I05-ARPES
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O. J.
Clark
,
F.
Mazzola
,
I.
Markovic
,
J. M.
Riley
,
J.
Feng
,
B.-J.
Yang
,
K.
Sumida
,
T.
Okuda
,
J.
Fujii
,
I.
Vobornik
,
T. K.
Kim
,
K.
Okawa
,
T.
Sasagawa
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[14927, 16262]
Abstract: The band inversions that generate the topologically non-trivial band gaps of topological insulators and the isolated Dirac touching points of three-dimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbital-manifold band inversions occurring along high-symmetry lines has been demonstrated, stabilising multiple bulk and surface Dirac fermions. Here, we discuss the underlying ingredients necessary to achieve such phases, and discuss their existence within the family of transition metal dichalcogenides. We show how their three-dimensional band structures naturally produce only small k z projected band gaps, and demonstrate how these play a significant role in shaping the surface electronic structure of these materials. We demonstrate, through spin- and angle-resolved photoemission and density functional theory calculations, how the surface electronic structures of the group-X TMDs PtSe2 and PdTe2 are host to up to five distinct surface states, each with complex band dispersions and spin textures. Finally, we discuss how the origin of several recently-realised instances of topological phenomena in systems outside of the TMDs, including the iron-based superconductors, can be understood as a consequence of the same underlying mechanism driving k z -mediated band inversions in the TMDs.
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Mar 2019
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I05-ARPES
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Abstract: 2D electron gases (2DEGs) in oxides show great potential for the discovery of new physical phenomena and at the same time hold promise for electronic applications. In this work, angle‐resolved photoemission is used to determine the electronic structure of a 2DEG stabilized in the (111)‐oriented surface of the strong spin–orbit coupling material KTaO3. The measurements reveal multiple sub‐bands that emerge as a consequence of quantum confinement and form a sixfold symmetric Fermi surface. This electronic structure is well reproduced by self‐consistent tight‐binding supercell calculations. Based on these calculations, the spin and orbital texture of the 2DEG is determined. It is found that the 2DEG Fermi surface is derived from bulk J = 3/2 states and exhibits an unconventional anisotropic Rashba‐like lifting of the spin‐degeneracy. Spin‐momentum locking holds only for high‐symmetry directions and a strong out‐of‐plane spin component renders the spin texture threefold symmetric. It is found that the average spin‐splitting on the Fermi surface is an order of magnitude larger than in SrTiO3, which should translate into an enhancement in the spin–orbitronic response of (111)‐KTaO3 2DEG‐based devices.
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Mar 2019
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I05-ARPES
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Diamond Proposal Number(s):
[13438, 16262, 18705]
Abstract: We investigate the electronic structure of a two-dimensional electron gas created at the surface of the multivalley semimetal 1T−PtSe2. Using angle-resolved photoemission and first-principles-based surface space-charge calculations, we show how the induced quantum well sub-band states form multiple Fermi surfaces, which exhibit highly anisotropic Rashba-like spin splittings. We further show how the presence of both electronlike and holelike bulk carriers causes the near-surface band bending potential to develop an unusual nonmonotonic form, with spatially segregated electron accumulation and hole accumulation regions, which in turn amplifies the induced spin splitting. Our results thus demonstrate the novel environment that semimetals provide for tailoring electrostatically induced potential profiles and their corresponding quantum sub-band states.
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Jan 2019
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I05-ARPES
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O. J.
Clark
,
M. J.
Neat
,
K.
Okawa
,
L.
Bawden
,
I.
Markovic
,
Federico
Mazzola
,
J.
Feng
,
V.
Sunko
,
J. M.
Riley
,
W.
Meevasana
,
J.
Fujii
,
I.
Vobornik
,
T. K.
Kim
,
M.
Hoesch
,
T.
Sasagawa
,
P.
Wahl
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[9500, 12469, 13438, 16262]
Abstract: We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe2 by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe2 with its sister compound PtSe2, 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.
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Apr 2018
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I05-ARPES
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M. S.
Bahramy
,
O. J.
Clark
,
B.-J.
Yang
,
J.
Feng
,
L.
Bawden
,
J. M.
Riley
,
I.
Markovic
,
F.
Mazzola
,
V.
Sunko
,
D.
Biswas
,
S. P.
Cooil
,
M.
Jorge
,
J. W.
Wells
,
M.
Leandersson
,
T.
Balasubramanian
,
J.
Fujii
,
I.
Vobornik
,
J. E.
Rault
,
T. K.
Kim
,
M.
Hoesch
,
K.
Okawa
,
M.
Asakawa
,
T.
Sasagawa
,
T.
Eknapakul
,
W.
Meevasana
,
P. D. C.
King
Diamond Proposal Number(s):
[2469, 9500, 13438, 14927]
Abstract: 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 co-existence 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.
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Nov 2017
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I05-ARPES
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Siobhan
Mckeown Walker
,
S.
Riccò
,
F.
Bruno
,
Alberto
De La Torre
,
Anna
Tamai
,
E.
Golias
,
A.
Varykhalov
,
D.
Marchenko
,
Moritz
Hoesch
,
M. S.
Bahramy
,
Philip
King
,
J.
Sánchez-Barriga
,
Felix
Baumberger
Diamond Proposal Number(s):
[11741]
Abstract: We reinvestigate the putative giant spin splitting at the surface of SrTiO3 reported by Santander–Syro et al. [Nat. Mater. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on fractured (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.
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Jun 2016
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I05-ARPES
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L.
Bawden
,
S. P.
Cooil
,
F.
Mazzola
,
J. M.
Riley
,
L. J.
Collins-Mcintyre
,
V.
Sunko
,
K. W. B.
Hunvik
,
M.
Leandersson
,
C. M.
Polley
,
T.
Balasubramanian
,
T. K.
Kim
,
M.
Hoesch
,
J. W.
Wells
,
G.
Balakrishnan
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[11383]
Open Access
Abstract: 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.
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May 2016
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I05-ARPES
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J.
Riley
,
W.
Meevasana
,
L.
Bawden
,
M.
Asakawa
,
T.
Takayama
,
T.
Eknapakul
,
T.
Kim
,
M.
Hoesch
,
S. K.
Mo
,
H.
Takagi
,
T.
Sasagawa
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[9500, 11383]
Abstract: Tunable bandgaps, extraordinarily large exciton-binding energies, strong light–matter coupling and a locking of the electron spin with layer and valley pseudospins have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications. Using angle-resolved photoemission (ARPES), we show here that doping electrons at the surface of the prototypical strong spin–orbit TMD WSe2 , akin to applying a gate voltage in a transistor-type device, induces a counterintuitive lowering of the surface chemical potential concomitant with the formation of a multivalley 2D electron gas (2DEG). These measurements provide a direct spectroscopic signature of negative electronic compressibility (NEC), a result of electron–electron interactions, which we find persists to carrier densities approximately three orders of magnitude higher than in typical semiconductor 2DEGs that exhibit this effect. An accompanying tunable spin splitting of the valence bands further reveals a complex interplay between single-particle band-structure evolution and many-body interactions in electrostatically doped TMDs. Understanding and exploiting this will open up new opportunities for advanced electronic and quantum-logic devices.
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Sep 2015
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I05-ARPES
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Siobhan
Mckeown Walker
,
Flavio
Bruno
,
Zhiming
Wang
,
Alberto
De La Torre
,
Sara
Ricco
,
Anna
Tamai
,
Timur
Kim
,
Moritz
Hoesch
,
Ming
Shi
,
Mohammad Saeed
Bahramy
,
Philip
King
,
Felix
Baumberger
Abstract: The origin of the 2D electron gas (2DEG)stabilized at the bare surface of SrTiO3 (001) is investigated. Using high-resolution angle-resolved photoemission and core-level spectroscopy, it is shown conclusively that this 2DEG arises from light-induced oxygen vacancies. The dominant mechanism driving vacancy formation is identified, allowing unprecedented control over the 2DEG carrier density.
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May 2015
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I05-ARPES
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J. M.
Riley
,
F.
Mazzola
,
M.
Dendzik
,
M.
Michiardi
,
T.
Takayama
,
L.
Bawden
,
C.
Granerød
,
M.
Leandersson
,
T.
Balasubramanian
,
M.
Hoesch
,
T. K.
Kim
,
H.
Takagi
,
W.
Meevasana
,
Ph.
Hoffmann
,
M. S.
Bahramy
,
J. W.
Wells
,
P. D. C.
King
Diamond Proposal Number(s):
[9427, 9500]
Abstract: Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices. This is generally accepted to require breaking global structural inversion symmetry. In contrast, here we report the observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide WSe2. Mediated by a lack of inversion symmetry in constituent structural units of the bulk crystal where the electronic states are localized we show how spin splittings up to ∼0.5 eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides of key importance for using these compounds in proposed valleytronic devices.
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Oct 2014
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