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Heavily electron-doped surfaces of Bi$_2$Se$_3$ have been studied by spin andangle resolved photoemission spectroscopy. Upon doping, electrons occupy aseries of {\bf k}-split pairs of states above the topological surface state.The {\bf k}-splitting originates from the large spin-orbit coupling and resultsin a Rashba-type behavior, unequivocally demonstrated here via the spinanalysis. The spin helicities of the lowest laying Rashba doublet and theadjacent topological surface state alternate in a left-right-left sequence.This spin configuration sets constraints to inter-band scattering channelsopened by electron doping. A detailed analysis of the scattering rates suggeststhat intra-band scattering dominates with the largest effect coming fromwarping of the Fermi surface.

In the present work, we investigate the electronic structure of the two-dimensional ferromagnet Cr2Ge2Te6 by photoemission spectroscopy and ab initio calculations. Our results demonstrate the presence of multi-hole–type bands in the vicinity of the Fermi level indicating that the material can support high electrical conductivity by manipulating the chemical potential. Also, our photon energy-dependent angle-resolved photoemission experiment shows that several of the hole bands exhibit weak dispersion with varied incident photon energy providing experimental signature for the two-dimensional nature of Cr2Ge2Te6. Thereby, these findings can pave the way for further studies towards the application of Cr2Ge2Te6 in electronic devices. [ABSTRACT FROM AUTHOR]

Presents a study which discussed the spin-polarized electron spectroscopy for chemical analysis on the oxidation of an iron surface with the combination of spin-polarization detection with core-level photoemission. Background on studies of magnetism at surfaces and thin films; Experimental method used; Description of the iron 3p photoemission spectra; Overview of the spin-resolved aspects of the spectra.

X-ray linear dichroism (XLD) effects at the Ni L2 edge of NiO films grown on Fe(001) were studied systematically as a function of NiO thickness (dN¡o), temperature, interfacial modulation, and Fe magnetization directions. We found that the Ni XLD signal exhibited an unusual dependence on dNi0. The XLD effect persists above the Néel temperature and exhibits a similar i/Ni0-dependence as the behavior at room temperature. On the other hand the Ni XLD signal reverses its sign as the Fe magnetization is switched by 90°. We propose a possible mechanism such that the Fe magnetostriction induces the NiO uniaxial crystal field to dominate the XLD magnitude in the NiO/Fe(001) system. [ABSTRACT FROM AUTHOR]

A beamline has been established at the National Synchrotron Light Source to perform angle-resolved photoemission experiments on magnetic surfaces with spin sensitivity. The system has two novel features: it uses a miniature electron-spin polarization analyzer and it also uses synchrotron radiation from an undulator rather than a bending magnet. [ABSTRACT FROM AUTHOR]

Ferromagnetic Co-doped rutile TiO2 single crystals were synthesized by high-temperature ion implantation and characterized by a variety of techniques. Co is uniformly distributed to a depth of ∼300 nm with an average concentration of ∼2 at. %, except in the near-surface region, where the concentration is ∼3 at. %. Ferromagnetic behavior is exhibited at room temperature with an effective saturation magnetization of ∼0.6 μB/Co atom. The Co is in a formal oxidation state of +2 throughout the implanted region, and no Co(O) is detected. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have grown epitaxial Fe-doped TiO2 rutile films on rutile TiO2(110) substrates, and have explored the resulting compositional, structural, morphological and magnetic properties. Clusters of mixed TiO2 rutile and Fe3O4 form on the surface of a continuous rutile epitaxial film during growth. Room-temperature ferromagnetism is observed, and is associated with the formation of secondary phase Fe3O4 rather than a true diluted magnetic oxide semiconductor. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Aluminum films oxidized in oxygen atmosphere are widely used in magnetic tunneling junctions (MTJs). We have determined the oxidation depth of these films by Fourier transform infrared (FTIR) spectroscopy in reflection-absorption mode. The oxide/Al interfaces are found to be abrupt and the thickness of the oxide layer increases slowly with exposure to oxygen, after a rapid onset. These results provide a physical picture of observed changes in the magnetotransport of MTJs as a function of Al thickness and oxidation time. The results also agree well with the empirically found optimum oxide growth condition which yields a maximum magnetoresistance.© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents an abstract of the study 'Spin-Polarized Photoelectron Diffraction: A New Probe of Short-Range Magnetic Order,' by C.S. Fadley, B. Sinkovic, B.D. Hermsmeier and J. Osterwalder.

Abstract: Single crystal La2NiO4 films were grown on STO (001) substrates by use of pulsed laser deposition (PLD). It was found that these films grow with the c-axis in the out of plane direction up to a certain critical thickness. Films thicker than that resulted in a structural reorientation from c-axis to a-axis out of plane orientation. The evolution of the c and a axis were measured by using a four circle X-ray diffractometer. [Copyright &y& Elsevier]

Three-dimensional topological insulators and topological crystalline insulators represent new quantum states of matter, which are predicted to have insulating bulk states and spin-momentum-locked gapless surface states. Experimentally, it has proven difficult to achieve the high bulk resistivity that would allow surface states to dominate the transport properties over a substantial temperature range. Here we report a series of indium-doped Pb1-xSnx vTe compounds that manifest huge bulk resistivities together with evidence consistent with the topological character of the surface states for x ≳ 0.35, based on thickness-dependent transport studies and magnetoresistance measurements. For these bulk-insulating materials, the surface states determine the resistivity for temperatures beyond 20 K. [ABSTRACT FROM AUTHOR]

Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional s-wave superconductors put hard constraints on these experiments. Significantly larger gaps and higher transition temperatures in cuprate superconductors might be an attractive alternative to considerably relax these constraints, but it is not clear whether the proximity effect would be effective in heterostructures involving cuprates and topological insulators. Here, we present angle-resolved photoemission studies of thin Bi2Se3 films grown in situ on optimally doped Bi2Sr2CaCu2O8δd substrates that show the absence of proximity-induced gaps on the surfaces of Bi2Se3 films as thin as a 1.5 quintuple layer. These results suggest that the superconducting proximity effect between a cuprate superconductor and a topological insulator is strongly suppressed, likely due to a very short coherence length along the c axis, incompatible crystal and pairing symmetries at the interface, small size of the topological surface state's Fermi surface, and adverse effects of a strong spin-orbit coupling in the topological material. [ABSTRACT FROM AUTHOR]

We report on the local electronic structure of oxygen incoöorated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, initially grown films are measured to be nonsuperconducting, but become superconducting following oxygen incorporation. In FeSe the opposite happens; initially grown films are measured to be superconducting, but experience a quenching of superconductivity following oxygen incorporation. X-ray photoemission and total fluorescence yield (TFY) x-ray absoötion experiments show that oxygen incoöoration changes the initial Fe valence state in both the as grown FeTe and FeSe films to mainly Fe3+ in the oxygen incoöorated films. In contrast we observe that while Te moves to a mixed Te0/Te4+ valence state, the Se always remains Se0. This work highlights how different responses of the electronic structure by the respective chalcogenides to oxidation could be related to the mechanisms which are inducing superconductivity in FeTe and quenching superconductivity in FeSe. [ABSTRACT FROM AUTHOR]

Abstract: Films of the parent compound FeTe can be made superconducting via the addition of interstitial oxygen. The process is reversible. We have characterized the new superconductors with a variety of experiments. X-ray diffraction shows that the superconductor has the same overall structure but a small lattice constant change compared to pure FeTe. X-ray absorption shows that superconducting FeTeO x has a nominal valence of 3+. DFT calculations show the most likely position for interstitial oxygen and confirm that such oxygen incorporation does not produce a large change in structure. [Copyright &y& Elsevier]

Nodal-line semimetals (NLSs) represent a new type of topological semimetallic phase beyond Weyl and Dirac semimetals in the sense that they host closed loops or open curves of band degeneracies in the Brillouin zone. Parallel to the classification of type-I and type-II Weyl semimetals, there are two types of NLSs. The type-I NLS phase has been proposed and realized in many compounds, whereas the exotic type-II NLS phase that strongly violates Lorentz symmetry has remained elusive. First-principles calculations show that Mg 3 Bi 2 is a material candidate for the type-II NLS. The band crossing is close to the Fermi level and exhibits the type-II nature of the nodal line in this material. Spin-orbit coupling generates only a small energy gap (≈35 meV) at the nodal points and does not negate the band dispersion of Mg 3 Bi 2 that yields the type-II nodal line. Based on this prediction, Mg 3 Bi 2 single crystals are synthesized and the presence of the type-II nodal lines in the material is confirmed. The angle-resolved photoemission spectroscopy measurements agree well with the first-principles results below the Fermi level and thus strongly suggest Mg 3 Bi 2 as an ideal material platform for studying the as-yet unstudied properties of type-II nodal-line semimetals.

Correction for 'Origin of the temperature dependence of the energy gap in Cr-doped Bi2Se3' by Turgut Yilmaz et al., Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/c7cp08049b.

Recent progress in impurity-doped topological insulators has shown that the gap at the Dirac point shrinks with reducing temperature. This is an obstacle for experimental realization of the quantum anomalous Hall effect at higher temperature due to the requirement of a larger energy gap. In order to solve this puzzle, we study the gap at the Dirac point by performing temperature-dependent photoemission spectroscopy and X-ray diffraction experiments in Cr-doped Bi2Se3. Our valence band photoemission study revealed that the gap alters with temperature due to residual gas condensation on the sample surface with cooling. Residual gas on the sample surface creates an electron doping effect that modifies the chemical potential of the system resulting in the change of the gap size with variable temperature. Furthermore, such electron doping can weaken the ferromagnetism and lead to a bulk band contribution in the transport measurements. Therefore, such effects can hinder the existence of the quantum anomalous Hall state at higher temperatures. Hence, this work can pave the way for future studies towards a high-temperature quantum anomalous Hall effect.

A systematic investigation of the thickness and oxygen pressure dependence for the structural properties of ultra-thin epitaxial magnetite (Fe3O4) films has been carried out; for such films, the structural properties generally differ from those for the bulk when the thickness  ⩽10 nm. Iron oxide ultra-thin films with thicknesses varying from 3 nm to 20 nm were grown on MgO (0 0 1) substrates using molecular beam epitaxy under different oxygen pressures ranging from 1  ×  10(-7) torr to 1  ×  10(-5) torr. The crystallographic and electronic structures of the films were characterized using low energy electron diffraction (LEED) and x-ray photoemission spectroscopy (XPS), respectively. The quality of the epitaxial Fe3O4 ultra-thin films was judged by magnetic measurements of the Verwey transition, along with complementary XPS spectra. It was observed that under the same growth conditions the stoichiometry of ultra-thin films under 10 nm transforms from the Fe3O4 phase to the FeO phase. In this work, a phase diagram based on thickness and oxygen pressure has been constructed to explain the structural phase transformation. It was found that high-quality magnetite films with thicknesses  ⩽20 nm formed within a narrow range of oxygen pressure. An optimal and controlled growth process is a crucial requirement for the accurate study of the magnetic and electronic properties for ultra-thin Fe3O4 films. Furthermore, these results are significant because they may indicate a general trend in the growth of other oxide films, which has not been previously observed or considered.

Proximity-induced superconductivity in a 3D topological insulator represents a new avenue for observing zero-energy Majorana fermions inside vortex cores. Relatively small gaps and low transition temperatures of conventional s-wave superconductors put hard constraints on these experiments. Significantly larger gaps and higher transition temperatures in cuprate superconductors might be an attractive alternative to considerably relax these constraints, but it is not clear whether the proximity effect would be effective in heterostructures involving cuprates and topological insulators. Here, we present angle-resolved photoemission studies of thin Bi(2)Se(3) films grown in situ on optimally doped Bi(2)Sr(2)CaCu(2)O(8+δ) substrates that show the absence of proximity-induced gaps on the surfaces of Bi(2)Se(3) films as thin as a 1.5 quintuple layer. These results suggest that the superconducting proximity effect between a cuprate superconductor and a topological insulator is strongly suppressed, likely due to a very short coherence length along the c axis, incompatible crystal and pairing symmetries at the interface, small size of the topological surface state's Fermi surface, and adverse effects of a strong spin-orbit coupling in the topological material.

The magnetic anisotropy in antiferromagnetic 500 A thick NiO films, before and after the establishment of an exchange bias field with Co84Fe16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both <111> textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.