I09-Surface and Interface Structural Analysis
|
Caio C.
Silva
,
Daniela
Dombrowski
,
Abdus
Samad
,
Jiaqi
Cai
,
Wouter
Jolie
,
Joshua
Hall
,
Paul
Ryan
,
Pardeep K.
Thakur
,
David A.
Duncan
,
Tien-Lin
Lee
,
Udo
Schwingenschlögl
,
Carsten
Busse
Diamond Proposal Number(s):
[14799, 16710, 19801]
Abstract: We determined the structure of epitaxial
2H-TaS2
on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moiré superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the
TaS2
layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.
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Nov 2021
|
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I10-Beamline for Advanced Dichroism
|
Yao
Guang
,
Kejing
Ran
,
Junwei
Zhang
,
Yizhou
Liu
,
Senfu
Zhang
,
Xuepeng
Qiu
,
Yong
Peng
,
Xixiang
Zhang
,
Markus
Weigand
,
Joachim
Gräfe
,
Gisela
Schütz
,
Gerrit
Van Der Laan
,
Thorsten
Hesjedal
,
Shilei
Zhang
,
Guoqiang
Yu
,
Xiufeng
Han
Diamond Proposal Number(s):
[23785]
Abstract: A three-dimensional singular point that consists of two oppositely aligned emergent monopoles is identified in continuous CoTb thin films, as confirmed by complementary techniques of resonant elastic x-ray scattering, Lorentz transmission electron microscopy, and scanning transmission x-ray microscopy. This new type of topological defect can be regarded as a superposition of an emergent magnetic monopole and an antimonopole, around which the source and drain of the magnetic flux overlap in space. We experimentally prove that the observed spin twist seen in Lorentz transmission electron microscopy reveals the cross section of the superimposed three-dimensional structure, providing a straightforward strategy for the observation of magnetic singularities. Such a quasiparticle provides an excellent platform for studying the rich physics of emergent electromagnetism.
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Nov 2021
|
|
|
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D. F.
Liu
,
Q. N.
Xu
,
E. K.
Liu
,
J. L.
Shen
,
C. C.
Le
,
Y. W.
Li
,
D.
Pei
,
A. J.
Liang
,
P.
Dudin
,
T. K.
Kim
,
C.
Cacho
,
Y. F.
Xu
,
Y.
Sun
,
L. X.
Yang
,
Z. K.
Liu
,
C.
Felser
,
S. S. P.
Parkin
,
Y. L.
Chen
Abstract: Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetries (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS
Co
3
Sn
2
S
2
, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structure evolution was clearly observed across the Curie temperature
(
T
C
=
177
K
)
, including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a method to control their exotic physical properties.
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Nov 2021
|
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I13-2-Diamond Manchester Imaging
|
Diamond Proposal Number(s):
[23760]
Abstract: We present a dynamic implementation of the beam-tracking x-ray imaging method providing absorption, phase, and ultrasmall angle scattering signals with microscopic resolution and high frame rate. We demonstrate the method’s ability to capture dynamic processes with 22-ms time resolution by investigating the melting of metals in laser additive manufacturing, which has so far been limited to single-modality synchrotron radiography. The simultaneous availability of three contrast channels enables earlier segmentation of droplets, tracking of powder dynamic, and estimation of unfused powder amounts, demonstrating that the method can provide additional information on melting processes.
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Nov 2021
|
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I09-Surface and Interface Structural Analysis
|
M.
Chrysler
,
J.
Gabel
,
T.-L.
Lee
,
A. N.
Penn
,
B. E.
Matthews
,
D. M.
Kepaptsoglou
,
Q. M.
Ramasse
,
J. R.
Paudel
,
R. K.
Sah
,
J. D.
Grassi
,
Z.
Zhu
,
A. X.
Gray
,
J. M.
Lebeau
,
S. R.
Spurgeon
,
S. A.
Chambers
,
P. V.
Sushko
,
J. H.
Ngai
Diamond Proposal Number(s):
[25582, 26487]
Abstract: We demonstrate that the interfacial dipole associated with bonding across the
SrTi
O
3
/
Si
heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into
SrTi
O
3
. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.
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Oct 2021
|
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I09-Surface and Interface Structural Analysis
|
F.
Offi
,
K.
Yamauchi
,
S.
Picozzi
,
V.
Lollobrigida
,
A.
Verna
,
C.
Schlueter
,
T.-L.
Lee
,
A.
Regoutz
,
D. J.
Payne
,
A.
Petrov
,
G.
Vinai
,
G. M.
Pierantozzi
,
T.
Pincelli
,
G.
Panaccione
,
F.
Borgatti
Diamond Proposal Number(s):
[11322]
Abstract: Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of
La
0.65
Sr
0.35
MnO
3
(LSMO), a landmark system for spintronics applications, a description based solely on Mn
3
d
and O
2
p
electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La
5
p
, Mn
4
s
, and O
2
s
orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.
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Oct 2021
|
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I15-Extreme Conditions
|
Diamond Proposal Number(s):
[11768]
Abstract: Gadolinium has long been believed to undergo a high-pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended
s
-orbital to the compact
f
-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the
f
-electrons [Fabbris et al., Phys. Rev. B 88, 245103 (2013)]. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high-pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using x-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes
h
c
p
→
9
R
→
dhcp
→
fcc
→
d
−
fcc
is reproduced. The so-called “volume-collapsed” high-pressure phase is shown to be an unusual stacking of close-packed planes, with
Fddd
symmetry and a density change of less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the
f
-electrons. The key result is that there is no XRD evidence for volume collapse in gadolinium. The sequence of phase transitions is well described by standard density functional theory. There is no need for special treatment of the
f
-electrons or evidence of
f
-electron bonding. Noting that in previous spectroscopic evidence there is no change in the
f
-electrons we conclude that high-pressure gadolinium has no complicated
f
-electron physics such as Mott-Hubbard, Kondo, or valence transitions.
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Oct 2021
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B18-Core EXAFS
|
Rajesh
Tripathi
,
D. T.
Adroja
,
M. R.
Lees
,
A.
Sundaresan
,
S.
Langridge
,
A.
Bhattacharyya
,
V. K.
Anand
,
D. D.
Khalyavin
,
J.
Sannigrahi
,
G.
Cibin
,
A. D.
Hillier
,
R. I.
Smith
,
H. C.
Walker
,
Y.
Muro
,
T.
Takabatake
Diamond Proposal Number(s):
[17953]
Abstract: We report a systematic study of the
5
d
-electron-doped system
Ce
(
Fe
1
−
x
Ir
x
)
2
Al
10
(
0
≤
x
≤
0.15
)
. With increasing
x
, the orthorhombic
b
axis decreases slightly while accompanying changes in
a
and
c
leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor
CeFe
2
Al
10
, the low-temperature energy gap, which is proposed to be a consequence of strong
c
−
f
hybridization, is suppressed by a small amount of Ir substitution for Fe and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at
x
=
0.04
) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Néel temperature
T
N
of 3.1(2) K for
x
=
0.15
. The ordered moment is estimated to be smaller than 0.07(1)
μ
B
/Ce, although the trivalent state of Ce is confirmed by Ce
L
3
-edge x-ray absorption near edge spectroscopy. It is suggested that the
c
−
f
hybridization gap, which plays an important role in the unusually high ordering temperatures observed in
Ce
T
2
Al
10
(
T
= Ru and Os), may not be necessary for the onset of magnetic order with a low
T
N
seen here in
Ce
(
Fe
1
−
x
Ir
x
)
2
Al
10
.
|
Oct 2021
|
|
I09-Surface and Interface Structural Analysis
|
X. C.
Huang
,
W.-W.
Li
,
S.
Zhang
,
F. E.
Oropeza
,
G.
Gorni
,
V. A.
De La Pena-O'Shea
,
T.-L.
Lee
,
M.
Wu
,
L.-S.
Wang
,
D.-C.
Qi
,
L.
Qiao
,
J.
Cheng
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[24219]
Abstract: In this paper, we report insights into the local atomic and electronic structure of
NiCo
2
O
4
epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined
NiCo
2
O
4
epitaxial thin films with controlled properties on
Mg
Al
2
O
4
(
001
)
substrates using pulsed laser deposition. Films grown at low temperatures (
<
400
∘
C
) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the
Ni
3
+
valence state promoted at low growth temperature introduces delocalized
Ni
3
d
-derived states at the Fermi level (
E
F
), responsible for the metallic state in
NiCo
2
O
4
, while the
Co
3
d
-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and
∼
+
2
. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the
Ni
2
+
valence state in
NiCo
2
O
4
. The
Ni
3
d
-related state becomes localized away from
E
F
, opening a band gap for a semiconducting state. The band gap of the semiconducting
NiCo
2
O
4
is estimated to be
<
0.8
eV
, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is
d
−
d
dipole forbidden, and therefore, the semiconducting
NiCo
2
O
4
still shows reasonable transparency in the infrared-visible light region. The present insights into the role of
Ni
3
+
in determining the electronic structure and defect chemistry of
NiCo
2
O
4
provide important guidance for use of
NiCo
2
O
4
in electrocatalysis and opto-electronics.
|
Sep 2021
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[18787]
Abstract: One of the most important functionalities of the atomically thin insulator hexagonal boron nitride (hBN) is its ability to chemically and electronically decouple functional materials from highly reactive surfaces. It is therefore of utmost importance to uncover its structural properties on surfaces on an atomic and mesoscopic length scale. In this paper, we quantify the relative coverages of structurally different domains of a hBN layer on the Ni(111) surface using low-energy electron microscopy and the normal incidence x-ray standing wave technique. We find that hBN nucleates on defect sites of the Ni(111) surface and predominantly grows in two epitaxial domains that are rotated by
60
∘
with respect to each other. The two domains reveal identical adsorption heights, indicating a similar chemical interaction strength with the Ni(111) surface. The different azimuthal orientations of these domains originate from different adsorption sites of N and B. We demonstrate that the majority (
≈
70
%
) of hBN domains exhibit a
(
N
,
B
)
=
(
top
,
fcc
)
adsorption site configuration while the minority (
≈
30
%
) show a
(
N
,
B
)
=
(
top
,
hcp
)
configuration. Our study hence underlines the crucial role of the atomic adsorption configuration in the mesoscopic domain structures of in situ fabricated two-dimensional materials on highly reactive surfaces.
|
Sep 2021
|
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