I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15856]
Abstract: Depositing disordered Al on top of
Sr
Ti
O
3
is a cheap and easy way to create a two-dimensional electron system in the
Sr
Ti
O
3
surface layers. To facilitate future device applications, we passivate the heterostructure by a disordered
La
Al
O
3
capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth.
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Jun 2021
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I11-High Resolution Powder Diffraction
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Abstract: Electronic phase separation into charge-ordered (CO) and charge-averaged (CA) phases in
Ca
Fe
3
−
x
M
x
O
5
samples for dopants
M
=
Mn
and Co has been investigated using powder neutron and x-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. Electronic phase separation is observed in lightly doped
Ca
Fe
3
O
5
samples, where
4
–
10
%
Ca is replaced by Mn, Fe, or Co, and the CA ground state is stabilized at higher doping levels. The CO and CA phases are found to emerge below a common magnetic ordering temperature at
300
–
320
K
providing strong evidence for a lattice strain-driven mechanism that couples their magnetic transitions.
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Feb 2021
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NONE-No attached Diamond beamline
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Kenji
Nawa
,
Demie
Kepaptsoglou
,
Arsham
Ghasemi
,
Philip
Hasnip
,
Guillermo
Bárcena-González
,
Giuseppe
Nicotra
,
Pedro L.
Galindo
,
Quentin M.
Ramasse
,
Kohji
Nakamura
,
Susannah C.
Speller
,
Balati
Kuerbanjiang
,
Thorsten
Hesjedal
,
Vlado K.
Lazarov
Abstract: We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator
Bi
2
Te
3
on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the
Bi
2
Te
3
quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.
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Feb 2021
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I15-Extreme Conditions
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Monika
Gamza
,
Paolo
Abrami
,
Lawrence V. D.
Gammond
,
Jake
Ayres
,
Israel
Osmond
,
Takaki
Muramatsu
,
Robert
Armstrong
,
Hugh
Perryman
,
Dominik
Daisenberger
,
Sitikantha
Das
,
Sven
Friedemann
Diamond Proposal Number(s):
[19319]
Abstract: Cadmium arsenide
(
Cd
3
As
2
)
hosts massless Dirac electrons in its ambient-condition tetragonal phase. We report x-ray diffraction and electrical resistivity measurements of
Cd
3
As
2
upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that, at room temperature, the transition between the low- and high-pressure phases results in large microstrain and reduced crystallite size, both on rising and falling pressure. This leads to nonreversible electronic properties, including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. This paper indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements, we demonstrate that annealing at
∼
200
∘
C
greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with
a
HP
≈
8.68
Å
,
b
HP
≈
17.15
Å
,
and
c
HP
≈
18.58
Å
. The diffraction study indicates that, during the structural transformation, a new phase with another primitive orthorhombic structure may also be stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in
Cd
3
As
2
.
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Feb 2021
|
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I05-ARPES
|
L.
Kang
,
L.
Shen
,
Y. J.
Chen
,
S. C.
Sun
,
X.
Gu
,
H. J.
Zheng
,
Z. K.
Liu
,
J. X.
Wu
,
H. L.
Peng
,
F. W.
Zheng
,
P.
Zhang
,
L. X.
Yang
,
Y. L.
Chen
Diamond Proposal Number(s):
[23648, 24827]
Abstract: Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of β-InSe, a van der Waals semiconductor with a direct band gap. Our measurements show a good agreement with ab initio calculations, which helps reveal the important impact of spin-orbit coupling on the electronic structure of β-InSe. Using surface potassium doping, we tune the chemical potential of the system and observe the unoccupied conduction band. The direct band gap is determined to be about 1.3 eV. Interestingly, we observe a global band shift when the sample is illuminated by a continuous-wave laser at 632.8 nm, which can be understood by an efficient surface photovoltaic effect. The surface photovoltaic can be tuned by in situ surface potassium doping. Our results not only provide important insights into the semiconducting properties of InSe, but also suggest a feasible method to study and engineer the surface photovoltaic effect in InSe-based devices.
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Dec 2020
|
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I06-Nanoscience
|
Federico
Motti
,
G.
Vinai
,
Valentina
Bonanni
,
Vincent
Polewczyk
,
Paola
Mantegazza
,
Thomas
Forrest
,
Francesco
Maccherozzi
,
Stefania
Benedetti
,
Christian
Rinaldi
,
Matteo
Cantoni
,
Damiano
Cassese
,
Stefano
Prato
,
Sarnjeet S.
Dhesi
,
Giorgio
Rossi
,
Giancarlo
Panaccione
,
Piero
Torelli
Diamond Proposal Number(s):
[18810]
Abstract: A ferromagnetic (FM) thin film deposited on a substrate of
Pb
(
Mg
1
/
3
Nb
2
/
3
)
O
3
−
PbTiO
3
(PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used “strain-mediated” description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.
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Nov 2020
|
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I13-1-Coherence
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Jiecheng
Diao
,
Xiaowen
Shi
,
Tadesse A.
Assefa
,
Longlong
Wu
,
Ana F.
Suzana
,
Daniel S.
Nunes
,
Darren
Batey
,
Silvia
Cipiccia
,
Christoph
Rau
,
Ross J.
Harder
,
Wonsuk
Cha
,
Ian K.
Robinson
Abstract: In this work, ferroelastic domain walls inside
BaTiO
3
(BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent x-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature.
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Oct 2020
|
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I06-Nanoscience
|
T.
Janda
,
J.
Godinho
,
T.
Ostatnicky
,
E.
Pfitzner
,
G.
Ulrich
,
A.
Hoehl
,
S.
Reimers
,
Z.
Šobáň
,
T.
Metzger
,
H.
Reichlová
,
V.
Novák
,
R. P.
Campion
,
J.
Heberle
,
P.
Wadley
,
K. W.
Edmonds
,
O. J.
Amin
,
J. S.
Chauhan
,
S. S.
Dhesi
,
F.
Maccherozzi
,
R. M.
Otxoa
,
P. E.
Roy
,
K.
Olejník
,
P.
Němec
,
T.
Jungwirth
,
B.
Kaestner
,
J.
Wunderlich
Diamond Proposal Number(s):
[22437, 16376, 20793]
Abstract: Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible
180
∘
switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent
90
∘
switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.
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Sep 2020
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I09-Surface and Interface Structural Analysis
|
Leanne A. H.
Jones
,
Wojciech M.
Linhart
,
Nicole
Fleck
,
Jack E. N.
Swallow
,
Philip A. E.
Murgatroyd
,
Huw
Shiel
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Laurence J.
Hardwick
,
Jonathan
Alaria
,
Frank
Jaeckel
,
Robert
Kudrawiec
,
Lee A.
Burton
,
Aron
Walsh
,
Jonathan M.
Skelton
,
Tim D.
Veal
,
Vin R.
Dhanak
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The effects of Sn
5
s
lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn
5
s
orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn
5
s
state contribution at the top of the valence band in the different Sn-sulphides, SnS,
Sn
2
S
3
, and
SnS
2
, to be clearly identified. SnS and
Sn
2
S
3
contain Sn(II) cations and the corresponding Sn
5
s
lone pairs are at the valence band maximum (VBM), leading to
∼
1.0
–1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast,
SnS
2
only contains Sn(IV) cations, no filled lone pairs, and therefore has a
∼
2.3
eV room-temperature band gap and much lower VBM compared with SnS and
Sn
2
S
3
. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS,
Sn
2
S
3
, and
SnS
2
, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn
5
s
lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with
n
s
2
configuration.
|
Jul 2020
|
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[22648]
Open Access
Abstract: We report on the electronic structure of doped LaFeO3 at the crossover from an insulating-to-metallic phase Transition. Comprehensive x-ray spectroscopic methodologies are used to understand core and valence electronic structure as well as crystal structure distortions associated with the electronic transition. Despite the antiferromagnetic (AFM) ordering at room temperature, we show direct evidence of itinerant carriers at the Fermi level revealed by resonant photoemission spectroscopy (RPES) at the Mo L3 edge. RPES data taken at the Fe L3 edge show spectral weight near the valence band edge and significant hybridization with O 2p states required for AFM ordering. Resonant inelastic x-ray scattering spectra taken across Fe L2, 3 edges show electron correlation effects (U) driven by Coulomb interactions of d electrons as well as broad charge-transfer excitations for x > 0.2 where the compound crosses over to a metallic state. Site substitution of Fe by Mo ions in the Fe-O6 octahedra enhances the separation of the two Fe-O bonds and Fe-O-Fe bonding angles relative to the orthorhombic
LaFeO3, but no considerable Distortions are present to the overall structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. This insulator-to-metal phase transition with AFM stability is primarily understood through intermediate interaction strengths between correlation (U) and bandwidth (W) at the Fe site, where an estimation of this ratio is given. These results highlight the important role of extrinsic carriers in stabilizing a unique phase transition that can guide future efforts in antiferromagnetic-metal spintronics.
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Mar 2020
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