As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

1 Two-dimensional electron gases (2DEGs) forming at the interfaces of transition metal oxides [1][2][3] display a range of properties including tunable insulatorsuperconductor-metal transitions [4][5][6], large magnetoresistance [7], coexisting ferromagnetism and superconductivity [8, 9], and a spin splitting of a few meV [10,11]. Strontium titanate (SrTiO 3 ), the cornerstone of such oxide-based electronics, is a transparent, nonmagnetic, wide-band-gap insulator in the bulk, and has recently been found to host a surface 2DEG [12][13][14][15]. The most strongly confined carriers within this 2DEG comprise two sub-bands, separated by an energy gap of 90 meV and forming concentric circular Fermi surfaces [12,13,15].Using spin-and angle-resolved photoemission spectroscopy (SARPES), we show that the electron spins in these sub-bands have opposite chiralities. Although the Rashba effect might be expected to give rise to such spin textures, the giant splitting of almost 100 meV at the Fermi level is far larger than anticipated [16,17].Moreover, in contrast to a simple Rashba system, the spin-polarized sub-bands are non-degenerate at the Brillouin zone centre. This degeneracy can be lifted by time-reversal symmetry breaking, implying the possible existence of magnetic order. These results show that confined electronic states at oxide surfaces can be endowed with novel, non-trivial properties that are both theoretically challenging to anticipate and promising for technological applications.The 2DEG at the surface of SrTiO 3 , formed by confined electrons of the t 2g conduction band with Ti-3d character, is universal in the sense that its constituent subbands, their fillings, and their Fermi surfaces are independent of the bulk sample doping and of whether the surfaces are prepared by cleaving [12,13] or by etching and in situ annealing [15]. This 2DEG consists on two light electron subbands dispersing down to about −180 meV and, respectively, −90 meV below the Fermi level (E F ), producing concentric Fermi surfaces around the Brillouin zone center, and heavy shallow subbands dispersing down to about −40 meV, forming ellipsoidal Fermi surfaces [12,13,15].In fact, the 2DEG in SrTiO 3 has been associated with a band-bending of ∼ 300 meV at the surface of the material [12,13,15]. In such a quantum well profile, the most bound light subbands are tightly confined near the surface, while the less bound heavy subbands are more delocalized towards the bulk [12,15]. The present work focuses on resolving the spin 2 structure of the strongly two-dimensional light subbands, which are the most technologically relevant in terms of their carrier concentration and mobility.The surface band-bending of ∼ 300 meV amounts to an electric field F ∼ 100 MV/m confining the conduction electrons near the surface. In a simple approximation, this field will induce a so-called "Rashba splitting" of the spin states in each subband, with the largest splitting occurring for the most bound subbands. In an ideal surface, the corresponding momentum separat...

2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3 -based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

We study, using angle-resolved photoemission spectroscopy, the two-dimensional electron gas (2DEG) at the surface of KTaO3 (KTO), a wide-gap insulator with strong spin-orbit coupling (SOC). We find that this 2DEG is a genuinely different physical state with respect to the bulk: the orbital symmetries of its subbands are entirely reconstructed and their masses are renormalized. This occurs because the values of the SOC, the Fermi energy, and the subband splittings become comparable in the 2DEG. Additionally, we identify an F-center-like heavy band resulting from the polar nature of the KTO surface

We report the existence of confined electronic states at the (110) and (111) surfaces of SrTiO 3 . Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of SrTiO 3 . This occurs because the crystallographic symmetries of the surface and subsurface planes and the effective electron masses along the confinement direction influence the symmetry of the electronic structure and the orbital ordering of the t 2g manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of SrTiO 3 echoes the tailoring of macroscopic (e.g., transport) properties reported recently in LaAlO 3 =SrTiO 3 (110) and (111) interfaces, and is promising for searching new types of two-dimensional electronic states in correlated-electron oxides.Two-dimensional electron gases (2DEGs) in transitionmetal oxides (TMOs) present remarkable phenomena that make them unique from a fundamental viewpoint and promising for applications [1,2]. For instance, heterostructures grown on the (001) surface of SrTiO 3 , a TMO insulator with a large band gap of approximately 3.5 eV, can develop 2DEGs showing metal-to-insulator transitions [3], superconductivity [4], or magnetism [5,6]. Recently, 2DEGs at the (111) and (110) interfaces of LaAlO 3 =SrTiO 3 were also reported [7]. The latter showed a highly anisotropic conductivity [8] and a superconducting state spatially more extended than the one at the (001) interface [9]. Interestingly, theoretical works have also predicted that exotic, possibly topological, electronic states might occur at interfaces composed of (111) bilayers of cubic TMOs [10-13], as two (111) planes of transition-metal ions form a honeycomb lattice, similar to the one found in graphene. In this context, the discoveries that 2DEGs can also be created at the bare (001) surfaces of SrTiO 3 [14-16] and KTaO 3 [17,18], and more recently at the (111) surface of KTaO 3 [19], open new roads in the fabrication and study of different types of 2DEGs in TMOs-in particular, using surface-sensitive spectroscopic techniques, which give direct information about the Fermi surface and subband structure of the confined states. The origin of the confinement is attributed to a local doping of the surface region due to oxygen vacancies and/or lattice distortions.Here we show that new types of 2DEGs can be directly tailored at the bare (110) and (111) surfaces of SrTiO 3 . Imaging their electronic structure via angle-resolved photoemission spectroscopy (ARPES), we find that their Fermi surfaces, subband masses, and orbital ordering are different from the ones of the 2DEG at the SrTiO 3 ð001Þ surface [14,15] and the ones predicted for the bulk, being thus uniquely sensitive to the confining crystallographic directi...

We report the existence of metallic two dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of dxy orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nano-engineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.In its pure stoichiometric form, the transition metal oxide (TMO) TiO 2 is a transparent insulator that crystallizes in mainly two different phases: rutile and anatase. Both phases have been studied extensively over the last decades, due to their photocatalytic properties discussed in several reviews [1][2][3][4]. Recently, a strong interest in the anatase phase of TiO 2 also surged, owing to its potential for applications in other research fields. For instance, networks of anatase nanoparticles are found in dye-sensitized solar cells [5,6], anatase thin films can be used as transparent conducting oxides [7], and devices based on anatase can be envisioned in spintronics [8,9]. To harness such a wide range of functionalities and guide potential applications using anatase, it is thus critical to understand its microscopic electronic structure, which will be ultimately responsible for the remarkable properties of this material. Moreover, as most applications in microelectronics or heterogeneous catalysis involve essentially the electronic states at the material's surface, it is crucial to directly measure and characterize such states.More generally, the study of two-dimensional electron gases (2DEGs) in TMO surfaces/interfaces has become a very active field of research. The archetypal example, SrTiO 3 -based heterostructures, display many fundamentally interesting properties [10][11][12], such as field-effect induced insulator-to-superconductor transitions [13], magnetism [14] and the coexistence of magnetism and superconductivity [15]. More recently, the discoveries of 2DEGs at the bare (001), (110) and (111) It is well established that the TiO atomic planes, and their ability to accommodate chemical doping by oxygen vacancies at the surface region, play a key role in the formation of the 2DEG at the surface of SrTiO 3 (001). Thus, as a step forward to understand the formation of 2DEGs in TMOs, it is natural to focus on pure TiO 2 crystals such as rutile or anatase. In fact, it is known that, for both rutile and anatase crystal sur...

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