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...

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin-and angle-resolved photoemission, we find that these generically host a coexistence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.The classification of electronic structures based on their topological properties has opened powerful routes for understanding solid state materials. 1 The nowfamiliar Z 2 topological insulators are most renowned for their spin-polarised Dirac surface states residing in inverted bulk band gaps. 1 In systems with rotational invariance, a band inversion on the rotation axis can generate protected Dirac cones with a point-like Fermi surface of the bulk electronic structure. 2-8 If either inversion or time-reversal symmetry is broken, a bulk Dirac point can split into a pair of spin-polarised Weyl points. 9-15 Unlike for elementary particles, Lorentz-violating Weyl fermions can also exist in the solid state, manifested as a tilting of the Weyl cone. If this tilt is sufficiently large, so-called type-II Weyl points can occur, now formed at the touching of open electron and hole pockets. [15][16][17][18][19][20][21][22] Realising such phases in solid-state materials not only offers unique environments and opportunities for studying the fundamental properties of fermions, but also holds potential for applications exploiting their exotic surface excitations and bulk electric and thermal transport properties. [23][24][25][26][27] Consequently, there is an intense current effort focused on identifying compounds which host the requisite band inversions. In many cases, however, this arXiv:1702.08177v2 [cond-mat.mtrl-sci]

We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe_{2} by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe_{2} with its sister compound PtSe_{2}, we demonstrate how enhanced interlayer hopping in the Te-based material drives a band inversion within the antibonding p-orbital manifold well above the Fermi level. We show how this mediates spin-polarized topological surface states which form rich multivalley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.

Semiconductors are typically considered weakly interacting systems, well described by conventional band theory. The exchange and correlation energies arising from electronelectron interactions can, however, dominate the kinetic energy in the dilute doping limit.This stabilises a small regime of negative electronic compressibility (NEC), κ = weak anti-localisation 17 and a density-tuned dome of superconductivity. 18 A detailed understanding of the underlying gate-induced electronic structure evolution driving such emergent properties has, however, remained elusive.Here, we mimic the effects of field-effect doping in the TMD WSe 2 by the sub-monolayer 2 deposition of alkali metals at the vacuum-cleaved surface. Such "chemical gating" leaves the surface accessible for detailed spectroscopic measurements. From angle-resolved photoemission (ARPES), we uncover how the resulting charge accumulation drives a pronounced reconstruction of the bulk electronic structure, not only mediating the formation of a multivalley 2DEG and a giant tuneable valence band spin splitting, but also inducing a pronounced decrease of the surface chemical potential with increasing electron doping. This direct spectroscopic observation of NEC, which we find persists to remarkably high electron densities, reveals a dominant role of many-body interactions shaping the underlying electronic landscape of electrostatically-tuned TMDs.In Figure 1, we show the occupied electronic structure of bulk and chemically-gated WSe 2 as measured by ARPES. No electronic states cross the Fermi level for the pristine cleaved material ( Fig. 1(b)), consistent with its semiconducting bulk. While the uppermost valence bands near the zone centre are strongly three-dimensional, those at the zone-corner K point have negligible dispersion along k z , with electronic wavefunctions localised to single Se-W-Se monlayers (half of the unit cell). 8,19 These two-dimensional states, which form the lowest energy band extrema in monolayer TMDs, are strongly spin-polarised even in the bulk. 6,8The spin is coupled to the valley degree of freedom, alternating sign at neighbouring corners of the Brillouin zone just as for monolayer MoS 2 and WSe 2 . 5,7 For the 2H structure, spin also becomes locked to the layer pseudospin, reversing sign for neighbouring Se-W-Se layers. 6-8An energetic degeneracy of the states in neighbouring layers thus enforces the total electronic structure to be spin degenerate, as required by the structural inversion symmetry of bulk WSe 2 ( Fig. 1(a)).We show that breaking such inversion symmetry, achieved here by our surface doping approach, drives a number of striking changes of the electronic structure ( Fig. 1(c)). Deposition of minute quantities of alkali metals, electron doping the surface, causes the conduction band states to become populated at the K point (only weakly visible) and approximately mid-way along the Γ − K direction (denoted here as T ). The latter have the larger occupied bandwidth, maintaining an indirect band gap as for bulk WSe 2 . Unl...

Engineering and enhancing inversion symmetry breaking in solids is a major goal in condensed matter physics and materials science, as a route to advancing new physics and applications ranging from improved ferroelectrics for memory devices to materials hosting Majorana zero modes for quantum computing. Here, we uncover a new mechanism for realising a much larger energy scale of inversion symmetry breaking at surfaces and interfaces than is typically achieved. The key ingredient is a pronounced asymmetry of surface hopping energies, i.e. a kinetic energy-driven inversion symmetry breaking, whose energy scale is pinned at a significant fraction of the bandwidth. We show, from spin-and angle-resolved photoemission, how this enables surface states of 3d and 4d-based transition-metal oxides to surprisingly develop some of the largest Rashba-like spin splittings that are known. Our findings open new possibilities to produce spin textured states in oxides which exploit the full potential of the bare atomic spin-orbit coupling, raising exciting prospects for oxide spintronics. More generally, the core structural building blocks which enable this are common to numerous materials, providing the prospect of enhanced inversion symmetry breaking at judiciously-chosen surfaces of a plethora of compounds, and suggesting routes to interfacial control of inversion symmetry breaking in designer heterostructures.The lifting of inversion symmetry is a key prerequisite for stabilising a wide range of striking physical properties such as chiral magnetism, ferroelectricity, odd-parity multipolar orders, and the creation of Weyl fermions and other spin-split electronic states without magnetism [1][2][3][4][5][6][7] . Inversion symmetry is naturally broken at surfaces and interfaces of materials, opening exciting routes to stabilise electronic structures distinct from those of the bulk [8][9][10][11][12][13][14][15] . A striking example is found in materials which also host significant spin-orbit interactions, where inversion symmetry breaking (ISB) underpins the formation of topologically-protected surface states 11,16 and Rashba 10,17 spin splitting of surface or interfacelocalised two-dimensional electron gases 14,15,[18][19][20][21][22] , generically characterised by a locking of the quasiparticle spin perpendicular to its momentum. Such effects lie at the heart of a variety of proposed applications in spin-based electronics 10,[23][24][25][26][27] , and provide new routes to stabilise novel physical regimes such as spiral RKKY interactions, enhanced electron-phonon coupling, localisation by a weak potential, large spin-transfer torques, and mixed singlet-triplet superconductivity [28][29][30][31][32][33][34][35] . Conventional wisdom about how to maximize the Rashba effect has been to work with heavy elements whose atomic spin-orbit coupling is large. However, the energetic spin splittings obtained are usually only a small fraction of the atomic spin-orbit energy scale. This is because the key physics is not exclusively that of spi...

Transport and ARPES reveal extremely good metallicity arising from almost free-electron behavior in single-crystal PtCoO2.

We report detailed investigations of the properties of a superconductor obtained by substituting In at the Sn site in the topological crystalline insulator (TCI), SnTe. Transport, magnetization and heat capacity measurements have been performed on crystals of Sn0.6In0.4Te, which is shown to be a bulk superconductor with T onset c at ∼ 4.70(5) K and T zero c at ∼ 3.50(5) K. The upper and lower critical fields are estimated to be µ0Hc2(0) = 1.42(3) T and µ0Hc1(0) = 0.90(3) mT respectively, while κ = 56.4(8) indicates this material is a strongly type II superconductor.

Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.