¦ van der Waals gap Septuple layer J § J || T 1/2 J 0,1 || J 0,2 || J 0,3 || J 0,4 ||

Oxygen vacancies created in anatase TiO2 by UV photons (80 -130 eV) provide an effective electrondoping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission (ARPES) reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.The anatase structural phase of titanium dioxide (TiO 2 ) can be the key element in novel applications. Whereas extensive work has been focused on its famous photocatalytic behavior [1-3], more and more proposed devices, such as memristors [4], spintronic devices [5], and photovoltaic cells [6][7][8], rely on its less well-known electronic properties. In particular, anatase has been recently suggested as a candidate for replacing the In-based technology for transparent conducting oxides [9] in a wide range of applications from solar cell elements, to light-emitting devices, to flat panels, to touch-screen controls [10]. The crucial quantity for the figure of merit in these devices is conductivity, and it is therefore of major interest to understand and control the electronic properties of pristine and doped anatase.Stoichiometric anatase is an insulator with a 3.2 eV band gap [11] but oxygen vacancies, typically present with concentrations in the 10 17 cm −3 range [12,13], create a shallow donor level ∼10 meV below the conduction band (CB) [14]. Since large single crystals became available for transport studies, a better insight has been gained on the influence of these donors on the electronic response of anatase. Above ∼60 K, the electrons thermally excited into the CB give rise to metallic-like transport. At lower temperatures, the anomalous increase of resistivity indicates that the charge carriers are not bare electrons but polarons [14], i.e., electrons coherently coupled to a lattice distorsion induced by the Coulomb interaction. Understanding the properties of such composite particles in anatase is important to better engineer the material for targeted applications, where the low electron mobility often represents the overall performance bottleneck. We will also demonstrate that, from the point of view of fundamental physics, anatase represents an excellent model compound to study the behavior of the "rare" large polaron quasiparticles (QPs), intermediate between localized small polarons and free electrons.We performed ARPES measurements on TiO 2 single crystals ( Fig. 1(a)) and thin films grown in situ on insulating LaAlO 3 and conducting Nb-doped SrTiO 3 substrates. Clean (001) surfaces were prepared as described in Suppl. Inf. The results presented have been obtained consistently both for single crystals and thin films, and therefore reflect intrinsic properties of the anatase phase, independent of the sample preparation method. While oxygen defects are always present to some extent after the surface preparation, we have found that exposure to UV photons...

Crystal growth of MnBi 2 Te 4 has delivered the first experimental corroboration of the 3D antiferromagnetic topological insulator state. Our present results confirm that the synthesis of MnBi 2 Te 4 can be scaled-up and strengthen it as a promising experimental platform for studies of a crossover between magnetic ordering and non-trivial topology. High-quality single crystals of MnBi 2 Te 4 are grown by slow cooling within a narrow range between the melting points of Bi 2 Te 3 (586 °C) and MnBi 2 Te 4 (600 °C). Single crystal X-ray diffraction and electron microscopy reveal ubiquitous antisite defects in both cation sites and, possibly, Mn vacancies. Powders of MnBi 2 Te 4 can be obtained at subsolidus temperatures, and a complementary thermochemical study establishes a limited high-temperature range of phase stability. Nevertheless, quenched powders are stable at room temperature and exhibit long-range antiferromagnetic ordering below 24 K. The expected Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption and linear dichroism data. MnBi 2 Te 4 exhibits a metallic type of resistivity in the range 4.5-300 K. The compound is an n-type conductor that reaches a thermoelectric figure of merit up to ZT = 0.17. Angle-resolved photoemission experiments provide evidence for a surface state forming a gapped Dirac cone.

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron–hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier–Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.