¦ 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.

We observe a giant spin-orbit splitting in bulk and surface states of the non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics -a large, robust spin splitting and ambipolar conduction -are present in this material.

Angle resolved photoemission spectroscopy (ARPES) is commonly known as a powerful probe of the one-electron removal spectral function in ordered solid state. With increasing efficiency of light sources and spectrometers, experiments over a wide range of emission angles become more and more common. Consequently, the angular variation of ARPES spectral weight-often times termed "matrix element effect"-enters as an additional source of information. In this tutorial, we develop a simple but instructive free electron final state approach based on the three-step model to describe the intensity distribution in ARPES. We find a compact expression showing that the ARPES spectral weight of a given Bloch band is essentially determined by the momentum distribution (the Fourier transform) of its associated Wannier orbital-times a polarization dependent pre-factor. While the former is giving direct information on the symmetry and shape of the electronic wave function, the latter can give rise to surprising geometric effects. We discuss a variety of modern and instructive experimental showcases for which this simplistic formalism works astonishingly well and discuss the limits of this approach.

In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable bandgaps 1-3 and strongly bound excitons and trions emerge from strong many-body effects [4][5][6] , beyond the spin and valley degrees of freedom induced by spin-orbit coupling and by lattice symmetry 7 . Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions [8][9][10] . Here, we use micro-focused angle-resolved photoemission spectroscopy (microARPES) and in situ surface doping to manipulate the electronic structure of single-layer WS 2 on hexagonal boron nitride (WS 2 /h-BN). Upon electron doping, we observe an unexpected giant renormalization of the spin-orbit splitting of the single-layer WS 2 valence band, from 430 meV to 660 meV, together with a bandgap reduction of at least 325 meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials [11][12][13] . Coulomb interactions in 2D materials are several times stronger than in their 3D counterparts. In 2D TMDs, this is most directly evidenced by the presence of excitons with binding energies an order of magnitude higher than in the bulk 4 . Although the excitons in these 2D materials have been widely studied by optical techniques 13 , the impact of strong electron-electron interactions on the quasiparticle band structure remains unclear. Theory predicts that many-body effects will influence the spin-orbit splitting around the valenceband maximum (VBM) and conduction-band minimum (CBM) 14 . Although these should be observable by ARPES, a direct probe of many-body effects 15 , measurements so far have mainly focused on the layer-dependence of the single-particle spectrum and the direct bandgap transition in 2D TMD systems, including epitaxial single- . Unfortunately, the lateral size of mechanically assembled heterostructures is usually of the order of 10 μ m, much smaller than the beam spot of typical ARPES setups (≳ 100 μ m). Furthermore, sample charging on insulating bulk h-BN substrates would complicate ARPES experiments.We overcome these challenges as follows. We realize a high-quality 2D semiconductor-insulator interface by mechanically transferring a relatively large (~100 μ m) single-layer WS 2 crystal onto a thin flake of h-BN that has itself been transferred onto a degenerately doped TiO 2 substrate, as depicted in Fig. 1a. Sample charging is avoided because there is electrical contact from the continuous single-layer WS 2 flake to both the h-BN and the conductive TiO 2 . Figure 1b is an optical microscope image of the sample, including a flake of h-BN, approximately 100 μ m wide, surrounded by several transferred flakes of single-layer WS 2 on the Ti...

The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators like Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin-and angle-resolved photoelectron spectroscopy (ARPES) experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.The hallmark of a topological insulator is a single spinpolarized Dirac cone at the surface which is protected by time reversal-symmetry and originates from a band inversion in the bulk [1,2]. Notably, breaking time-reversal symmetry by magnetic order does not necessarily destroy the non-trivial topology but instead may drive the system into another topological phase. One example is the quantum anomalous Hall (QAH) state that has been observed in magnetically doped topological insulators [3]. The QAH state, in turn, may form the basis for yet more exotic electronic states, such as axion insulators [4,5] and chiral Majorana fermions [6]. Another example is the antiferromagnetic topological insulator state which is protected by a combination of time-reversal and lattice translational symmetries [7].Magnetic order in a topological insulator has mainly been achieved by doping with 3d impurities [3,8], which however inevitably gives rise to increased disorder. By contrast, the layered van der Waals material MnBi 2 Te 4 [9, 10] has recently been proposed to realize an intrinsic magnetic topological insulator [11][12][13][14], i.e. a compound that features magnetic order and a topologically non-trivial bulk band structure at the arXiv:1903.11826v2 [cond-mat.str-el]