states, and the force constants. The resulting nearest-neighbor force constants are first compared to those of other ordered structures of different stoichiometry, and then used to generate the inelastic scattering cross sections within the itinerant coherent potential approximation. We find that structural relaxation substantially affects the computed force constants and resulting inelastic cross sections, and that the effect is much more pronounced in random alloys than in ordered alloys.Comment: 8 pages, 3 eps figures, uses revtex

First-principles investigation of initial stages of surfactant mediated growth on the Si(111) substrate

The atomic and electronic structure of single adatoms, adatom-clusters and steps an clean or surfactant covered Si(111) surfaces are studied with ab-initio methods. All calculations are performed using the self-developed program package EStCoMPP which is based an density functional theory combined with pseudopotentials. The paths and energy barriers for diffusion of single Si ad-atoms are studied an the clean (3 x 3)-reconstructed Si(111), which shows most of the features of the (7 x 7)-reconstruction. At the most stable position the extra ad-atom sits at a bridge position between the ad-atom and one atom of the second layer. The barrier for diffusion from the faulted to the unfaulted unit half or vice versa is 0.44 eV. The saddle-point is located an the boundary between the unit-halfs, close to the dimer. Also the structure of small Si-clusters an this surface is investigated, showing a preferred cluster formation in either one of the unit halfs. The structure and development of small Si-clusters is calculated an surfactant covered Si(111). The investigated cluster sizes range from single ad-atoms to 4 atoms. The results explain the experimentally determined features of the initial stages of homoepitaxial growth. We find that doublelayer growth starts at a cluster size of 4 an As covered Si(111) while no doublelayer growth occurs when Sb is used as surfactant. The atomic structures of the two closed-packed step-edges an As terminated Si(111) are calculated. The results show that the exposed Si atoms at the stepedges are replaced by As. Thus, the step-edges are fully terminated by surfactant atoms like the surface. While at the (112) step-edge the As atoms are in a naturally threefold coordinated position, the As atoms form dimers at the (112) step-edge. STM-Images are simulated to compare the calculated structures with experiments. We can explain the measured step-characteristics with the calculated electronic local density of states. The key-features of the simulated images are in agreement with the experimental findings. The apparent outward relaxation of the upper terrace As atoms at the step-edge can be explained by the higher electronic density of states in the p-channel of these atoms. The stability of different surface reconstructions of Sb-covered strained Ge(111) is investigated as a function of the lateral lattice constant. We find for high compressive strain the ($\sqrt{3}$ x $\sqrt{3}$) reconstruction to be stable. Over a large region of lattice constants (a$_{Si}$-a$_{Ge}$) we find the (2 x 1) reconstruction to be preferred, and for dilated surfaces the (1 x 1) reconstruction is favored. This explains the experimentally determined structures of Sb-covered Ge-films grown on Si(111):Sb, and even the peculiar surface structure with (1 x 1) surface in a (6$\sqrt{3}$ x 6$\sqrt{3}$) superstructure of overrelaxed islands, which is found for 3 Ge layers