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]
Open Access
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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Veronika
Sunko
,
Edgar
Abarca Morales
,
Igor
Markovic
,
Mark E.
Barber
,
Dijana
Milosavljević
,
Federico
Mazzola
,
Dmitry A.
Sokolov
,
Naoki
Kikugawa
,
Cephise
Cacho
,
Pavel
Dudin
,
Helge
Rosner
,
Clifford
Hicks
,
Philip D. C.
King
,
Andrew P.
Mackenzie
Diamond Proposal Number(s):
[20427]
Open Access
Abstract: Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.
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Dec 2019
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I05-ARPES
I10-Beamline for Advanced Dichroism
|
Matthew D.
Watson
,
Igor
Markovic
,
Federico
Mazzola
,
Akhil
Rajan
,
Edgar A.
Morales
,
David
Burn
,
Thorsten
Hesjedal
,
Gerrit
Van Der Laan
,
Saumya
Mukherjee
,
Timur K.
Kim
,
Chiara
Bigi
,
Ivana
Vobornik
,
Monica
Ciomaga Hatnean
,
Geetha
Balakrishnan
,
Philip D. C.
King
Diamond Proposal Number(s):
[21986, 22794, 23785]
Abstract: We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, CrGeTe3. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. The t2g states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.
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May 2020
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I05-ARPES
|
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
|
Jiagui
Feng
,
Deepnarayan
Biswas
,
Akhil
Rajan
,
Matthew D.
Watson
,
Federico
Mazzola
,
Oliver J.
Clark
,
Kaycee
Underwood
,
I.
Markovic
,
Martin
Mclaren
,
Andrew
Hunter
,
David M.
Burn
,
Liam B.
Duffy
,
Sourabh
Barua
,
Geetha
Balakrishnan
,
Francois
Bertran
,
Patrick
Le Fevre
,
Timur
Kim
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Peter
Wahl
,
Phil D. C.
King
Diamond Proposal Number(s):
[19771]
Abstract: How the interacting electronic states and phases of layered transition-metal dichalcogenides
evolve when thinned to the single-layer limit is a key open question in the study of two-dimensional
materials. Here, we use angle-resolved photoemission to investigate the electronic structure of monolayer VSe2 grown on bi-layer graphene/SiC. While the global electronic structure is similar to that of bulk VSe2, we show that, for the monolayer, pronounced energy gaps develop over the entire Fermi surface with decreasing temperature below Tc = 140 5 K, concomitant with the emergence
of charge-order superstructures evident in low-energy electron diffraction. These observations point
to a charge-density wave instability in the monolayer which is strongly enhanced over that of the bulk. Moreover, our measurements of both the electronic structure and of x-ray magnetic circular dichroism reveal no signatures of a ferromagnetic ordering, in contrast to the results of a recent experimental study as well as expectations from density-functional theory. Our study thus points
to a delicate balance that can be realised between competing interacting states and phases in
monolayer transition-metal dichalcogenides.
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Jun 2018
|
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I05-ARPES
|
Igor
Markovic
,
Matthew D.
Watson
,
Oliver J.
Clark
,
Federico
Mazzola
,
Edgar
Abarca Morales
,
Chris A.
Hooley
,
Helge
Rosner
,
Craig M.
Polley
,
Thiagarajan
Balasubramanian
,
Saumya
Mukherjee
,
Naoki
Kikugawa
,
Dmitry A.
Sokolov
,
Andrew P.
Mackenzie
,
Phil D. C.
King
Diamond Proposal Number(s):
[21986, 25040]
Abstract: The interplay between spin–orbit coupling and structural inversion symmetry breaking in solids has generated much interest due to the nontrivial spin and magnetic textures which can result. Such studies are typically focused on systems where large atomic number elements lead to strong spin–orbit coupling, in turn rendering electronic correlations weak. In contrast, here we investigate the temperature-dependent electronic structure of Ca3Ru2O7
, a 4d
oxide metal for which both correlations and spin–orbit coupling are pronounced and in which octahedral tilts and rotations combine to mediate both global and local inversion symmetry-breaking polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridization mediated by a hidden Rashba-type spin–orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridization is actually the key driver for the phase transition, reflecting a delicate interplay between spin–orbit coupling and strong electronic correlations and revealing a route to control magnetic ordering in solids.
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Jun 2020
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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
|
Federico
Mazzola
,
Veronika
Sunko
,
Seunghyun
Khim
,
Helge
Rosner
,
Pallavi
Kushwaha
,
Oliver J.
Clark
,
Lewis
Bawden
,
Igor
Markovic
,
Timur K.
Kim
,
Moritz
Hoesch
,
Andrew P.
Mackenzie
,
Phil D. C.
King
Diamond Proposal Number(s):
[12469, 14927, 16262]
Abstract: The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability to itinerant ferromagnetism at the Pd-terminated surface of the nonmagnetic delafossite oxide metal PdCoO2. Combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO2, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.
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Dec 2018
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I05-ARPES
|
Veronika
Sunko
,
H.
Rosner
,
P.
Kushwaha
,
S.
Khim
,
F.
Mazzola
,
L.
Bawden
,
O. J.
Clark
,
J. M.
Riley
,
D.
Kasinathan
,
M. W.
Haverkort
,
T. K.
Kim
,
M.
Hoesch
,
J.
Fujii
,
I.
Vobornik
,
A. P.
Mackenzie
,
Philip
King
Diamond Proposal Number(s):
[12469, 14927, 18267]
Abstract: Engineering and enhancing the breaking of inversion symmetry in solids—that is, allowing electrons to differentiate between ‘up’ and ‘down’—is a key goal in condensed-matter physics and materials science because it can be used to stabilize states that are of fundamental interest and also have potential practical applications. Examples include improved ferroelectrics for memory devices and materials that host Majorana zero modes for quantum computing1, 2. Although inversion symmetry is naturally broken in several crystalline environments, such as at surfaces and interfaces, maximizing the influence of this effect on the electronic states of interest remains a challenge. Here we present a mechanism for realizing a much larger coupling of inversion-symmetry breaking to itinerant surface electrons than is typically achieved. The key element is a pronounced asymmetry of surface hopping energies—that is, a kinetic-energy-coupled inversion-symmetry breaking, the energy scale of which is a substantial fraction of the bandwidth. Using spin- and angle-resolved photoemission spectroscopy, we demonstrate that such a strong inversion-symmetry breaking, when combined with spin–orbit interactions, can mediate Rashba-like3, 4 spin splittings that are much larger than would typically be expected. The energy scale of the inversion-symmetry breaking that we achieve is so large that the spin splitting in the CoO2- and RhO2-derived surface states of delafossite oxides becomes controlled by the full atomic spin–orbit coupling of the 3d and 4d transition metals, resulting in some of the largest known Rashba-like3, 4 spin splittings. The core structural building blocks that facilitate the bandwidth-scaled inversion-symmetry breaking are common to numerous materials. Our findings therefore provide opportunities for creating spin-textured states and suggest routes to interfacial control of inversion-symmetry breaking in designer heterostructures of oxides and other material classes.
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Sep 2017
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I05-ARPES
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Diamond Proposal Number(s):
[19771, 18555, 16262]
Abstract: We revisit the enduring problem of the 2 × 2 × 2 charge density wave (CDW) order in TiSe2, utilizing photon energy-dependent angle-resolved photoemission spectroscopy to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridization that is strongly kz dependent. While such a momentum-selective coupling can provide the energy gain required to form the CDW, we show how additional “passenger” states remain, which couple only weakly to the CDW and thus dominate the low-energy physics in the ordered phase of TiSe2.
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Feb 2019
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