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...

Abstract.Using monochromated Electron Energy Loss Spectroscopy (EELS) in a probecorrected Scanning Transmission Electron Microscope (STEM) we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

Band gap variations in thin film structures, across grain boundaries, and in embedded nanoparticles are of increasing interest in the materials science community. As many common experimental techniques for measuring band gaps do not have the spatial resolution needed to observe these variations directly, probe-corrected Scanning Transmission Electron Microscope (STEM) with monochromated Electron Energy-Loss Spectroscopy (EELS) is a promising method for studying band gaps of such features. However, extraction of band gaps from EELS data sets usually requires heavy user involvement, and makes the analysis of large data sets challenging. Here we develop and present methods for automated extraction of band gap maps from large STEM-EELS data sets with high spatial resolution while preserving high accuracy and precision.

Mixed oxide hydride anion systems constitute a novel class of materials exhibiting intriguing properties such as solid-state hydride ion conduction and fast anion exchange. In this contribution we derive the kinetics of hydride ion transport in a mixed oxide-hydride system, SrTiO 3-x H x through isotope exchange and depth profiling. DFT calculations indicate that migration of Hto neighboring vacant oxygen lattice sites is fast, but that long-range transport is impeded by slow reorganization of the oxygen sublattice. From measured hydride tracer diffusion coefficients and the correlation factors derived from DFT, we are able to derive the hydrogen self-diffusion coefficients in SrTiO 3-x H x . More generally, the explicit description of hydride ion transport in SrTiO 3-x H x through combination of experimental and computational methods reported in this work can be applied to explore anion diffusion in other mixed anion systems.

It is known that (ZnO) 1−x (GaN) x alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of (ZnO) 1−x (GaN) x homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle GW approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic "tail" in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in (ZnO) 1−x (GaN) x alloys is due to the defect band formation, presumably at the top of the valence band maximum.