Two-dimensional electron gases (2DEGs) in SrTiO 3 have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the d-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO 3 -based 2DEGs, and yield new microscopic insights on their functional properties.

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 coexistence 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.The classification of electronic structures based on their topological properties has opened powerful routes for understanding solid state materials. 1 The nowfamiliar Z 2 topological insulators are most renowned for their spin-polarised Dirac surface states residing in inverted bulk band gaps. 1 In systems with rotational invariance, a band inversion on the rotation axis can generate protected Dirac cones with a point-like Fermi surface of the bulk electronic structure. 2-8 If either inversion or time-reversal symmetry is broken, a bulk Dirac point can split into a pair of spin-polarised Weyl points. 9-15 Unlike for elementary particles, Lorentz-violating Weyl fermions can also exist in the solid state, manifested as a tilting of the Weyl cone. If this tilt is sufficiently large, so-called type-II Weyl points can occur, now formed at the touching of open electron and hole pockets. [15][16][17][18][19][20][21][22] Realising such phases in solid-state materials not only offers unique environments and opportunities for studying the fundamental properties of fermions, but also holds potential for applications exploiting their exotic surface excitations and bulk electric and thermal transport properties. [23][24][25][26][27] Consequently, there is an intense current effort focused on identifying compounds which host the requisite band inversions. In many cases, however, this arXiv:1702.08177v2 [cond-mat.mtrl-sci]

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO3. 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 KTaO3-and SrTiO3-based 2DEGs. The polar nature of the KTaO3(100) surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO3(100). Today's electronic devices largely rely on control of the conductivity of two-dimensional (2D) electron channels in semiconductor hosts. Creating such 2D electron gases (2DEGs) in oxides, which in bulk form generally show much larger and more diverse responses to external stimuli, holds the potential for devices with functionalities well beyond what we have experienced to date [1]. The prototypical example of an oxide 2DEG, formed when SrTiO 3 is interfaced to the polar surface of another perovskite oxide [2], has indeed proved a very rich system [3,4]. Recently, it was discovered that oxygen vacancies could mediate formation of a similar 2DEG at the bare surface of SrTiO 3 [5,6], providing an exciting opportunity for the realization of 2DEGs in more exotic parent materials than has been achieved via interface engineering.Of particular interest are 5d transition metal oxides (TMOs), whose large spin-orbit interactions offer the potential to incorporate the spintronic functionality sought in emerging schemes of semiconductor electronics [7][8][9] into all-oxide devices. Despite the extended nature of 5d atomic orbitals, the interplay of their large spinorbit interactions with even a modest Coulomb repulsion yields pronounced signatures of electron correlations in these materials, such as the formation of Mott insulating states [10][11][12] and possible correlated topological insulators [13,14], suggesting 5d TMOs could provide a novel and potentially very rich host for engineering of artificial 2D electron systems. Understanding the interplay of strong spin-orbit coupling, quantum confinement, and correlations within such a 2DEG is an essential step towards realizing their potential for practical applications. To date, this has been hampered by the difficulty of generating 2DEGs in 5d oxides via interface engineering.Here, we show that a 2DEG can be created at the (100) surface of the 5d perovskite KTaO 3 . Exploiting its surface-localized nature, we utilize angle-resolved photoemission (ARPES) to provide the first direct measurement of the subband structure of a 5d-oxide 2DEG. Our measurements, supported by model calculations, reveal an important role of multi-orbital physics in this system. Surprisingly, however, they do not show any significant Rashba spin-splitting of the 2DEG, which might naturally be expected.Single-crystal...