Subband Structure of a Two-Dimensional Electron Gas Formed at the Polar Surface of the Strong Spin-Orbit Perovskite KTaO3

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO{sub 3}. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, we find no experimental signatures of a Rashba spin splitting, which has important implications for the interpretation of transport measurements in both KTaO{sub 3}- and SrTiO{sub 3}-based 2DEGs. The polar nature of the KTaO{sub 3}(100) surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO{sub 3}(100)

Band bending at In-rich InGaN surfaces

The band bending and carrier concentration profiles as a function of depth below the surface for oxidized InxGa1−xN alloys with a composition range of 0.39 ≤ x ≤ 1.00 are investigated using x-ray photoelectron, infrared reflection, and optical absorption spectroscopies, and solutions of Poisson’s equation within a modified Thomas–Fermi approximation. All of these InGaN samples exhibit downward band bending ranging from 0.19 to 0.66 eV and a high surface sheet charge density ranging from 5.0×1012 to 1.5×1013 cm−2. The downward band bending is more pronounced in the most In-rich InGaN samples, resulting in larger near-surface electron concentrations

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Formation of heavy d-electron quasiparticles in Sr3Ru2O7

The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets

Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserve