I12-JEEP: Joint Engineering, Environmental and Processing
|
Diamond Proposal Number(s):
[28087]
Abstract: The structural properties of two Ge-As-Se glass compositions (
Ge
10
As
10
Se
80
and
Ge
21
As
21
Se
58
) are investigated from a combination of density-functional-based molecular dynamics simulations and neutron/x-ray scattering experiments. We first focus on structural characteristics, including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions, and compare our results with the experimental data. Results leave anticipated coordinations from the octet rule (
Se
II
,
As
III
, and
Ge
IV
) unchanged, and these are contrasted with respect to glasses having the same average coordination number
¯
r
such as binary
As
30
Se
70
and
Ge
33
Se
67
. The increase of (As,Ge) content induces a growth of ring structures that are dominated by edge-sharing motifs (four-membered rings) having mostly heteropolar bonds, while As-As and As-Ge homopolar bonds are clearly more favored than Ge-Ge. These features signal that both topological (rings) and chemical (bonds) features are different with respect to related binaries. The validity of the so-called vibrational isocoordination rule stating that properties of multicomponent chalcogenides depend solely on
¯
r
is checked, and results from a vibrational analysis indicates that this rule is merely satisfied for the Se-rich composition. An inspection of correlations via the Bhatia-Thornton formalism shows that topological ordering is not only different between
Ge
10
As
10
Se
80
and
Ge
21
As
21
Se
58
but also radically contrasts with respect to the isocoordinated binary glasses and displays an obvious reduced directional bonding.
|
Jul 2022
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
M.
Hepting
,
M.
Bejas
,
A.
Nag
,
H.
Yamase
,
N.
Coppola
,
D.
Betto
,
C.
Falter
,
Mirian
Garcia-Fernandez
,
S.
Agrestini
,
K.-J.
Zhou
,
M.
Minola
,
C.
Sacco
,
L.
Maritato
,
P.
Orgiani
,
H. I.
Wei
,
K. M.
Shen
,
D. G.
Schlom
,
A.
Galdi
,
A.
Greco
,
B.
Keimer
Diamond Proposal Number(s):
[23933]
Open Access
Abstract: We use resonant inelastic x-ray scattering to probe the propagation of plasmons in the electron-doped cuprate superconductor
Sr
0.9
La
0.1
CuO
2
. We detect a plasmon gap of
∼
120
meV
at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in
Sr
0.9
La
0.1
CuO
2
are not strictly acoustic. The plasmon dispersion, including the gap, is accurately captured by layered
t
−
J
−
V
model calculations. A similar analysis performed on recent resonant inelastic x-ray scattering data from other cuprates suggests that the plasmon gap is generic and its size is related to the magnitude of the interlayer hopping
t
z
. Our work signifies the three dimensionality of the charge dynamics in layered cuprates and provides a new method to determine
t
z
.
|
Jul 2022
|
|
Theoretical Physics
|
Abstract: Circular dichroism in resonant elastic x-ray scattering (CD-REXS) has recently been observed in chiral
structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CD-REXS for one-dimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CD-REXS for a proper-screw-shaped Bloch-type helix and cycloid-shaped Néel-type helix, we found that CD-REXS for both magnetic and polar helices can discriminate between both types of helices.We also found that the x-ray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CD-REXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch- and Néel-type helices correspond to truly chiral and achiral structures, respectively, and why CD-REXS is able to distinguish between these two types of helices.
|
Jul 2022
|
|
I09-Surface and Interface Structural Analysis
|
You-Ron
Lin
,
Markus
Franke
,
Shayan
Parhizkar
,
Miriam
Raths
,
Victor
Wen-Zhe Yu
,
Tien-Lin
Lee
,
Serguei
Soubatch
,
Volker
Blum
,
F. Stefan
Tautz
,
Christian
Kumpf
,
Francois C.
Bocquet
Diamond Proposal Number(s):
[17737]
Abstract: In the field of van der Waals heterostructures, the twist angle between stacked two-dimensional layers has been identified to be of utmost importance for the properties of the heterostructures. In this context, we previously reported the growth of a single layer of unconventionally oriented epitaxial graphene that forms in a surfactant atmosphere [F. C. Bocquet et al., Phys. Rev. Lett. 125, 106102 (2020)]. The resulting G-
R
0
∘
layer is aligned with the SiC lattice, and hence represents an important milestone towards high-quality twisted bilayer graphene, a frequently investigated model system in this field. Here, we focus on the surface structures obtained in the same surfactant atmosphere, but at lower preparation temperatures at which a boron nitride template layer forms on SiC(0001). In a comprehensive study based on complementary experimental and theoretical techniques, we find—in contrast to the literature—that this template layer is a hexagonal
B
x
N
y
layer, but not high-quality hBN. It is aligned with the SiC lattice and gradually replaced by low-quality graphene in the
0
∘
orientation of the
B
x
N
y
template layer upon annealing.
|
Jun 2022
|
|
I10-Beamline for Advanced Dichroism
|
Diamond Proposal Number(s):
[19173]
Abstract: We present an investigation on the structural and magnetic properties of the interfaces of
Fe
3
O
4
/
MgO
(
001
)
and
Fe
3
O
4
/
NiO
/
MgO
(
001
)
by extracting cation-selective magneto-optical depth profiles by means of x-ray resonant magnetic reflectivity in combination with charge-transfer multiplet simulations of x-ray magnetic circular dichroism data. For
Fe
3
O
4
/
MgO
(
001
)
, the magneto-optical depth profiles at the
Fe
2
+
oct
and the
Fe
3
+
oct
resonant energies follow exactly the structural profile, while the magneto-optical depth profile at the
Fe
3
+
tet
resonance is offset by
3.2
±
1.3
Å from the interface, consistent with a
B
-site interface termination of
Fe
3
O
4
with fully intact magnetic order. In contrast, for
Fe
3
O
4
/
NiO
(
001
)
, the magneto-optical depth profiles at the
Fe
2
+
oct
and the
Ni
2
+
resonances agree with the structural profile, but the interface positions of the magneto-optical depth profiles at the
Fe
3
+
oct
and the
Fe
3
+
tet
resonances are spatially shifted by
3.3
±
1.4
and
2.7
±
0.9
Å, respectively, not consistent with a magnetically ordered stoichiometric interface. This may be related to an intermixed
(
Ni
,
Fe
)
O
layer at the interface. The dichroic depth profile at the Ni
L
3
edge might hint at uncompensated magnetic moments throughout the NiO film.
|
Jun 2022
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Nimrod
Bachar
,
Kacper
Koteras
,
Jakub
Gawraczynski
,
Waldemar
Trzciński
,
Józef
Paszula
,
Riccardo
Piombo
,
Paolo
Barone
,
Zoran
Mazej
,
Giacomo
Ghiringhelli
,
Abhishek
Nag
,
Ke-Jin
Zhou
,
José
Lorenzana
,
Dirk
Van Der Marel
,
Wojciech
Grochala
Diamond Proposal Number(s):
[24869]
Open Access
Abstract: Charge-transfer insulators are the parent phase of a large group of today's unconventional high-temperature superconductors. Here we study experimentally and theoretically the interband excitations of the charge-transfer insulator silver fluoride
AgF
2
, which has been proposed as an excellent analog of oxocuprates. Optical conductivity and resonant inelastic x-ray scattering on
AgF
2
polycrystalline sample show a close similarity with that measured on undoped
La
2
CuO
4
. While the former shows a charge-transfer gap
∼
3.4
eV, larger than in the cuprate,
d
d
excitations are nearly at the same energy in the two materials. Density functional theory and exact diagonalization cluster computations of the multiplet spectra show that
AgF
2
is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that
AgF
2
is at the verge of a charge-transfer instability. The overall resemblance of our data on
AgF
2
to those published previously on
La
2
CuO
4
suggests that the underlying charge-transfer insulator physics is the same, while
AgF
2
could also benefit from a proximity to a charge density wave phase as in
BaBiO
3
. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.
|
May 2022
|
|
I06-Nanoscience
|
G.
Awana
,
R.
Fujita
,
A.
Frisk
,
P.
Chen
,
Q.
Yao
,
A. J.
Caruana
,
C. J.
Kinane
,
N.-J.
Steinke
,
S.
Langridge
,
P.
Olalde-Velasco
,
S. S.
Dhesi
,
G.
Van Der Laan
,
X. F.
Kou
,
S. L.
Zhang
,
T.
Hesjedal
,
D.
Backes
Diamond Proposal Number(s):
[23748]
Open Access
Abstract: An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in
MnTe
/
Bi
2
Te
3
heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the
Bi
2
Te
3
layer. Furthermore, the interface between MnTe and
Bi
2
Te
3
is not abrupt, but extending over
∼
2.2
nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.
|
May 2022
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[20076]
Abstract: X-ray standing-wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for ½ monolayer (ML) Pt grown on a
SrTiO
3
(001) surface. The XSW induced modulations in the core level (CL), and VB photoemission from the surface and substrate atoms were monitored for three
h
k
l
substrate Bragg reflections. The XSW CL analysis shows the Pt to have a face-centered-cubic-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Overall, this Letter represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.
|
May 2022
|
|
I10-Beamline for Advanced Dichroism
|
Sheng
Jiang
,
Zhaocong
Huang
,
Qi
An
,
Wen
Zhang
,
Yuli
Yin
,
Dong
Zhang
,
Jun
Du
,
Biao
You
,
Jian-Guo
Zheng
,
Wenqing
Liu
,
Ya
Zhai
Abstract: The significance of spin transport over an interface in energy-efficient spintronic devices has stimulated interest in the spintronic society during the last few decades. Here, interfaces of
permalloy
/
Cu
1
−
x
Tb
x
(Py/Cu-Tb) were investigated in depth. As the Cu-Tb thickness increases, we found that the saturation magnetization of the bilayers falls and then plateaus. Element-specific x-ray magnetic circular dichroism studies suggest that the Tb moment aligns opposite to the Fe and Ni moments, forming a self-assembled antiferromagnetic interface. As a result, the Cu-Tb adjacent layer to Py and the interface have a significant impact on spin transport. Relevant parameters, such as spin mixing conductance, spin diffusion length, and damping, can be tuned by inserting a thin Cu layer between Py and Tb or varying the compositions of Cu-Tb alloys. Using rare-earth Tb, we provide an effective method for controlling the spin transport and magnetism of ferromagnet/normal-metal interfaces. This approach is expected to have a great deal of potential in spintronic applications.
|
May 2022
|
|
I05-ARPES
|
M.
Berben
,
S.
Smit
,
C.
Duffy
,
Y.-T.
Hsu
,
L.
Bawden
,
F.
Heringa
,
F.
Gerritsen
,
S.
Cassanelli
,
X.
Feng
,
S.
Bron
,
E.
Van Heumen
,
Y.
Huang
,
F.
Bertran
,
T. K.
Kim
,
C.
Cacho
,
A.
Carrington
,
M. S.
Golden
,
N. E.
Hussey
Abstract: Once doped away from their parent Mott insulating state, the hole-doped cuprates enter into many varied and exotic phases. The onset temperature of each phase is then plotted versus
p
—the number of doped holes per copper atom—to form a representative phase diagram. Apart from differences in the absolute temperature scales among the various families, the resultant phase diagrams are strikingly similar. In particular, the
p
values corresponding to optimal doping (
p
opt
∼
0.16
) and to the end of the pseudogap phase
(
p
∗
∼
0.19
–
0.20
)
are essentially the same for all cuprate families bar one: the single-layer Bi-based cuprate
Bi
2
+
z
−
y
Pb
y
Sr
2
−
x
−
z
La
x
CuO
6
+
δ
(Bi2201). This anomaly arises partly due to the complex stoichiometry of this material and also to the different
p
values inferred from disparate (e.g., bulk or surface) measurements performed on samples with comparable superconducting transition temperatures
T
c
. Here, by combining measurements of the in-plane resistivity in zero and high magnetic fields with angle-resolved photoemission spectroscopy studies in the superconducting and normal state, we argue that the phase diagram of Bi2201 may in fact be similar to that realized in other families. This study therefore brings Bi2201 into the fold and supports the notion of universality of
p
opt
and
p
∗
in all hole-doped cuprates.
|
Apr 2022
|
|
