Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor

Abstract

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...

Spin-resolved photoemission study of epitaxially grown MoSe2and WSe2thin films

“…The reversal of the measured spin polarization by changing the flip angle suggests that the spin texture prominent in our measurement condition, 55eV, C+ polarized, is sensitive to the orbitals that changes their parity when reflected with respect to the mirror plane. According to the first principle calculations [19,39], the orbital character of the K-point VB is mainly in-plane, a mixture of d xy and d x 2 −y 2 , from which we expect the change of signs with the change in flip angle in our measurement geometry. A more detailed studies on the spin texture depending on the orbital characters, in both theory and experiment, as was done for the topological surface state in topological insulators [48,49], are necessary to disentangle the nature of measured spin texture in SARPES with respect to the initial state spin.…”

“…Of particular interest, Ref. [39] reports spin polarized signal just from regular bulk WSe 2 , and interprets their results in terms of strong electron localization at the K/K'-point of WSe 2 .…”

“…There has been only a few SARPES studies on MX 2 [37,38,39] despite the large interest on the material class, mainly due to the difficulty in preparing large area samples suitable for measurements in ultra-high vacuum environment, as current SARPES setups at the synchrotron source are bound to large beam spot size to allow maximum photon flux. Moreover, the previous SARPES works are still essentially on bulk samples of MX 2 , such as bulk inversion symmetry broken 3R phase of MoS 2 [37] or spin polarization induced by the space charge residing in the overlayer [38].…”

A First-Principles Time-Dependent Density Functional Theory Framework for Spin and Time-Resolved Angular-Resolved Photoelectron Spectroscopy in Periodic Systems

“…By comparing this spectrum to the experiments reported in Refs. 56,57 it becomes clear that the system probed by the experiment is composed of more than two layers as indicated, for instance, by the presence of an ARPES signal filling the space between the two topmost valence bands at Γ.…”

Physical sciences at Diamond: past achievements and future opportunities

“…It uses a technique known as angle resolved photo-emission spectroscopy (ARPES) [1]. The first branch of I05 ARPES beamline began operations in July 2013 and has already had some spectacular early successes [4][5][6].…”