V. Holy

Strain-induced nonsymmorphic symmetry breaking and removal of Dirac semimetallic nodal line in an orthoperovskite iridate

By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite SrIrO3 can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the Pbnm mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the n-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.

Picosecond inverse magnetostriction in galfenol thin films

Coherent high-amplitude precession of the magnetization and spin waves with frequencies up to 40 GHz are generated by injecting picosecond compressive and shear acoustic pulses into nanometer-sized galfenol (Fe81Ga19) films. The magnetization modulation is due to the picosecond inverse magnetostrictive effect. The oscillations of the magnetization measured by magneto-optical Kerr rotation last for several nanoseconds, and the maximum modulation of the in-plane effective magnetic field is as high as 40 mT. These results in combination with a comprehensive theoretical analysis show that galfenol films possess excellent properties for ultrafast magnetization control based on the picosecond inverse magnetostrictive effect.

Structural characterization of self-assembled quantum dot structures by x-ray diffraction techniques

We have investigated, by means of X-ray diffraction reciprocal space mapping and X-ray reflectivity, multilayers of self-organized InGaAs quantum dots grown on GaAs by MBE. An anisotropy of the average inter-dot spacings in the [100]and [110]directions was found, consistent with an ordering of the dots in a two-dimensional square lattice with main axes along the -directions and a lattice parameter of 55 nm. The nearly perfect vertical alignment (stacking) of the dots was deduced consistently from the diffraction peak shape and from measurements of the resonant diffuse scattering in the X-ray reflection regime.

Obtaining the structure factors for an epitaxial film using Cu X-ray radiation

Determining atomic positions in thin films by X-ray diffraction is, at present, a task reserved for synchrotron facilities. Here an experimental method is presented which enables the determination of the structure factor amplitudes of thin films using laboratory-based equipment (Cu Kα radiation). This method was tested using an epitaxial 130 nm film of CuMnAs grown on top of a GaAs substrate, which unlike the orthorhombic bulk phase forms a crystal structure with tetragonal symmetry. From the set of structure factor moduli obtained by applying this method, the solution and refinement of the crystal structure of the film has been possible. The results are supported by consistent high-resolution scanning transmission electron microscopy and stoichiometry analyses.

Shape and composition of buried PbSe quantum dots determined by scanning tunneling microscopy

Capping of self-assembled semiconductor quantum dots usually alters their shape and composition due to alloying with the matrix material. To determine the structure of capped dots, a method is developed based on the analysis of surface displacements induced by buried dots measured by scanning tunneling microscopy. For self-assembled PbSe dots overgrown with PbTe layers, the buried dots are found to be highly truncated and extended in the lateral direction, and due to intermixing their composition is changed to PbSexTe1−x, with xSe of only 55%.

Three-dimensional reciprocal space mapping of diffuse scattering for the study of stacking faults in semipolar (\bf 11{\overline 2}2) GaN layers grown from the sidewall of an r-patterned sapphire substrate

Three-dimensional reciprocal space mapping of semipolar (11{\overline 2}2) GaN grown on stripe-patterned r-plane (1{\overline 1}02) sapphire substrates is found to be a powerful and crucial method for the analysis of diffuse scattering originating from stacking faults that are diffracting in a noncoplanar geometry. Additionally, by measuring three-dimensional reciprocal space maps (3D-RSMs) of several reflections, the transmission electron microscopy visibility criteria could be confirmed. Furthermore, similar to cathodoluminescence, the 3D-RSM method could be used in future as a reliable tool to distinguish clearly between the diffuse scattering signals coming from prismatic and from basal plane stacking faults and from partial dislocations in semipolar (11{\overline 2}2) GaN. The fitting of the diffuse scattering intensity profile along the stacking fault streaks with a simulation based on the Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. A reduction of the stacking fault density due to the intercalation of an SiN interlayer in the GaN layer deposited on the sidewall of the pre-patterned sapphire substrate has led to an improvement of the optoelectronic properties, influenced by the crystal quality, as has been demonstrated by a locally resolved cathodoluminescence investigation.

Structural characterization of self-assembled Ge dot multilayers by X-ray diffraction and reflectivity methods

We have studied the structural properties of multiple layers of self-assembled Ge dots non-destructively by conventional X-ray diffraction and grazing incidence diffraction using reciprocal space mapping techniques. In particular, the influence of the thickness of the Si spacer between the individual dot layers on the strain relaxation in the dots and the lateral and vertical correlation of the dot positions was investigated. Whereas conventional X-ray diffraction averages over the whole multilayer stack, grazing incidence diffraction allows for a position-resolved determination of the strain relaxation.

The instrumental resolution of a moire extensometer in light of its recent automatisation

This paper assesses the instrumental resolution of a mechanical extensometer in light of its recent automatisation. The instrument takes advantage of the moire phenomenon of optical interference to measure angular rotation in two perpendicular planes and displacement in three dimensions. Our assessment systematically defines an analytical solution for the complete interpretation of a generic moire pattern and a set of mathematical approximations for the moire patterns used to measure rotation and displacement. The ultimate sensitivity of the automated instrument is determined on the basis of a generic least square differences fitting procedure while the instrumental resolution is defined on the basis of realistic, rather than optimal, scenarios: the resolution of the rotation measurements are in the order of 8.7*10-5 rad while the resolution of the displacement measurements are better than 5 μm. This assessment represents the first step towards a global numerical repository for processed data recorded by the automated extensometers.

Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

The magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.

Ferroelectric phase transitions in multiferroic Ge1-xMnx Te driven by local lattice distortions

The evolution of local ferroelectric lattice distortions in multiferroic Ge1-x Mn x Te is studied by x-ray diffraction, x-ray absorption spectroscopy and density functional theory. We show that the anion/cation displacements smoothly decrease with increasing Mn content, thereby reducing the ferroelectric transition from 700 to 100 K at x = 0.5, where the ferromagnetic Curie temperature reaches its maximum. First principles calculations explain this quenching by different local bond contributions of the Mn 3d shell compared to the Ge 4s shell in excellent quantitative agreement with the experiments.