10 research outputs found
Tuning two-dimensional electron (hole) gases at LaInO/BaSnO interfaces: Impact of polar distortions, termination, and thickness
Two-dimensional election gases (2DEG), arising due to quantum confinement at
interfaces between transparent conducting oxides, have received tremendous
attention in view of electronic applications. The challenge is to find a
material system that exhibits both a high charge-carrier density and mobility,
at and even above room temperature. Here, we explore the potential of
interfaces formed by two lattice-matched wide-gap oxides of emerging interest,
, the polar, orthorhombic perovskite LaInO and the
non-polar, cubic perovskite BaSnO, employing density-functional theory
and many-body theory. We demonstrate that this material combination exhibits
all the key features for reaching the goal. For periodic heterostructures, we
find that the polar discontinuity at the interface is mainly compensated by
electronic relaxation through charge transfer from the LaInO to the
BaSnO side. This leads to the formation of a 2DEG hosted by the
highly-dispersive Sn--derived conduction band and a 2D hole gas of O-
character, strongly localized inside LaInO. Remarkably, structural
distortions through octahedra tilts induce a depolarization field counteracting
the polar discontinuity, and thus increasing the (minimal)
LaInO thickness, , required for the formation of a 2DEG. These polar
distortions decrease with increasing LaInO thickness, enhancing the polar
discontinuity and leading to a 2DEG density of 0.5 electron per unit-cell
surface. Interestingly, in non-periodic heterostructures, these distortions
lead to a decrease of , thereby enhancing and delocalizing the 2DEG. We
rationalize how polar distortions, termination, and thickness can be exploited
in view of tailoring the 2DEG characteristics, and why this material is
superior to the most studied prototype LaAlO/SrTiO
Similarity of materials and data-quality assessment by fingerprinting
Identifying similar materials, i.e., those sharing a certain property or
feature, requires interoperable data of high quality. It also requires means to
measure similarity. We demonstrate how a spectral fingerprint as a descriptor,
combined with a similarity metric, can be used for establishing quantitative
relationships between materials data, thereby serving multiple purposes. This
concerns, for instance, the identification of materials exhibiting electronic
properties similar to a chosen one. The same approach can be used for assessing
uncertainty in data that potentially come from different sources. Selected
examples show how to quantify differences between measured optical spectra or
the impact of methodology and computational parameters on calculated
properties, like the the density of states or excitonic spectra. Moreover,
combining the same fingerprint with a clustering approach allows us to explore
materials spaces in view of finding (un)expected trends or patterns. In all
cases, we provide physical reasoning behind the findings of the automatized
assessment of data
Excitations in cubic BaSnO: a consistent picture revealed by combining theory and experiment
Among the transparent conducting oxides, the perovskite barium stannate is
most promising for various electronic applications due to its outstanding
carrier mobility achieved at room temperature. Most important characteristics
however, , its band gap, effective masses, and absorption edge remain
controversial. Here, we provide a fully consistent picture by combining
state-of-the-art methodology with forefront electron energy-loss
spectroscopy (EELS) and optical absorption measurements. On- and off-axis
valence EELS spectra, featuring signals originating from band gap transitions,
are acquired on defect-free sample regions of a BaSnO single crystal.
These high-energy-resolution measurements are able to capture also very weak
excitations below the optical gap, attributed to indirect transitions. By
temperature-dependent optical absorption measurements, we assess band-gap
renormalization effects induced by electron-phonon coupling. Overall, we find
for the effective electronic mass, the direct and the indirect gap, the optical
gap as well as the absorption onsets and spectra excellent agreement between
both experimental techniques and the theoretical many-body results, supporting
also the picture of a phonon-mediated mechanism where indirect transitions are
activated by phonon-induced symmetry lowering. This work demonstrates a
fruitful connection between different high-level theoretical and experimental
methods for exploring the characteristics of advanced materials
Enhanced Light–Matter Interaction in Graphene/h-BN van der Waals Heterostructures
By
investigating the optoelectronic properties of prototypical
graphene/hexagonal boron nitride (h-BN) heterostructures, we demonstrate
how a nanostructured combination of these materials can lead to a
dramatic enhancement of light–matter interaction and give rise
to unique excitations. In the framework of ab initio many-body perturbation
theory, we show that such heterostructures absorb light over a broad
frequency range, from the near-infrared to the ultraviolet (UV), and
that each spectral region is characterized by a specific type of excitations.
Delocalized electron–hole pairs in graphene dominate the low-energy
part of the spectrum, while strongly bound electron–hole pairs
in h-BN are preserved in the near-UV. Besides these features, characteristic
of the pristine constituents, charge-transfer excitations appear across
the visible region. Remarkably, the spatial distribution of the electron
and the hole can be selectively tuned by modulating the stacking arrangement
of the individual building blocks. Our results open up unprecedented
perspectives in view of designing van der Waals heterostructures with
tailored optoelectronic features
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Melt Growth and Physical Properties of Bulk LaInO3 Single Crystals
Large bulk LaInO3 single crystals are grown from the melt contained within iridium crucibles by the vertical gradient freeze (VGF) method. The obtained crystals are undoped or intentionally doped with Ba or Ce, and enabled wafer fabrication of size 10 × 10 mm2. High melting point of LaInO3 (≈1880 °C) and thermal instability at high temperatures require specific conditions for bulk crystal growth. The crystals do not undergo any phase transition up to 1300 °C, above which a noticeable thermal decomposition takes place. The good structural quality of the crystals makes them suitable for epitaxy. The onset of strong optical absorption shows orientation-dependent behavior due to the orthorhombic symmetry of the LaInO3 crystals. Assuming direct transitions, optical bandgaps of 4.35 and 4.39 eV are obtained for polarizations along the [010] and the [100], [001] crystallographic directions, respectively. There is an additional weak absorption in the range between 2.8 and 4 eV due to oxygen vacancies. Density-functional-theory calculations support the interpretation of the optical absorption data. Cathodoluminescence spectra show a broad, structured emission band peaking at ≈2.2 eV. All bulk crystals are electrically insulating. The relative static dielectric constant is determined at a value of 24.6 along the [001] direction