763 research outputs found
Atomic resolution mapping of phonon excitations in STEM-EELS experiments
Atomically resolved electron energy-loss spectroscopy experiments are
commonplace in modern aberrationcorrected transmission electron microscopes.
Energy resolution has also been increasing steadily with the continuous
improvement of electron monochromators. Electronic excitations however are
known to be delocalised due to the long range interaction of the charged
accelerated electrons with the electrons in a sample. This has made several
scientists question the value of combined high spatial and energy resolution
for mapping interband transitions and possibly phonon excitation in crystals.
In this paper we demonstrate experimentally that atomic resolution information
is indeed available at very low energy losses around 100 meV expressed as a
modulation of the broadening of the zero loss peak. Careful data analysis
allows us to get a glimpse of what are likely phonon excitations with both an
energy loss and gain part. These experiments confirm recent theoretical
predictions on the strong localisation of phonon excitations as opposed to
electronic excitations and show that a combination of atomic resolution and
recent developments in increased energy resolution will offer great benefit for
mapping phonon modes in real space
Barrier efficiency of sponge-like La2Zr2O7 buffer layers for YBCO-coated conductors
Solution derived La2Zr2O7 films have drawn much attention for potential
applications as thermal barriers or low-cost buffer layers for coated conductor
technology. Annealing and coating parameters strongly affect the microstructure
of La2Zr2O7, but different film processing methods can yield similar
microstructural features such as nanovoids and nanometer-sized La2Zr2O7 grains.
Nanoporosity is a typical feature found in such films and the implications for
the functionality of the films is investigated by a combination of scanning
transmission electron microscopy, electron energy-loss spectroscopy and
quantitative electron tomography. Chemical solution based La2Zr2O7 films
deposited on flexible Ni-5at.%W substrates with a {100} biaxial texture
were prepared for an in-depth characterization. A sponge-like structure
composed of nanometer sized voids is revealed by high-angle annular dark-field
scanning transmission electron microscopy in combination with electron
tomography. A three-dimensional quantification of nanovoids in the La2Zr2O7
film is obtained on a local scale. Mostly non-interconnected highly facetted
nanovoids compromise more than one-fifth of the investigated sample volume. The
diffusion barrier efficiency of a 170 nm thick La2Zr2O7 film is investigated by
STEM-EELS yielding a 1.8 \pm 0.2 nm oxide layer beyond which no significant
nickel diffusion can be detected and intermixing is observed. This is of
particular significance for the functionality of YBa2Cu3O7-{\delta} coated
conductor architectures based on solution derived La2Zr2O7 films as diffusion
barriers.Comment: Accepted for publication in Superconductor Science and Technolog
Magnetic monopole field exposed by electrons
Magnetic monopoles have provided a rich field of study, leading to a wide
area of research in particle physics, solid state physics, ultra-cold gases,
superconductors, cosmology, and gauge theory. So far, no true magnetic
monopoles were found experimentally. Using the Aharonov-Bohm effect, one of the
central results of quantum physics, shows however, that an effective monopole
field can be produced. Understanding the effects of such a monopole field on
its surroundings is crucial to its observation and provides a better grasp of
fundamental physical theory. We realize the diffraction of fast electrons at a
magnetic monopole field generated by a nanoscopic magnetized ferromagnetic
needle. Previous studies have been limited to theoretical semiclassical optical
calculations of the motion of electrons in such a monopole field. Solid state
systems like the recently studied 'spin ice' provide a constrained system to
study similar fields, but make it impossible to separate the monopole from the
material. Free space diffraction helps to understand the dynamics of the
electron-monopole system without the complexity of a solid state system. The
use of a simple object such as a magnetized needle will allow various areas of
physics to use the general dynamical effects of monopole fields without
requiring a monopole particle or specific solids which have internal
monopole-like properties. The experiment performed here shows that even without
a true magnetic monopole particle, the theoretical background on monopoles
serves as a basis for experiments and can be applied to efficiently create
electron vortices. Various predictions about angular momentum and general field
effects can readily be studied using the available equipment. This realization
provides insights for the scientific community on how to detect magnetic
monopoles in high energy collisions, cosmological processes, or novel
materials.Comment: 5 pages, 3 figures + 7 pages of supplementary information, 8 figure
Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7
We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that
was controversially described with respect to its structural organization and
magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction,
electron diffraction, transmission electron microscopy, and band structure
calculations, we solve the room-temperature structure of this compound
[alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The
gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889
A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7
structure, the Cu and Cl atoms are randomly displaced from the special
positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x
c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub),
space group Pbam] are stable between 640 K and 500 K and below 500 K,
respectively. The structural changes at 500 K and 640 K are identified as
order-disorder phase transitions. The displacement of the Cl atoms is frozen
upon the gamma --> beta transformation, while a cooperative tilting of the NbO6
octahedra in the alpha-phase further eliminates the disorder of the Cu atoms.
The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types
of the atomic displacements that interfere due to the bonding between the Cu
atoms and the apical oxygens of the NbO6 octahedra. The precise structural
information resolves the controversy between the previous computation-based
models and provides the long-sought input for understanding the magnetic
properties of (CuCl)LaNb2O7.Comment: 12 pages, 10 figures, 5 tables; crystallographic information (cif
files) include
Electronically coupled complementary interfaces between perovskite band insulators
Perovskite oxides exhibit a plethora of exceptional electronic properties,
providing the basis for novel concepts of oxide-electronic devices. The
interest in these materials is even extended by the remarkable characteristics
of their interfaces. Studies on single epitaxial connections between the two
wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either
high-mobility electron conductors or insulating, depending on the atomic
stacking sequences. In the latter case they are conceivably positively charged.
For device applications, as well as for basic understanding of the interface
conduction mechanism, it is important to investigate the electronic coupling of
closely-spaced complementary interfaces. Here we report the successful
realization of such electronically coupled complementary interfaces in SrTiO3 -
LaAlO3 thin film multilayer structures, in which the atomic stacking sequence
at the interfaces was confirmed by quantitative transmission electron
microscopy. We found a critical separation distance of 6 perovskite unit cell
layers, corresponding to approximately 2.3 nm, below which a decrease of the
interface conductivity and carrier density occurs. Interestingly, the high
carrier mobilities characterizing the separate electron doped interfaces are
found to be maintained in coupled structures down to sub-nanometer interface
spacing
Задачи глобальной экологии
Changes in the size distribution and composition of bimetallic Pd-Au nanoclusters have been observed after hydrogen exposure. This effect is caused by hydrogen-induced Ostwald ripening whereby the hydrogen reduces the binding energy of the cluster atoms leading to their detachment from the cluster. The composition changes due to a difference in mobility of the detached palladium and gold atoms on the surface. Fast palladium atoms contribute to the formation of larger nanoclusters, while the slower gold atoms are confined to the smaller nanoclusters. These transformations in the Pd-Au nanocluster size and composition set a limit for chemical reactions in which such nanoclusters are involved together with hydrogen
Synthesis of a 3D network of Pt nanowires by atomic layer deposition on carbonaceous template
The formation of a 3D network composed of free standing and interconnected Pt
nanowires is achieved by a two-step method, consisting of conformal deposition
of Pt by atomic layer deposition (ALD) on a forest of carbon nanotubes and
subsequent removal of the carbonaceous template. Detailed characterization of
this novel 3D nanostructure was carried out by transmission electron microscopy
(TEM) and electrochemical impedance spectroscopy (EIS). These characterizations
showed that this pure 3D nanostructure of platinum is self-supported and offers
an enhancement of the electrochemically active surface area by a factor of 50
Optimized fabrication of high quality La0.67Sr0.33MnO3 thin films considering all essential characteristics
In this article, an overview of the fabrication and properties of high
quality La0.67Sr0.33MnO3 (LSMO) thin films is given. A high quality LSMO film
combines a smooth surface morphology with a large magnetization and a small
residual resistivity, while avoiding precipitates and surface segregation. In
literature, typically only a few of these issues are adressed. We therefore
present a thorough characterization of our films, which were grown by pulsed
laser deposition. The films were characterized with reflection high energy
electron diffraction, atomic force microscopy, x-ray diffraction, magnetization
and transport measurements, x-ray photoelectron spectroscopy and scanning
transmission electron microscopy. The films have a saturation magnetization of
4.0 {\mu}B/Mn, a Curie temperature of 350 K and a residual resistivity of 60
{\mu}{\Omega}cm. These results indicate that high quality films, combining both
large magnetization and small residual resistivity, were realized. A comparison
between different samples presented in literature shows that focussing on a
single property is insufficient for the optimization of the deposition process.
For high quality films, all properties have to be adressed. For LSMO devices,
the thin film quality is crucial for the device performance. Therefore, this
research is important for the application of LSMO in devices.Comment: Accepted for publication in Journal of Physics D - Applied Physic
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