I09-Surface and Interface Structural Analysis
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G.
Vinai
,
C.
Bigi
,
A.
Rajan
,
M. D.
Watson
,
T.-l.
Lee
,
F.
Mazzola
,
S.
Modesti
,
S.
Barua
,
M.
Ciomaga Hatnean
,
G.
Balakrishnan
,
P. D. C.
King
,
P.
Torelli
,
G.
Rossi
,
G.
Panaccione
Diamond Proposal Number(s):
[21429]
Abstract: Among transition-metal dichalcogenides, mono and few-layers thick
VSe
2
has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of
Fe
/
VSe
2
heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk
VSe
2
do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the
Fe
/
VSe
2
interface and its relation with interfacial magnetic coupling.
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Jan 2020
|
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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
|
|
I05-ARPES
I09-Surface and Interface Structural Analysis
|
Veronika
Sunko
,
F.
Mazzola
,
S.
Kitamura
,
S.
Khim
,
P.
Kushwaha
,
O. J.
Clark
,
M. D.
Watson
,
I.
Markovic
,
D.
Biswas
,
L.
Pourovskii
,
T. K.
Kim
,
T.-l.
Lee
,
P. K.
Thakur
,
H.
Rosner
,
A.
Georges
,
R.
Moessner
,
T.
Oka
,
A. P.
Mackenzie
,
P. D. C.
King
Diamond Proposal Number(s):
[19479, 17699]
Abstract: A nearly free electron metal and a Mott insulating state can be thought of as opposite ends of the spectrum of possibilities for the motion of electrons in a solid. Understanding their interaction lies at the heart of the correlated electron problem. In the magnetic oxide metal PdCrO2, nearly free and Mott-localized electrons exist in alternating layers, forming natural heterostructures. Using angle-resolved photoemission spectroscopy, quantitatively supported by a strong coupling analysis, we show that the coupling between these layers leads to an “intertwined” excitation that is a convolution of the charge spectrum of the metallic layer and the spin susceptibility of the Mott layer. Our findings establish PdCrO2 as a model system in which to probe Kondo lattice physics and also open new routes to use the a priori nonmagnetic probe of photoemission to gain insights into the spin susceptibility of correlated electron materials.
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Feb 2020
|
|
I05-ARPES
|
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
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[14927]
Abstract: We report on a combined study of the de Haas-van Alphen effect and angle-resolved photoemission spectroscopy on single crystals of the metallic delafossite PdRhO2 rounded off by ab initio band structure calculations. A high-sensitivity torque magnetometry setup with superconducting quantum interference device readout and synchrotron-based photoemission with a light spot size of 50μm enabled high-resolution data to be obtained from samples as small as 150×100×20(μm)3. The Fermi surface shape is nearly cylindrical with a rounded hexagonal cross section enclosing a Luttinger volume of 1.00(1) electrons per formula unit.
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Aug 2017
|
|
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
|
|
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.
|
Apr 2018
|
|
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.
|
Jun 2018
|
|
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.
|
Dec 2018
|
|
I05-ARPES
|
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.
|
Dec 2019
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