I09-Surface and Interface Structural Analysis
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David A.
Duncan
,
Nicolae
Atodiresei
,
Simone
Lisi
,
Phil J.
Blowey
,
Vasile
Caciuc
,
James
Lawrence
,
Tien-lin
Lee
,
Maria Grazia
Betti
,
Pardeep Kumar
Thakur
,
Ada
Della Pia
,
Stefan
Blügel
,
Giovanni
Costantini
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[13625]
Open Access
Abstract: Theoretical formulations capable of modeling chemical interactions over 3–4 orders of magnitude of bond strength, from covalent to van der Waals (vdW) forces, are one of the primary goals in materials physics, and chemistry. Development of vdW corrections for density-functional theory has thus been a major research field for two decades. While many of these corrections are semiempirical, more theoretically rigorous ab initio functionals have been developed. The ab initio functional vdW-DF2, when coupled with the reoptimized B86 exchange function (vdW-DF2-rB86), has typically performed as well, if not better than most semiempirical formulations. Here we present a system, Co intercalation of graphene on Ir(111), for which a semiempirical correction predicts local corrugation maxima in locations at which the vdW-DF2-rB86 functional predicts global minima. Sub-angstrom precision quantitative structural measurements show better agreement with the semiempirical correction. We posit that it is balancing the weak vdW interaction with the stronger, even covalent, interactions that proves a challenge for the vdW-DF2-rB86 functional.
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Dec 2019
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[12975]
Abstract: A combination of scanning tunneling microscopy, low-energy electron diffraction,and low-energy electron microscopy (LEEM) has been used to identify the structural phases formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(111). These comprise a two-dimensional gas phase, a low-density commensurate (LDC) phase, and a higher-density incommensurate (HDI) phase. LEEM also shows the presence of an additional “precursor-HDI” phase with a surface unit mesh area only ≈3% less than the HDI phase. Normal incidence x-ray standing-wave measurements of the HDI phase yield almost identical structural parameters to the LDC phase for which a full structure determination has been previously reported. The results show TCNQ does not adopt the inverted bowl distortion favored in earlier density functional theory calculations of TCNQ on coinage metal surfaces, but the N atoms are twisted out of the molecular plane, an effect found for the LDC phase to be due to incorporation of Ag adatoms. The possible role of Ag adatoms in the HDI phase, and in the transition from the precursor-HDI phase, is discussed.
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Nov 2019
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I09-Surface and Interface Structural Analysis
|
Abstract: The electronic properties of
CaCuO
2
/
La
0.7
Sr
0.3
MnO
3
(LSMO) superlattices are determined by the electronic structure of the structural units and in particular their interfaces. The electronic structure of LSMO is governed by a metal-insulator transition, which is controlled by the thickness of the units and the sample temperature, resulting in a systematic downward band shift for metallic samples (i.e., thick LSMO units, low temperature). We present a systematic study of the changes in the valence-band structure and screening features in Mn
2
p
and Cu
2
p
core-level spectra. The results show that hybridization of Cu
3
d
orbitals with out-of-plane O
2
p
orbitals can be systematically tuned by controlling the band alignment at the interface via the metal-to-insulator transition of the LSMO units. This opens a new route to rational design of functional interfaces and control of orbital reconstructions.
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Sep 2019
|
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I09-Surface and Interface Structural Analysis
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Matthew J.
Wahila
,
Galo
Paez
,
Christopher N.
Singh
,
Anna
Regoutz
,
Shawn
Sallis
,
Mateusz J.
Zuba
,
Jatinkumar
Rana
,
M. Brooks
Tellekamp
,
Jos E.
Boschker
,
Toni
Markurt
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Tim D.
Veal
,
Wanli
Yang
,
Tien-lin
Lee
,
Fanny
Rodolakis
,
Jerzy T.
Sadowski
,
David
Prendergast
,
Wei-cheng
Lee
,
W. Alan
Doolittle
,
Louis F. J.
Piper
Diamond Proposal Number(s):
[20647, 21430]
Abstract: The metal-insulator transition of
NbO
2
is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed
NbO
x
that may be inherently different to crystalline
NbO
2
. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial
NbO
2
thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at
∼
810
∘
C
. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline
NbO
2
as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.
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Jul 2019
|
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I09-Surface and Interface Structural Analysis
|
Tomáš
Rauch
,
Victor A.
Rogalev
,
Maximilian
Bauernfeind
,
Julian
Maklar
,
Felix
Reis
,
Florian
Adler
,
Simon
Moser
,
Johannes
Weis
,
Tien-lin
Lee
,
Pardeep K.
Thakur
,
Jörg
Schäfer
,
Ralph
Claessen
,
Jürgen
Henk
,
Ingrid
Mertig
Diamond Proposal Number(s):
[19512]
Abstract: The diamond and zinc-blende semiconductors are well-known and have been widely studied for decades. Yet, their electronic structure still surprises with unexpected topological properties of the valence bands. In this joint theoretical and experimental investigation, we demonstrate for the benchmark compounds InSb and GaAs that the electronic structure features topological surface states below the Fermi energy. Our parity analysis shows that the spin-orbit split-off band near the valence band maximum exhibits a strong topologically nontrivial behavior characterized by the
Z
2
invariants
(
1
;
000
)
. The nontrivial character is a consequence of the nonzero spin-orbit coupling and is imposed by the chosen constituents, in contrast to the conventional topological phase transition mechanism which relies on tuning parameters in the system Hamiltonian. Ab initio-based tight-binding calculations resolve topological surface states in the occupied electronic structure of InSb and GaAs, further confirmed experimentally by soft x-ray angle-resolved photoemission from both materials. Our findings are valid for all other materials whose valence bands are adiabatically linked to those of InSb, i.e., many diamond and zinc-blende semiconductors, as well as other related materials, such as half-Heusler compounds.
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Jun 2019
|
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I13-1-Coherence
|
Diamond Proposal Number(s):
[10117]
Abstract: Imaging ordered materials with coherent x rays holds great potential to improve our understanding of phenomena in complex materials systems where emergent behavior can arise due to coupling of spin, lattice, and orbital degrees of freedom. Coherent diffractive imaging (CDI) is a lensless imaging technique for probing the structure of materials in three dimensions. Central to the success of the CDI method is the inversion of propagated wave field information to recover a quantitative image of the illuminated crystalline structure. Present challenges faced with existing approaches to image recovery are often due to nonuniqueness of wave propagated forms of the electron density information that can cause prohibitive stagnation of the reconstruction algorithm. Here we report on a major advancement in image recovery that is able to recover the three-dimensional image of a 492 nm gold single crystal undergoing progressive deformation to a highly strained condition without the use of a priori information. Our findings also demonstrate the significance of robust image recovery techniques for revealing high resolution topological structure.
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Apr 2019
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I05-ARPES
|
Diamond Proposal Number(s):
[18586]
Abstract: Using angle-resolved photoemission spectroscopy and density functional theory (DFT) we study the electronic structure of layered BaZnBi2. Our experimental results show no evidence of Dirac states in BaZnBi2 originated either from the bulk or the surface. The calculated band structure without spin-orbit interaction shows linear band dispersions at X along the X−M high-symmetry line. In addition, the calculations suggest a gapless band crossing point along the Γ−M high-symmetry line. However, as soon as the spin-orbit interaction is turned on, the band crossing point is significantly gapped out. These observations suggest that the Dirac fermions in BaZnBi2 are trivial similar to the Dirac states observed in grapheme. The experimental observations are in good agreement with the DFT calculations.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Anna
Regoutz
,
Alex M.
Ganose
,
Lars
Blumenthal
,
Christoph
Schlueter
,
Tien-lin
Lee
,
Gregor
Kieslich
,
Anthony K.
Cheetham
,
Gwilherm
Kerherve
,
Ying-sheng
Huang
,
Ruei-san
Chen
,
Giovanni
Vinai
,
Tommaso
Pincelli
,
Giancarlo
Panaccione
,
Kelvin H. L.
Zhang
,
Russell G.
Egdell
,
Johannes
Lischner
,
David O.
Scanlon
,
David J.
Payne
Diamond Proposal Number(s):
[12673]
Abstract: Theory and experiment are combined to gain an understanding of the electronic properties of OsO2, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of OsO2 single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory.
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Feb 2019
|
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I19-Small Molecule Single Crystal Diffraction
|
A.
Stefancic
,
S. H.
Moody
,
T. J.
Hicken
,
M. T.
Birch
,
G.
Balakrishnan
,
S. A.
Barnett
,
M.
Crisanti
,
J. S. O.
Evans
,
S. J. R.
Holt
,
K. J. A.
Franke
,
P. D.
Hatton
,
B. M.
Huddart
,
M. R.
Lees
,
F. L.
Pratt
,
C. C.
Tang
,
M. N.
Wilson
,
F.
Xiao
,
T.
Lancaster
Abstract: We present an investigation into the structural and magnetic properties of Zn-substituted Cu2OSeO3, a system in which the skyrmion lattice (SkL) phase in the magnetic field–temperature phase diagram was previously seen to split as a function of increasing Zn concentration. We find that splitting of the SkL is only observed in polycrystalline samples and reflects the occurrence of several coexisting phases with different Zn content, each distinguished by different magnetic behavior. No such multiphase behavior is observed in single-crystal samples.
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Nov 2018
|
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I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[17449]
Abstract: The electronic properties of epitaxial heterojunctions consisting of the prototypical perovskite oxide semiconductor,
n
−
SrTiO
3
, and the high-mobility Group IV semiconductor
p
-Ge have been investigated. Hard x-ray photoelectron spectroscopy with a new method of analysis has been used to determine band alignment while at the same time quantifying a large built-in potential found to be present within the Ge. Accordingly, the built-in potential within the Ge has been mapped in a layer-resolved fashion. Electron transfer from donors in the
n
−
SrTi
O
3
to the
p
-Ge creates a space-charge region in the Ge resulting in downward band bending, which spans most of the Ge gap. This strong downward band bending facilitates visible light, photogenerated electron transfer from Ge to STO, favorable to drive the hydrogen evolution reaction associated with water splitting. Ti 2p and
Sr
3
d
core-level line shapes reveal that the STO bands are flat despite the space-charge layer therein. Inclusion of the effect of Ge band bending on band alignment is significant, amounting to a
∼
0.4
eV
reduction in valence band offset compared to the value resulting from using spectra averaged over all layers. Density functional theory allows candidate interface structural models deduced from scanning transmission electron microscopy images to be simulated and structurally optimized. These structures are used to generate multislice simulations that reproduce the experimental images quite well. The calculated band offsets for these structures are in good agreement with experiment.
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Sep 2018
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