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Superconductivity in topological insulators is expected to show very unconventional features such as $p+ip$ order parameter, Majorana fermions etc. However, the intrinsic superconductivity has been observed in a very limited number of materials in which the pairing symmetry is still a matter of debate. Here, we study the topological crystalline insulator (Pb$_{1-x}$Sn$_x$)$_{1-y}$In$_y$Te, where a peculiar insulator to superconductor transition was previously reported near the gap inversion transition, when the system is nearly a 3-dimensional Dirac semimetal. Both the existence of superconductivity near the 3-dimensional Dirac semimetal and the occurrence of insulator to superconductor transition in an isotropic material is highly unusual. We suggest that the observed phenomena are related to an intrinsic instability of a 3-dimensional Dirac semimetal state in (Pb$_{1-x}$Sn$_x$)$_{1-y}$In$_y$Te and "flattening" of the bulk valence and conduction bands as they acquire a Mexican hat-like dispersion on the inverted side of the phase diagram. This favors the pairing instability if the chemical potential is pinned to these flat regions., Comment: 9 pages, 5 figures

In the present work, we investigate the electronic structure of the two-dimensional (2D) ferromagnet Cr2Ge2Te6 by photoemission spectroscopy and ab initio calculations. Our results demonstrate the presence of multiple 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 revealed that several of the hole bands exhibit weak dispersion with varied incident photon energy providing experimental signature for its two dimensionality. These findings can pave the way for further studies towards the application of Cr2Ge2Te6 in electronic devices., Comment: 8 pages, 3 figures

In this report, we identify the origin of the temperature dependence of the surface energy gap in impurity-doped topological insulators. The gap at the Dirac point and its variation with temperature were studied by using angle-resolved photoemission spectroscopy in Cr-doped Bi2Se3. Our valence band photoemission results revealed that the gap varies with temperature due to residual gas condensation on the sample surface with cooling. Adsorbate on the surface of the sample creates an electron doping effect that modifies the chemical potential of the system resulting in the change of the gap with variable temperature. Such electron doping can weaken the ferromagnetism and lead to a bulk band contribution in the transport measurements. Also, a larger energy gap is required to suppress the thermal excitations for the quantum anomalous Hall effect. Therefore, such effects can hinder the quantum anomalous Hall state at higher temperatures. Resolving this issue can pave the way for enhancing the observation temperature of the quantum anomalous Hall effect. Therefore, our findings can play a significant role in the discovery of the high-temperature quantum anomalous Hall effect in impurity-doped topological insulators., Comment: 8 pages, 6 figures

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 the 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 Bi2Sr2CaCu2O8 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 Fermi surface and adverse effects of a strong spin-orbit coupling in the topological material., Comment: Work presented at the March Meeting of American Physical Society in Denver on March 3, 2014

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

We report on the local electronic structure of oxygen incorporated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, intially grown films are measured to be non-superconducting, 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. Total Fluorescence Yield (TFY) X-ray absorption experiments show that oxygen incorporation changes the initial Fe valence state in both the initially grown FeTe and FeSe films to mainly Fe3+ in the oxygen incorporated 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., Comment: 4 pages, 5 figures