E. van Heumen

Hard x-ray photoemission and density functional theory study of the internal electric field in SrTiO3/LaAlO3 oxide heterostructures

A combined experimental and theoretical investigation of the electronic structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is presented. High-resolution, hard x-ray photoemission is used to uncover the occupation of Ti 3d states and the relative energetic alignment—and hence internal electric fields—within the LaAlO3 layer. First, the Ti 2p core-level spectra clearly show occupation of Ti 3d states already for two unit cells of LaAlO3. Second, the LaAlO3 core levels were seen to shift to lower binding energy as the LaAlO3 overlayer thickness, n, was increased, agreeing with the expectations from the canonical electron transfer model for the emergence of conductivity at the interface. However, not only is the energy offset of only ∼300 meV between n=2 (insulating interface) and n=6 (metallic interface) an order of magnitude smaller than the simple expectation, but it is also clearly not the sum of a series of unit-cell-by-unit-cell shifts within the LaAlO3 block. Both of these facts argue against the simple charge-transfer picture involving a cumulative shift of the LaAlO3 valence bands above the SrTiO3 conduction bands, resulting in charge transfer only for n≥4. We discuss effects which could frustrate this elegant and simple charge-transfer model, concluding that although it cannot be ruled out, photodoping by the x-ray beam is unlikely to be the cause of the observed behavior. Turning to the theoretical data, our density functional simulations show that the presence of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the direction of the internal field in the LaAlO3. Therefore, taking the experimental and theoretical results together, a consistent picture emerges for real-life samples in which nature does not wait until n=4 and already for n=2 mechanisms other than internal-electric-field-driven electron transfer from idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active in heading off the incipient polarization catastrophe that drives the physics in these systems.

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe2-xCoxAs2

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.

Cleavage surfaces of the BaFe2−xCoxAs2 and FeySe1−xTex superconductors: A combined STM plus LEED study

We elucidate the termination surface of cleaved single crystals of the BaFe2−xCoxAs2 and FeySe1−xTex families of the high temperature iron based superconductors. By combining scanning tunneling microscopic data with low energy electron diffraction we prove that the termination layer of the Ba122 systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, FeySe1−xTex possesses only a single termination structure that of the tetragonally ordered Se1−xTex layer.

Optical properties of BaFe2-xCoxAs2

We present detailed temperature-dependent optical data on BaFe2- xCoxAs2 (BCFA), with x=0.14, between 4 meV and 6.5 eV. We analyze our spectra to determine the main optical parameters and show that in this material the interband conductivity already starts at energies as low as 10 meV. We determine the superfluid density ρs/(2πc)2=2.2±0.5·107 cm-2, which places optimally doped BFCA close to the Uemura line. Our experimental data shows clear signs of a superconducting gap with 2Δ1=6.2±0.8 meV. In addition, from comparing the experimental spectra to model calculations we obtain indications for an additional band of strongly scattered carriers with a larger gap, 2Δ2=14±2 meV.

Low carrier concentration crystals of the topological insulator Bi2-xSbxTe3-ySey: a magnetotransport study

In 3D topological insulators achieving a genuine bulk-insulating state is an important research topic. Recently, the material system (Bi,Sb)2(Te,Se)3 (BSTS) has been proposed as a topological insulator with high resistivity and a low carrier concentration (Ren et al 2011 Phys. Rev. B 84 165311). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized BSTS single crystals with compositions around x = 0.5 and y = 1.3. Resistance and Hall effect measurements show high resistivity and record low bulk carrier density for the composition Bi

Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFe2As2, an Fe-pnictide superconductor parent compound

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Epitaxial growth and thermodynamic stability of SrIrO3/SrTiO3 heterostructures

Sb

Dissimilarities between the electronic structure of chemically doped and chemically pressurized iron pnictides from an angle-resolved photoemission spectroscopy study

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Dirac states with knobs on: Interplay of external parameters and the surface electronic properties of three-dimensional topological insulators

Te

Pseudogap-less high-Tc superconductivity in BaCoxFe2−xAs2

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