I05-ARPES
|
Edgar
Abarca Morales
,
Gesa-R.
Siemann
,
Andela
Zivanovic
,
Philip A. E.
Murgatroyd
,
Igor
Markovic
,
Brendan
Edwards
,
Chris A.
Hooley
,
Dmitry A.
Sokolov
,
Naoki
Kikugawa
,
Cephise
Cacho
,
Matthew D.
Watson
,
Timur K.
Kim
,
Clifford W.
Hicks
,
Andrew P.
Mackenzie
,
Phil D. C.
King
Diamond Proposal Number(s):
[27471, 28412]
Abstract: We report the evolution of the electronic structure at the surface of the layered perovskite
Sr
2
RuO
4
under large in-plane uniaxial compression, leading to anisotropic
B
1
g
strains of
ϵ
x
x
−
ϵ
y
y
=
−
0.9
±
0.1
%
. From angle-resolved photoemission, we show how this drives a sequence of Lifshitz transitions, reshaping the low-energy electronic structure and the rich spectrum of van Hove singularities that the surface layer of
Sr
2
RuO
4
hosts. From comparison to tight-binding modeling, we find that the strain is accommodated predominantly by bond-length changes rather than modifications of octahedral tilt and rotation angles. Our study sheds new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of state singularities to the Fermi level, in turn paving the way to the possible realization of rich collective states at the
Sr
2
RuO
4
surface.
|
Feb 2023
|
|
I05-ARPES
|
F.
Mazzola
,
C.-M.
Yim
,
V.
Sunko
,
S.
Khim
,
P.
Kushwaha
,
O. J.
Clark
,
L.
Bawden
,
I.
Markovic
,
D.
Chakraborti
,
T. K.
Kim
,
M.
Hoesch
,
A. P.
Mackenzie
,
P.
Wahl
,
P. D. C.
King
Diamond Proposal Number(s):
[12469, 14927, 1626]
Open Access
Abstract: Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong electron–magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling is enhanced sevenfold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds light on electron–magnon interactions in solid-state materials, and the ways in which these can be controlled.
|
Feb 2022
|
|
I05-ARPES
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[19479, 17699]
Abstract: An international group of researchers wanted to study the electronic properties of an unusual coupled system. It has one layer that supports very mobile carriers, and one that supports Mott insulating and magnetically-ordered states. They performed initial studies using angle-resolved photoemission spectroscopy (ARPES) on beamline I05 at Diamond Light Source, observing some unusual spectral signatures. Theoretical calculations suggested that correlations and interlayer interactions had an unusual role in their generation. To understand the origin of these unusual spectral signatures, the team used Diamond’s Surface and Interface Structural Analysis beamline (I09) to perform ARPES measurements in the soft X-ray regime. The combined nature of the study, with closely integrated work on I09 and I05, applying photoemission measurements in different regimes, was crucial to the success of this work. By combining theory with the results from I09 and I05, the team were able to show that an intertwining of the spin and charge response in the system dominates the photoemission response.
ARPES is usually a non-magnetic probe, but this work demonstrated that in certain circumstances it becomes sensitive to spin-correlations in solids. Scientists could use the same approach to study materials that are incompatible with other, more conventional probes of spin- spin correlation functions. This potentially opens up new systems to experimental study, and an improved understanding of their magnetic properties would be important for real-world applications such as 2D spintronics.
|
Jul 2021
|
|
|
|
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.
|
Jul 2021
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[18705]
Open Access
Abstract: We report a combined experimental and theoretical study of the surface and bulk electronic structure of aluminium diboride, a nonsuperconducting sister compound of the superconductor
MgB
2
. We perform angle-resolved photoemission measurements with variable photon energy, and compare them to density functional theory calculations to disentangle the surface and bulk contributions to the measured spectra. Aluminium diboride is known to be aluminium deficient,
Al
1
−
δ
B
2
, which would be expected to lead to a hole doping as compared to the nominally stoichimoetric compound. Nonetheless, we find that the bulk
σ
states, which mediate superconductivity in
MgB
2
, remain more than
600
meV
below the Fermi level. However, we also observe
σ
states originating from the boron terminated surface, with an order of magnitude smaller binding energy of
70
meV
, and demonstrate how surface hole-doping can bring these across the Fermi level.
|
Jul 2020
|
|
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.
|
Jun 2020
|
|
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.
|
May 2020
|
|
I09-Surface and Interface Structural Analysis
|
Chiara
Bigi
,
Zhenkun
Tang
,
Gian Marco
Pierantozzi
,
Pasquale
Orgiani
,
Pranab Kumar
Das
,
Jun
Fujii
,
Ivana
Vobornik
,
Tommaso
Pincelli
,
Alessandro
Troglia
,
Tien-Lin
Lee
,
Regina
Ciancio
,
Goran
Drazic
,
Alberto
Verdini
,
Anna
Regoutz
,
Phil D. C.
King
,
Deepnarayan
Biswas
,
Giorgio
Rossi
,
Giancarlo
Panaccione
,
Annabella
Selloni
Diamond Proposal Number(s):
[16041]
Abstract: Two-dimensional (2D) metallic states induced by oxygen vacancies (
V
O
s
) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of
V
O
-induced states at the (001) surface of anatase
TiO
2
, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular
O
2
: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface
V
O
s
and remain localized at the surface, where they can promptly react with
O
2
. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with
O
2
is kinetically hindered on anatase
TiO
2
(001), thus making them much less sensitive to oxygen dosing.
|
Feb 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]
Open Access
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.
|
Feb 2020
|
|
I09-Surface and Interface Structural Analysis
|
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.
|
Jan 2020
|
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