Protective capping of topological surface states of intrinsically insulating Bi2_2Te3_3

We have identified epitaxially grown elemental Te as a capping material that is suited to protect the topological surface states of intrinsically insulating Bi$_2$Te$_3$. By using angle-resolved photoemission, we were able to show that the Te overlayer leaves the dispersive bands of the surface states intact and that it does not alter the chemical potential of the Bi$_2$Te$_3$ thin film. From in-situ four-point contact measurements, we observed that the conductivity of the capped film is still mainly determined by the metallic surface states and that the contribution of the capping layer is minor. Moreover, the Te overlayer can be annealed away in vacuum to produce a clean Bi$_2$Te$_3$ surface in its pristine state even after the exposure of the capped film to air. Our findings will facilitate well-defined and reliable ex-situ experiments on the properties of Bi$_2$Te$_3$ surface states with nontrivial topology.Comment: 5 pages, 5 figures, 2 pages supplemental material accepted for publication in AIP Advance

SmO thin films: a flexible route to correlated flat bands with nontrivial topology

Using density functional theory based calculations, we show that the correlated mixed-valent compound SmO is a 3D strongly topological semi-metal as a result of a 4$f$-5$d$ band inversion at the X point. The [001] surface Bloch spectral density reveals two weakly interacting Dirac cones that are quasi-degenerate at the M_bar-point and another single Dirac cone at the Gamma_bar-point. We also show that the topological non-triviality in SmO is very robust and prevails for a wide range of lattice parameters, making it an ideal candidate to investigate topological nontrivial correlated flat bands in thin-film form. Moreover, the electron filling is tunable by strain. In addition, we find conditions for which the inversion is of the 4f-6s type, making SmO to be a rather unique system. The similarities of the crystal symmetry and the lattice constant of SmO to the well studied ferromagnetic semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender towards realizing the quantum anomalous Hall effect in a strongly correlated electron system.Comment: Paper+supplemen

Nature of magnetism in Ca3_3Co2_2O6_6

We find using LSDA+U band structure calculations that the novel one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ is not a ferromagnetic half-metal but a Mott insulator. Both the octahedral and the trigonal Co ions are formally trivalent, with the octahedral being in the low-spin and the trigonal in the high-spin state. The inclusion of the spin-orbit coupling leads to the occupation of the minority-spin $d_{2}$ orbital for the unusually coordinated trigonal Co, producing a giant orbital moment (1.57 $\mu_{B}$). It also results in an anomalously large magnetocrystalline anisotropy (of order 70 meV), elucidating why the magnetism is highly Ising-like. The role of the oxygen holes, carrying an induced magnetic moment of 0.13 $\mu_{B}$ per oxygen, for the exchange interactions is discussed.Comment: 5 pages, 4 figures, and 1 tabl

Intrinsic conduction through topological surface states of insulating Bi2_2Te3_3 epitaxial thin films

Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet, their execution have been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ bulk single crystals as well as impurities on their surfaces. Here we present the preparation of Bi$_2$Te$_3$ thin films that are insulating in the bulk and the four-point probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of order 10$^{12}$ cm$^{-2}$ only and mobilities up to 4,600 cm$^2$/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and x-ray photoemission analysis, and the temperature dependent four-point probe conductivity measurement all in-situ under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states.Comment: accepted for publication in Proceedings of the National Academy of Sciences of the United States of America (PNAS

Insulating state and the importance of the spin-orbit coupling in Ca3_3CoRhO6_6

We have carried out a comparative theoretical study of the electronic structure of the novel one-dimensional Ca$_3$CoRhO$_6$ and Ca$_3$FeRhO$_6$ systems. The insulating antiferromagnetic state for the Ca$_3$FeRhO$_6$ can be well explained by band structure calculations with the closed shell high-spin $d^5$ (Fe$^{3+}$) and low-spin $t_{2g}^{6}$ (Rh$^{3+}$) configurations. We found for the Ca$_3$CoRhO$_6$ that the Co has a strong tendency to be $d^7$ (Co$^{2+}$) rather than $d^6$ (Co$^{3+}$), and that there is an orbital degeneracy in the local Co electronic structure. We argue that it is the spin-orbit coupling which will lift this degeneracy thereby enabling local spin density approximation + Hubbard U (LSDA+U) band structure calculations to generate the band gap. We predict that the orbital contribution to the magnetic moment in Ca$_3$CoRhO$_6$ is substantial, i.e. significantly larger than 1 $\mu_B$ per formula unit. Moreover, we propose a model for the contrasting intra-chain magnetism in both materials.Comment: 7 pages, 4 figures, and 1 tabl