I05-ARPES
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Diamond Proposal Number(s):
[15074, 19041]
Open Access
Abstract: A fundamental part of the puzzle of unconventional superconductivity in the Fe-based superconductors is the understanding of the magnetic and nematic instabilities of the parent compounds. The issues of which of these can be considered the leading instability, and whether weak- or strong-coupling approaches are applicable, are both critical and contentious. Here, we revisit the electronic structure of BaFe2As2 using angle-resolved photoemission spectroscopy (ARPES). Our high-resolution measurements of samples “detwinned” by the application of a mechanical strain reveal a highly anisotropic 3D Fermi surface in the low-temperature antiferromagnetic phase. By comparison of the observed dispersions with ab initio calculations, we argue that overall it is magnetism, rather than orbital/nematic ordering, which is the dominant effect, reconstructing the electronic structure across the Fe 3d bandwidth. Finally, using a state-of-the-art nano-ARPES system, we reveal how the observed electronic dispersions vary in real space as the beam spot crosses domain boundaries in an unstrained sample, enabling the measurement of ARPES data from within single antiferromagnetic domains, and showing consistence with the effective mono-domain samples obtained by detwinning.
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Jul 2019
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I05-ARPES
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Philipp
Braeuninger-weimer
,
Oliver
Burton
,
Robert S.
Weatherup
,
Ruizhi
Wang
,
Pavel
Dudin
,
Barry
Brennan
,
Andrew J.
Pollard
,
Bernhard C.
Bayer
,
Vlad P.
Veigang-radulescu
,
Jannik C.
Meyer
,
Billy J.
Murdoch
,
Peter J.
Cumpson
,
Stephan
Hofmann
Diamond Proposal Number(s):
[17381]
Open Access
Abstract: We explore a number of different electrochemical, wet chemical, and gas phase approaches to study intercalation and oxidation at the buried graphene-Ge interface. While the previous literature focused on the passivation of the Ge surface by chemical vapor deposited graphene, we show that particularly via electrochemical intercalation in a 0.25 N solution of anhydrous sodium acetate in glacial acetic acid, this passivation can be overcome to grow GeO2 under graphene. Angle resolved photoemission spectroscopy, Raman spectroscopy, He ion microscopy, and time-of-flight secondary ion mass spectrometry show that the monolayer graphene remains undamaged and its intrinsic strain is released by the interface oxidation. Graphene acts as a protection layer for the as-grown Ge oxide, and we discuss how these insights can be utilized for new processing approaches.
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Jul 2019
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I05-ARPES
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Soeren
Ulstrup
,
Cristina E.
Giusca
,
Jill A.
Miwa
,
Charlotte
Sanders
,
Alex
Browning
,
Pavel
Dudin
,
Cephise
Cacho
,
Olga
Kazakova
,
D. Kurt
Gaskill
,
Rachael L.
Myers-ward
,
Tianyi
Zhang
,
Mauricio
Terrones
,
Philip
Hofmann
Diamond Proposal Number(s):
[19260]
Open Access
Abstract: Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS2). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS2.
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Jul 2019
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I05-ARPES
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Susanne
Schulz
,
Ilya A.
Nechaev
,
Monika
Güttler
,
Georg
Poelchen
,
Alexander
Generalov
,
Steffen
Danzenbächer
,
Alla
Chikina
,
Silvia
Seiro
,
Kristin
Kliemt
,
Alexandra Yu.
Vyazovskaya
,
Timur K.
Kim
,
Pavel
Dudin
,
Evgueni V.
Chulkov
,
Clemens
Laubschat
,
Eugene E.
Krasovskii
,
Christoph
Geibel
,
Cornelius
Krellner
,
Kurt
Kummer
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
Open Access
Abstract: The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr2Si2, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.
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Jun 2019
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I05-ARPES
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Niels B. M.
Schröter
,
Ding
Pei
,
Maia G.
Vergniory
,
Yan
Sun
,
Kaustuv
Manna
,
Fernando
De Juan
,
Jonas A.
Krieger
,
Vicky
Süss
,
Marcus
Schmidt
,
Pavel
Dudin
,
Barry
Bradlyn
,
Timur K.
Kim
,
Thorsten
Schmitt
,
Cephise
Cacho
,
Claudia
Felser
,
Vladimir N.
Strocov
,
Yulin
Chen
Diamond Proposal Number(s):
[19883, 21400]
Abstract: Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge1, long Fermi arc surface states2,3, unusual magnetotransport4 and lattice dynamics5, as well as a quantized response to circularly polarized light6. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.
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May 2019
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I05-ARPES
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Diamond Proposal Number(s):
[21083]
Abstract: Identifying the electron dynamics in CuO chains has been elusive in Y-Ba-Cu-O (YBCO) cuprate systems when using standard angle-resolved photoemission spectroscopy (ARPES); a cleaved sample exhibits areas terminated by both CuO chains or BaO layers, and the size of a typical beam results in ARPES signals that are superposed from both terminations. Here, we employ spatially resolved ARPES with a submicrometric beam (nano-ARPES) to reveal the surface-termination-dependent electronic structures of the double CuO chains in
YBa
2
Cu
4
O
8
. We present an observation of sharp metallic dispersions and Fermi surfaces of the double CuO chains buried underneath the
CuO
2
-plane block on a BaO-terminated surface. While the observed Fermi surfaces of the CuO chains are highly one dimensional, the electrons in the CuO chains do not undergo significant electron correlations and no signature of a Tomonaga-Luttinger liquid or a marginal Fermi liquid is found. Our work represents an important experimental step toward understanding the charge dynamics and provides a starting basis for modeling the high-
T
c
superconductivity in YBCO cuprate systems.
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Apr 2019
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I05-ARPES
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Diamond Proposal Number(s):
[16764, 14488]
Open Access
Abstract: This article reports on the fabrication and testing of dedicated Fresnel zone plates for use at the nano-ARPES branch of the I05-ARPES beamline of Diamond Light Source to perform angle-resolved photoelectron spectroscopy with sub-micrometre resolution in real space. The aim of the design was to provide high photon flux combined with sub-micrometre spot sizes. The focusing lenses were tested with respect to efficiency and spatial resolution in the extreme ultraviolet between 50 eV and 90 eV. The experimentally determined diffraction efficiencies of the zone plates are as high as 8.6% at 80 eV, and a real-space resolution of 0.4 µm was demonstrated. Using the zone-plate-based setup, monolayer flakes of the two-dimensional semiconductor WS2 were investigated. This work demonstrates that the local electronic structure can be obtained from an area of a few micrometres across a two-dimensional heterostructure.
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Mar 2019
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I05-ARPES
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Ryo
Noguchi
,
T.
Takahashi
,
K.
Kuroda
,
M.
Ochi
,
T.
Shirasawa
,
M.
Sakano
,
C.
Bareille
,
M.
Nakayama
,
M. D.
Watson
,
K.
Yaji
,
A.
Harasawa
,
H.
Iwasawa
,
P.
Dudin
,
T. K.
Kim
,
M.
Hoesch
,
V.
Kandyba
,
A.
Giampietri
,
A.
Barinov
,
S.
Shin
,
R.
Arita
,
T.
Sasagawa
,
Takeshi
Kondo
Diamond Proposal Number(s):
[15095, 16161]
Abstract: The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator—a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either ‘strong’ or ‘weak’ and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Notably, the crystal has naturally cleavable top and side planes—stacked via van der Waals forces—which have long been desirable for the experimental realization of the WTI state. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase—viewed as quantum spin Hall insulators stacked three-dimensionally—will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.
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Feb 2019
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I05-ARPES
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Q.
Yao
,
D.
Kaczorowski
,
P.
Swatek
,
D.
Gnida
,
C. H. P.
Wen
,
X. H.
Niu
,
R.
Peng
,
H. C.
Xu
,
P.
Dudin
,
S.
Kirchner
,
Q. Y.
Chen
,
D. W.
Shen
,
D. L.
Feng
Diamond Proposal Number(s):
[16345]
Abstract: The localized-to-itinerant transition of f electrons lies at the heart of heavy-fermion physics, but has only been directly observed in single-layer Ce-based materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce 4f electrons in the heavy-fermion superconductor Ce2PdIn8, a typical n=2 CenMmIn3n+2m compound, using high-resolution and 4d−4f resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between f and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localized-to-itinerant transition is around 120 K, which is much higher than its coherence temperature Tcoh∼30K.
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Feb 2019
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I05-ARPES
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Diamond Proposal Number(s):
[19816]
Open Access
Abstract: The discovery in 2008 of the iron-based superconducting pnictide and
chalcogenide compounds has provided an entirely new family of materials for studying
the crucial interplay between superconductivity and magnetism in unconventional
superconductors. The alkali-metal-intercalated iron selenide (AxFe2-ySe2, A =
alkali metal) superconductors are of particular interest owing to their relatively high
transition temperatures of 30 K and the co-existence of the superconducting state
with antiferromagnetic ordering. Intrinsic phase separation on the mesoscopic scale is
known to occur in "single crystals" of these compounds, adding to the complexity of
interpretation of bulk property measurements. In this study, we investigate the local
electronic structure and chemistry of RbxFe2-ySe2 crystals using scanning microscopy
techniques. Nano-focussed angle-resolved photoemission spectroscopy (NanoARPES)
has enabled the band structure of the minority superconducting phase and the non-
superconducting matrix to be measured independently and linked to the surface
chemistry from the same regions using core-level spectroscopy. Valence band mapping
reveals the characteristic microstructure of these crystals, but does not have sucient
spatial resolution to enable the precise morphology of the superconducting phase
to be elucidated. Cryogenic magnetic force microscopy (MFM) has shown that the
superconducting phase has a fine-scale stripey morphology that was not resolved in
the SPEM experiment. The correlation of these findings with previous microstructural
studies, bulk measurements and first-principles DFT calculations paves the way for
understanding the intriguing electronic and magnetic properties of these compounds.
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Jan 2019
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