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
|
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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
|
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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
|
|
I07-Surface & interface diffraction
|
Diamond Proposal Number(s):
[20426]
Open Access
Abstract: We report on the characterization of the growth of vacuum-deposited zinc phthalocyanine (ZnPc) thin films on glass through a combination of in situ grazing incidence x-ray scattering, x-ray reflectivity, and atomic force microscopy. We found that the growth at room temperature proceeds via the formation of two structurally unique substrate-induced interfacial layers, followed by the growth of the
γ
-ZnPc polymorph thereafter (thickness
≈
1.0
nm). As the growth of the bulk
γ
-ZnPc progresses, a substantial out-of-plane lattice strain (
≈
15
%
relative to
γ
-ZnPc powder) is continually relaxed during the thin film growth. The rate of strain relaxation was slowed after a thickness of
≈
13
nm, corresponding to the transition from layer growth to island growth. The findings reveal the real-time microstructural evolution of ZnPc and highlight the importance of substrate-induced strain on thin film growth.
|
Mar 2022
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[25582]
Abstract: The electronic structures of semiconducting heterojunctions are critically dependent on composition including the presence and concentrations of dopants, both intended and unintended. Dopant profiles in the interfacial region can have major effects on band energies which in turn drive transport properties. Here we use core-level photoelectron line shapes excited with hard x rays to extract information about electric fields resulting from internal charge transfer in epitaxial
La
0.03
Sr
0.97
Zr
x
Ti
1
–
x
O
3
/
Ge
(
001
)
(
0.1
≤
x
≤
0.7
)
heterostructures. Experiments were carried out for heterojunctions involving both
n
- and
p
-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure, and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform
n
- and
p
-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.
|
Jan 2022
|
|
I16-Materials and Magnetism
|
Diamond Proposal Number(s):
[10248]
Open Access
Abstract: Demonstrating both the intrinsic and extrinsic nature of the giant piezoelectric effect (GPE) in complex solid solutions, near the morphotropic phase boundary, has been extremely challenging until now, because such materials exhibit multiple phases on the order of tens of microns across, meaning important information is lost due to averaging when using established high resolution diffraction techniques to extract three dimensional structural information. We have used a different approach proposed by Nisbet et al. [Acta Crystallogr. Sect. A 71, 20 (2015)], which has been adapted to differentiate between spatially adjacent phases and simultaneously track the evolution of those phases in response to electric fields. As a result, we have identified three environment specific GPEs. The first of these is a GPE which is an order of magnitude greater than previously reported for a given change in field. This is observed during a tetragonal-monoclinic transition in a multiphasic environment. A secondary, large GPE is observed in the neighboring, nontransitioning, monoclinic phase due to stress biasing, and a more typical GPE is observed when the system becomes monophasic. Our results demonstrate the simultaneous and complex interplay of intrinsic and extrinsic factors contributing to the GPE which is likely to have implications for device manufacture and miniaturization.
|
Dec 2021
|
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I15-Extreme Conditions
|
Nanci Prado
Sabalisck
,
Gerardo
Gil-De-Cos
,
Cristina
Gonzalez-Silgo
,
Candelaria
Guzman-Afonso
,
Victor
Lavin
,
Juan Javier
Lopez-Solano
,
Isabel Teresa
Martín-Mateos
,
Lourdes
Mestres
,
Andres
Mujica
,
David
Santamaria-Perez
,
Manuel
Eulalio Torres
,
Xavier
Vendrell
Diamond Proposal Number(s):
[8617]
Abstract: X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire α-
R
E
2
(
WO
4
)
3
(
RE
= La-Ho) family with modulated scheelite structure (α phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters
a
,
c
, and β was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter
a
decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the
a
axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the
WO
2
−
4
tetrahedrons. These changes correlate with those in the alternating RE…RE…RE chains and blocks of cationic vacancies arranged along the
a
axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having “hinge frames.”
|
Dec 2021
|
|
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.
|
Oct 2021
|
|
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.
|
Oct 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
|
|