Ondřej Caha

Magneto-Optical Signature of Massless Kane Electrons in Cd3As2

We report on optical reflectivity experiments performed on Cd_{3}As_{2} over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

Structure and composition of bismuth telluride topological insulators grown by molecular beam epitaxy

The structure and composition of Bi2Te3-delta topological insulator layers grown by molecular beam epitaxy is studied as a function of beam flux composition. It is demonstrated that, depending on the Te/Bi2Te3 flux ratio, different layer compositions are obtained corresponding to a Te deficit delta varying between 0 and 1. On the basis of X-ray diffraction analysis and a theoretical description using a random stacking model, it is shown that for delta >= 0 the structure of the epilayers is described well by a random stacking of Te-Bi-Te-Bi-Te quintuple layers and Bi-Bi bilayers sharing the same basic hexagonal lattice structure. The random stacking model accounts for the observed surface step structure of the layers and compares very well with the measured X-ray data, from which the lattice parameters a and c as a function of the chemical composition were deduced. In particular, the in-plane lattice parameter a is found to continuously increase and the average distance of the (0001) hexagonal lattice planes is found to decrease from the Bi2Te3 to the BiTe phase. Moreover, the lattice plane distances agree well with the linear interpolation between the Bi2Te3 and BiTe values taking the strain in the epilayers into account. Thus, the chemical composition Bi2Te3-delta can be directly determined by X-ray diffraction. From analysis of the X-ray diffraction data, quantitative information on the randomness of the stacking sequence of the Bi and Te layers is obtained. According to these findings, the layers represent random one-dimensional alloys of Te-Bi-Te-Bi-Te quintuple and Bi-Bi bilayers rather than a homologous series of ordered compounds.

Structural and electronic properties of manganese-doped Bi2Te3 epitaxial layers

We show that in manganese-doped topological insulator bismuth telluride layers, Mn atoms are incorporated predominantly as interstitials in the van der Waals gaps between the quintuple layers and not substitutionally on Bi sites within the quintuple layers. The structural properties of epitaxial layers with Mn concentration of up to 13% are studied by high-resolution x-ray diffraction, evidencing a shrinking of both the in-plane and out-of plane lattice parameters with increasing Mn content. Ferromagnetism sets in for Mn contents around 3% and the Curie temperatures rises up to 15 K for a Mn concentration of 9%. The easy magnetization axis is along the c-axis perpendicular to the (0001) epilayer plane. Angle-resolved photoemission spectroscopy reveals that the Fermi level is situated in the conduction band and no evidence for a gap opening at the topological surface state with the Dirac cone dispersion is found within the experimental resolution at temperatures close to the Curie temperature. From the detailed analysis of the extended x-ray absorption fine-structure experiments (EXAFS) performed at the MnK-edge, we demonstrate that the Mn atoms occupy interstitial positions within the van der Waals gap and are surrounded octahedrally by Te atoms of the adjacent quintuple layers.

Giant Rashba Splitting in Pb1-xSnxTe (111) Topological Crystalline Insulator Films Controlled by Bi Doping in the Bulk

The topological properties of lead-tin chalcogenide topological crystalline insulators can be widely tuned by temperature and composition. It is shown that bulk Bi doping of epitaxial Pb1-x Snx Te (111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface.

Surface morphology and magnetic anisotropy in (Ga,Mn)As

Atomic Force Microscopy and Grazing incidence X-ray diffraction measurements have revealed the presence of ripples aligned along the

Raman and interband optical spectra of epitaxial layers of the topological insulators Bi2Te3 and Bi2Se3 on BaF2 substrates

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Nonlinear evolution of surface morphology in InAs/AlAs superlattices via surface diffusion

direction on the surface of (Ga,Mn)As layers grown on GaAs(001) substrates and buffer layers, with periodicity of about 50 nm in all samples that have been studied. These samples show the strong symmetry breaking uniaxial magnetic anisotropy normally observed in such materials. We observe a clear correlation between the amplitude of the surface ripples and the strength of the uniaxial magnetic anisotropy component suggesting that these ripples might be the source of such anisotropy.

Density of Mn interstitials in (Ga,Mn)As epitaxial layers determined by anomalous x-ray diffraction

We report results of Raman and ellipsometric spectroscopy of the topological insulators Bi2Te3 and Bi2Se3 grown by molecular beam epitaxy on BaF2 (111) substrates. Surfaces and interfaces of the films are probed by Raman scattering from the front and back sides of the samples, which is possible owing to the transparent substrate. Surface modifications induced by intense illumination with exciting laser light have been detected, with excess tellurium at the surface during and after exposure. We also report data for thin epilayers containing a fractional number of unit cells and/or incomplete Bi2Te3 and Bi2Se3 quintuples. We have used spectroellipsometric measurements to obtain response functions and have derived the penetration depth of light in the 1.0–6.5 eV range.

Nucleation of lateral compositional modulation in InGaP epitaxial films grown on (001) GaAs

Continuum simulations of self-organized lateral compositional modulation growth in InAs/AlAs short-period superlattices on InP substrate are presented. The results of the simulations correspond quantitatively to the results of synchrotron x-ray diffraction experiments. The time evolution of the compositional modulation during epitaxial growth can be explained only including a nonlinear dependence of the elastic energy of the growing epitaxial layer on its thickness. From the fit of the experimental data to the growth simulations we have determined the parameters of this nonlinear dependence. It was found that the modulation amplitude does not depend on the values of the surface diffusion constants of particular elements.

Analysis of vacancy and interstitial nucleation kinetics in Si wafers during rapid thermal annealing.

Densities of Mn ions in epitaxial layers of (Ga,Mn)As were determined by anomalous x-ray diffraction, i.e., by a measurement of the dependence of the intensity of weak diffraction 002 on the photon energy around the Mn K absorption edge. From the measured data it was possible to determine the density of Mn ions in substitutional positions and the difference in the Mn densities in two possible interstitial positions in the GaAs lattice. The data demonstrate that the rate of the out-diffusion of the Mn interstitials from the Ga tetrahedrons significantly exceeds that from the As tetrahedrons.