I05-ARPES
|
A. Garrison
Linn
,
Peipei
Hao
,
Kyle N.
Gordon
,
Dushyant
Narayan
,
Bryan S.
Berggren
,
Nathaniel
Speiser
,
Sonka
Reimers
,
Richard P.
Campion
,
Vít
Novák
,
Sarnjeet S.
Dhesi
,
Timur K.
Kim
,
Cephise
Cacho
,
Libor
Šmejkal
,
Tomáš
Jungwirth
,
Jonathan D.
Denlinger
,
Peter
Wadley
,
Daniel S.
Dessau
Diamond Proposal Number(s):
[24224]
Open Access
Abstract: Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of ≈− 390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. In addition, 2×1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.
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May 2023
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I05-ARPES
|
Diamond Proposal Number(s):
[20573, 28919, 32737]
Open Access
Abstract: Diverse emergent correlated electron phenomena have been observed in twisted-graphene layers. Many electronic structure predictions have been reported exploring this new field, but with few momentum-resolved electronic structure measurements to test them. We use angle-resolved photoemission spectroscopy to study the twist-dependent (1° < θ < 8°) band structure of twisted-bilayer, monolayer-on-bilayer, and double-bilayer graphene (tDBG). Direct comparison is made between experiment and theory, using a hybrid k·p model for interlayer coupling. Quantitative agreement is found across twist angles, stacking geometries, and back-gate voltages, validating the models and revealing field-induced gaps in twisted graphenes. However, for tDBG at θ = 1.5 ± 0.2°, close to the magic angle θ = 1.3°, a flat band is found near the Fermi level with measured bandwidth Ew = 31 ± 5 meV. An analysis of the gap between the flat band and the next valence band shows deviations between experiment (Δh = 46 ± 5 meV) and theory (Δh = 5 meV), indicative of lattice relaxation in this regime.
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May 2023
|
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I05-ARPES
|
Abstract: In recent years, chromium sulphur bromide (CrSBr) has emerged as a promising highly- anisotropic semiconducting two-dimensional (2D) magnetic material to explore spintronics and quantum transport due to its strongly correlated quasiparticle interactions [1]. CrSBr is an A-type layered antiferromagnet; in the bulk material, above the Néel temperature (TN = 132K) it transitions to an intermediate ferromagnetic phase before becoming paramagnetic at high temperature. Experimental work on its fascinating optoelectronic properties has been heavily supported by electronic structure calculations using a variety of methods [2,3], but direct band structure measurements to test these predictions are still lacking. Recent angle- resolved photoemission microscopy (ARPES) measurements of bulk CrSBr were unable to measure below TN due to charging effects [4]. Here, we overcome this limitation through exfoliation of CrSBr flakes onto a template-stripped gold surface (Figure 1a) [5]. Using the nanoARPES endstation of the i05 beamline at Diamond Light Source, ARPES was acquired without charging from thin flakes (~10 nm thick) at temperatures down to < 40 K. Photon energy, and polarisation, dependent measurements confirm a strongly 2D dispersion and link the band dispersions to different atomic orbitals. Temperature-dependent measurements highlight electronic structure changes through the magnetic phase transitions, including shifts of the low energy valence bands and band splitting suggestive of spin-ordering (Figure 1b,c). These results also demonstrate a simple approach for the measurement of the low- temperature band structure of insulating layered materials.
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May 2023
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I05-ARPES
|
Q. Q.
Zhang
,
Y.
Shi
,
K. Y.
Zhai
,
W. X.
Zhao
,
X.
Du
,
J. S.
Zhou
,
X.
Gu
,
R. Z.
Xu
,
Y. D.
Li
,
Y. F.
Guo
,
Z. K.
Liu
,
C.
Chen
,
S.-K.
Mo
,
T. K.
Kim
,
C.
Cacho
,
J. W.
Yu
,
W.
Li
,
Y. L.
Chen
,
J.-H.
Chu
,
L. X.
Yang
Diamond Proposal Number(s):
[22375]
Abstract: EuTe
4
is a van der Waals material exhibiting a charge density wave (CDW) with a large thermal hysteresis in the resistivity and CDW gap. In this paper, we systematically study the electronic structure and transport properties of
EuTe
4
using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetoresistance (MR) measurements, and scanning tunneling microscopy (STM). We observe a CDW gap of
∼
200
meV
at low temperatures that persists up to 400 K, suggesting that the CDW transition occurs at a much higher temperature. The ARPES intensity near the Fermi level shows large thermal hysteretic behavior, consistent with the resistivity measurement. The hysteresis in the resistivity measurement does not change under a magnetic field up to 7 T, excluding the thermal magnetic hysteretic effect. Instead, the surface topography measured with STM shows surface domains with different CDW trimerization directions, which may be important for the thermal hysteretic behavior. Interestingly, we reveal a large negative MR at low temperatures that can be associated with the canting of magnetically ordered Eu spins. Our results shed light on the understanding of magnetic, transport, and electronic properties of
EuTe
4
.
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Mar 2023
|
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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.
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Feb 2023
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I05-ARPES
|
Kate
Reidy
,
Paulina Ewa
Majchrzak
,
Benedikt
Haas
,
Joachim Dahl
Thomsen
,
Andrea
Konečná
,
Eugene
Park
,
Julian
Klein
,
Alfred J. H.
Jones
,
Klara
Volckaert
,
Deepnarayan
Biswas
,
Matthew D.
Watson
,
Cephise
Cacho
,
Prineha
Narang
,
Christoph T.
Koch
,
Soeren
Ulstrup
,
Frances M.
Ross
,
Juan Carlos
Idrobo
Diamond Proposal Number(s):
[25368, 29607]
Abstract: The integration of metallic contacts with two-dimensional (2D) semiconductors is routinely required for the fabrication of nanoscale devices. However, nanometer-scale variations in the 2D/metal interface can drastically alter the local optoelectronic properties. Here, we map local excitonic changes of the 2D semiconductor MoS2 in contact with Au. We utilize a suspended and epitaxially grown 2D/metal platform that allows correlated electron energy-loss spectroscopy (EELS) and angle resolved photoelectron spectroscopy (nanoARPES) mapping. Spatial localization of MoS2 excitons uncovers an additional EELS peak related to the MoS2/Au interface. NanoARPES measurements indicate that Au–S hybridization decreases substantially with distance from the 2D/metal interface, suggesting that the observed EELS peak arises due to dielectric screening of the excitonic Coulomb interaction. Our results suggest that increasing the van der Waals distance could optimize excitonic spectra of mixed-dimensional 2D/3D interfaces and highlight opportunities for Coulomb engineering of exciton energies by the local dielectric environment or moiré engineering.
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Jan 2023
|
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Optics
|
Open Access
Abstract: Angle-resolved photoemission spectroscopy (ARPES) is a powerful method for measuring the electronic band structure of solids. Diamond Light Source is planning to build a multibend-achromat (MBA) synchrotron – Diamond-II - which will provide an almost diffraction-limited photon source in the vacuum-ultraviolet photon energy range. The improved emittance and higher coherence of MBA synchrotrons means that samples with features smaller than 1 µm can be readily studied using ARPES, provided the beamline is designed to take full advantage of the new photon source. We have developed an analytical method for optimising the optical design of a future Nano-ARPES beamline for Diamond-II. Our method enables one to explore large regions of parameter space for a beamline design in an unbiased and systematic way, with minimal requirements on computing power. We believe that the analytical method presented here will be a useful tool for synchrotron beamline designers, as it allows many beamline characteristics to be simulated quickly while working within any practical limitations.
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Dec 2022
|
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I05-ARPES
|
Diamond Proposal Number(s):
[15074, 23890]
Open Access
Abstract: The electronic structures of the iron-based superconductors have been intensively studied by using angle-resolved photoemission spectroscopy (ARPES). A considerable amount of research has been focused on the LaFeAsO family, showing the highest transition temperatures, where previous ARPES studies have found much larger Fermi surfaces than bulk theoretical calculations would predict. The discrepancy has been attributed to the presence of termination-dependent surface states. Here, using photoemission spectroscopy with a sub-micron focused beam spot (nano-ARPES) we have successfully measured the electronic structures of both the LaO and FeAs terminations in LaFeAsO. Our data reveal very different band dispersions and core-level spectra for different surface terminations, showing that previous macro-focus ARPES measurements were incomplete. Our results give direct evidence for the surface-driven electronic structure reconstruction in LaFeAsO, including formation of the termination-dependent surface states at the Fermi level. This experimental technique, which we have shown to be very powerful when applied to this prototypical compound, can now be used to study various materials with different surface terminations.
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Oct 2022
|
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I05-ARPES
|
J.
Küspert
,
R.
Cohn Wagner
,
C.
Lin
,
K.
Von Arx
,
Q.
Wang
,
K.
Kramer
,
W. R.
Pudelko
,
N. C.
Plumb
,
C. E.
Matt
,
C. G.
Fatuzzo
,
D.
Sutter
,
Y.
Sassa
,
J.-Q.
Yan
,
J.-S.
Zhou
,
J. B.
Goodenough
,
S.
Pyon
,
T.
Takayama
,
H.
Takagi
,
T.
Kurosawa
,
N.
Momono
,
M.
Oda
,
M.
Hoesch
,
C.
Cacho
,
T. K.
Kim
,
M.
Horio
,
J.
Chang
Diamond Proposal Number(s):
[27768, 10550]
Open Access
Abstract: We carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems
La
2
−
x
Sr
x
CuO
4
,
La
1.6
−
x
Nd
0.4
Sr
x
CuO
4
, and
La
1.8
−
x
Eu
0.2
Sr
x
CuO
4
display slightly different pseudogap critical points in the temperature versus doping phase diagram. We studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for antinodal spectral weight.
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Oct 2022
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I05-ARPES
|
Diamond Proposal Number(s):
[25201]
Abstract: Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moiré structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal
NbSe
2
and square BiSe layers that terminate the surface of the
(
BiSe
)
1
+
δ
NbSe
2
misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal
NbSe
2
layers, leading to quasi-one-dimensional electronic features. The electronic structure of the
NbSe
2
layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of band is mainly due to the atomic and orbital character of the states involved, opening a promising way to design electronic states that exploit the partially incommensurate character of the misfit compounds.
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Sep 2022
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