I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[16141]
Open Access
Abstract: Owing to the unique chemical and electronic properties arising from 3d-electrons, substitution with transition metal ions is one of the key routes for engineering new functionalities into materials. While this approach has been used extensively in complex metal oxide perovskites, metal halide perovskites have largely resisted facile isovalent substitution. In this work, it is demonstrated that the substitution of Co2+ into the lattice of methylammonium lead triiodide imparts magnetic behavior to the material while maintaining photovoltaic performance at low concentrations. In addition to comprehensively characterizing its magnetic properties, the Co2+ ions themselves are utilized as probes to sense the local electronic environment of Pb in the perovskite, thereby revealing the nature of their incorporation into the material. A comprehensive understanding of the effect of transition metal incorporation is provided, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.
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Mar 2023
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I10-Beamline for Advanced Dichroism
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N.-J.
Steinke
,
S. L.
Zhang
,
P. J.
Baker
,
L. B.
Duffy
,
F.
Kronast
,
J.
Krieger
,
Z.
Salman
,
T.
Prokscha
,
A.
Suter
,
S.
Langridge
,
Gerrit
Van Der Laan
,
T.
Hesjedal
Diamond Proposal Number(s):
[11503]
Abstract: Chromium-doped
Sb
2
Te
3
is a magnetic topological insulator (MTI), which belongs to the
(
Sb
,
Bi
)
2
(
Se
,
Te
)
3
family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below
≈
2
K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation
(
μ
SR
)
, and magnetic soft x-ray spectroscopy and imaging.
μ
SR
provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static
(
≈
μ
s
)
and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.
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Dec 2022
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I05-ARPES
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V. A.
Rogalev
,
F.
Reis
,
F.
Adler
,
M.
Bauernfeind
,
J.
Erhardt
,
A.
Kowalewski
,
M. R.
Scholz
,
L.
Dudy
,
L. B.
Duffy
,
T.
Hesjedal
,
M.
Hoesch
,
G.
Bihlmayer
,
J.
Schäfer
,
R.
Claessen
Diamond Proposal Number(s):
[10244, 10289, 12892, 15285]
Abstract: We report on the electronic structure of
α
-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of
α
-Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.
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Dec 2019
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Abstract: Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena
building on time-reversal symmetry-broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment Dy:Bi2Te3 with high transition temperature Cr:Sb2Te3 in a superlattice, and we found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the Dy:Bi2Te3 layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin-film stack or is surface-sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry, which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.
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Aug 2019
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Open Access
Abstract: Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic
topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure.
Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular
magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and
soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films.
X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the
local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.
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Jul 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[16162]
Abstract: The recently confirmed monolayer ferromagnet CrI3 is a frisky example of a two-dimensional ferromagnetic material with great application potential in van der Waals heterostructures. Here we present a soft x-ray absorption spectroscopy study of the magnetic bulk properties of CrI3, giving insight into the magnetic coupling scenario which is relevant for understanding its thickness-dependent magnetic properties. The experimental Cr
L2,3
x-ray magnetic circular dichroism spectra show a good agreement with calculated spectra for a hybridized ground state. In this high-spin Cr ground state the Cr–I bonds show a strongly covalent character. This is responsible for the strong superexchange interaction and increased spin-orbit coupling, resulting in the large magnetic anisotropy of the two-dimensionally layered CrI3 crystal.
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Aug 2018
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I05-ARPES
I10-Beamline for Advanced Dichroism
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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
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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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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[17480]
Abstract: The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped three-dimensional topological insulator (TI). The pioneering transition-metal doped (Sb,Bi)2(Se,Te)3 thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth-doped (Sb,Bi)2(Se,Te)3 thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allowfor the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:Bi2Te3/Cr:Sb2Te3 thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:Sb2Te3 can introduce long-range magnetic order in high-moment Dy:Bi2Te3—upto a temperature of 17 K—in excellent agreement with first-principles calculations,which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.
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May 2018
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L. B.
Duffy
,
N.-J.
Steinke
,
J. A.
Krieger
,
A. I.
Figueroa
,
K.
Kummer
,
T.
Lancaster
,
S. R.
Giblin
,
F. L.
Pratt
,
S. J.
Blundell
,
T.
Prokscha
,
A.
Suter
,
Sean
Langridge
,
V. N.
Strocov
,
Z.
Salman
,
G.
Van Der Laan
,
T.
Hesjedal
Abstract: Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI’s exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped Bi2Te3 remained elusive.Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.
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May 2018
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