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Paulina
Majchrzak
,
Klara
Volckaert
,
Antonija Grubišić
Čabo
,
Deepnarayan
Biswas
,
Marco
Bianchi
,
Sanjoy K.
Mahatha
,
Maciej
Dendzik
,
Federico
Andreatta
,
Signe S.
Grønborg
,
Igor
Markovic
,
Jonathon M.
Riley
,
Jens C.
Johannsen
,
Daniel
Lizzit
,
Luca
Bignardi
,
Silvano
Lizzit
,
Cephise
Cacho
,
Oliver
Alexander
,
Dan
Matselyukh
,
Adam S.
Wyatt
,
Richard T.
Chapman
,
Emma
Springate
,
Jeppe V.
Lauritsen
,
Phil D. C.
King
,
Charlotte
Sanders
,
Jill A.
Miwa
,
Philip
Hofmann
,
Soeren
Ulstrup
Open Access
Abstract: The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS
. These timescales are explained by Auger recombination involving MoS
and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.
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Jul 2021
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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
I10-Beamline for Advanced Dichroism - scattering
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J. M.
Riley
,
F.
Caruso
,
C.
Verdi
,
L. B.
Duffy
,
M. D.
Watson
,
L.
Bawden
,
K.
Volckaert
,
G.
Van Der Laan
,
T.
Hesjedal
,
M.
Hoesch
,
F.
Giustino
,
P. D. C.
King
Diamond Proposal Number(s):
[15481, 13539, 16162]
Open Access
Abstract: Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio
many-body calculations, we demonstrate how a strong coupling of conduction electrons with
collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.
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Jun 2018
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I05-ARPES
|
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
|
D.
Biswas
,
Alex M.
Ganose
,
R.
Yano
,
J. M.
Riley
,
L.
Bawden
,
O. J.
Clark
,
J.
Feng
,
L.
Collins-Mcintyre
,
M. T.
Sajjad
,
W.
Meevasana
,
T. K.
Kim
,
M.
Hoesch
,
J. E.
Rault
,
T.
Sasagawa
,
David O.
Scanlon
,
P. D. C.
King
Diamond Proposal Number(s):
[9500, 11383]
Abstract: We have used angle-resolved photoemission spectroscopy to investigate the band structure of ReS2, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along kz. These experiments are in good agreement with our density-functional theory calculations, shedding light on the bulk electronic structure of ReS2, and how it can be expected to evolve when thinned to a single layer.
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Aug 2017
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I05-ARPES
|
Matthew D.
Watson
,
Yu
Feng
,
Christopher
Nicholson
,
Claude
Monney
,
Jonathon
Riley
,
Hideaki
Iwasawa
,
Keith
Refson
,
Vincent
Sacksteder
,
Devashibhai
Adroja
,
Jun
Zhao
,
Moritz
Hoesch
Diamond Proposal Number(s):
[13797, 14572]
Open Access
Abstract: We present angle-resolved photoemission spectroscopy measurements of the quasi-one-dimensional superconductor K2Cr3As3. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ∼200 meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superconductivity in this family of compounds.
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Mar 2017
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I05-ARPES
|
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
|
P.
Kushwaha
,
V.
Sunko
,
P. J. W.
Moll
,
L.
Bawden
,
J. M.
Riley
,
N.
Nandi
,
H.
Rosner
,
M. P.
Schmidt
,
F.
Arnold
,
E.
Hassinger
,
T. K.
Kim
,
M.
Hoesch
,
A. P.
Mackenzie
,
P. D. C
King
Diamond Proposal Number(s):
[10040]
Open Access
Abstract: Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit–assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along kz. Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14me. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below EF, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free–electron system in a 5d delafossite transition-metal oxide.
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Oct 2015
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I05-ARPES
|
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
|
Lewis
Bawden
,
Jonathon
Riley
,
C. H.
Kim
,
R.
Sankar
,
E. J.
Monkman
,
D. E.
Shai
,
H. I.
Wei
,
E. B.
Lochocki
,
J. W.
Wells
,
W.
Meevasana
,
Timur
Kim
,
M
Hoesch
,
Y.
Ohtsubo
,
P.
Le Fevre
,
C. J.
Fennie
,
K. M.
Shen
,
F.
Chou
,
P. D. C.
King
Diamond Proposal Number(s):
[9427]
Open Access
Abstract: The Rashba effect is one of the most striking manifestations of spin-orbit coupling in solids, and provides a cornerstone for the burgeoning field of semiconductor spintronics. It is typically assumed to manifest as a momentum-dependent splitting of a single initially spin-degenerate band into two branches with opposite spin polarisation. Here, combining polarisation-dependent and resonant angle-resolved photoemission measurements with density-functional theory calculations, we show that the two "spin-split" branches of the model giant Rashba system BiTeI additionally develop disparate orbital textures, each of which is coupled to a distinct spin configuration. This necessitates a re-interpretation of spin splitting in Rashba-like systems, and opens new possibilities for controlling spin polarisation through the orbital sector.
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Sep 2015
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