Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity, colossal magnetoresistance or multiferroicity . The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO(3) and SrTiO(3) (ref. 4) represents an important milestone towards exploiting such properties in all-oxide devices. This conducting interface shows a number of appealing properties, including a high electron mobility, superconductivity and large magnetoresistance, and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×10(13)  cm(-2), can be formed at the bare SrTiO(3) surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.

The surface of a topological insulator plays host to an odd number of linearly-dispersing Dirac fermions, protected against back-scattering by time-reversal symmetry. such characteristics make these materials attractive not only for studying a range of fundamental phenomena in both condensed matter and particle physics, but also for applications ranging from spintronics to quantum computation. Here, we show that the single Dirac cone comprising the topological state of the prototypical topological insulator Bi 2 se 3 can co-exist with a two-dimensional electron gas (2DEG), a cornerstone of conventional electronics. Creation of the 2DEG is tied to a surface band-bending effect, which should be general for narrow-gap topological insulators. This leads to the unique situation where a topological and a non-topological, easily tunable and potentially superconducting, metallic state are confined to the same region of space.

Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z 2 topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement.

Methods to generate spin-polarised electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices. 1 This is generally accepted to require breaking global structural inversion symmetry. [1][2][3][4][5] In contrast, here we present direct evidence from spin-and angleresolved photoemission spectroscopy for a strong spin polarisation of bulk states in the centrosymmetric transition-metal dichalcogenide WSe 2 . We show how this arises due to a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localised, leading to enormous spin splittings up to ∼ 0.5 eV, with a spin texture that is strongly modulated in both real and momentum space. As well as providing the first experimental evidence for a recently-predicted 'hidden' spin polarisation in inversion-symmetric materials, 6 our study sheds new light on a putative spin-valley coupling in transition-metal dichalcogenides, 7-9 of key importance for using these compounds in proposed valleytronic devices.The powerful combination of inversion symmetryensures that electronic states of non-magnetic centrosymmetric materials must be doubly spin-degenerate. If inversion symmetry is broken, however, relativistic spin-orbit interactions can induce a momentum-dependent spin splitting via an effective magnetic field imposed by spatially-varying potentials. If the * To whom correspondence should be addressed: philip.king@standrews.ac.uk resulting spin polarisations can be controllably created and manipulated, they hold enormous promise to enable a range of new quantum technologies. These include routes towards electrical control of spin precession for spin-based electronics, 1,10 new ways to engineer topological states 11,12 and possible hosts of Majorana fermions for use in quantum computation. 5 To date, there are two generally-accepted categories of materials in which spinpolarised states can be stabilised without magnetism. The first exploits the breaking of structural inversion symmetry of a centrosymmetric host by imposing an electrostatic potential gradient, for example within an asymmetric quantum well, leading to Rashba-split 13 states localised at surfaces or interfaces. [14][15][16][17] In the second, a lack of global inversion symmetry in the unit cell mediates spin splitting of the bulk electronic states, either through a Dresselhaus-type interaction, 18 or a recently discovered bulk form of the Rashba effect. 4,19 Here, we present the first experimental observation of a third distinct class: a material which has bulk inversion symmetry but nonetheless exhibits a large spin polarisation of its bulk electronic states. We demonstrate this for the transition-metal dichalcogenide 2H-WSe 2 . This layered compound is composed of stacked Se-W-Se planes (Fig. 1(a)), each of which contains an in-plane net dipole moment which is proposed to lead to a strong spin-valley coupling for an isolated monolayer. 7,8,20 The bulk unit cell contains two s...

We report a Rashba spin splitting of a two-dimensional electron gas in the topological insulator Bi(2)Se(3) from angle-resolved photoemission spectroscopy. We further demonstrate its electrostatic control, and show that spin splittings can be achieved which are at least an order-of-magnitude larger than in other semiconductors. Together these results show promise for the miniaturization of spintronic devices to the nanoscale and their operation at room temperature.

Surfaces and interfaces o er new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1-4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5-7 to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8-13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.Carrier concentration is a key parameter defining the ground state of correlated electron systems. At the LaAlO 3 /SrTiO 3 interface, the 2DEL density can be tailored by field-effect gating. As the system is depleted of carriers, its ground state evolves from a high-mobility 2DEL 4 into a two-dimensional superconductor 8-10 with pseudogap behaviour 11 and possible pairing above T c (ref. 12). An analogous 2DEL can be induced by doping the (001) surface of SrTiO 3 . As for the interface, the surface 2DEL is confined by a band-bending potential in SrTiO 3 and consists of an orbitally polarized ladder of quantum confined Ti t 2g electrons that are highly mobile in the surface plane [5][6][7]14 . Thus far, the surface 2DEL has been studied only at carrier densities around 2 × 10 14 cm −2 , approximately a factor of five higher than typically observed at the LaAlO 3 /SrTiO 3 interface [5][6][7] . In the following, we present ARPES data extending to lower carrier densities that are directly comparable to the LaAlO 3 /SrTiO 3 interface. We achieve this by preparing SrTiO 3 (001) wafers in situ, which results in well-ordered clean surfaces that can be studied by ARPES over extended timescales, as they are less susceptible to the ultraviolet-induced formation of charged oxygen vacancies reported for cleaved SrTiO 3 5,7,15,16 . Details of the sample preparation are given in Methods. Figure 1a shows an energy-momentum intensity map for a 2DEL with a carrier density of n 2D ≈ 2.9 × 10 13 cm −2 estimated from the Luttinger volume of the first light subband and the two equivalent heavy subbands (see Supplementary Section 2). The most striking features of this data are replica bands at higher binding energy following the dispersion of the primary quasiparticle (QP) bands. The replica bands are all separated by approximately 100 meV and progressively lose intensity, but can be visualized up to the third replica in the curvature plot shown in Fi...

Intrinsic topological insulators are realized by alloying Bi(2)Te(3) with Bi(2)Se(3). Angle-resolved photoemission and bulk transport measurements reveal that the Fermi level is readily tuned into the bulk bandgap. First-principles calculations of the native defect landscape highlight the key role of anti-site defects for achieving this, and predict optimal growth conditions to realize maximally resistive topological insulators.

How the interacting electronic states and phases of layered transition-metal dichalcogenides evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe grown on bilayer graphene/SiC. While the global electronic structure is similar to that of bulk VSe, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below T = 140 ± 5 K, concomitant with the emergence of charge-order superstructures evident in low-energy electron diffraction. These observations point to a charge-density wave instability in the monolayer that is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of X-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density functional theory. Our study thus points to a delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides.