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Register a new userThe role of 2s Bloch wave state excitations on STEM-HAADF intensity in quantitative analysis of alloys
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In this work, we emphasize the important contribution of the 2s Bloch wave state to the properties of a STEM electron probe propagating on an atomic column. For a strong enough column potential, the confinement of the 2s state leads to a long-period oscillation of the electron wave function, which is reflected in the resulting STEM-HAADF intensity. We show how this influences STEM composition quantification even at large thicknesses. We found additionally that the excitation of the 2s state affects the intensity of alloys where long-range order phenomena are present, which in turn provides a way to probe the degree of order in alloys.
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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.
We investigate the effect of short-range order (SRO) on the electronic structure in alloys from the theoretical point of view using density of states (DOS) data. In particular, the interaction between the atoms at different lattice sites is affected by chemical disorder, which in turn is reflected in the fine structure of the DOS and, hence, in the outcome of spectroscopic measurements. We aim at quantifying the degree of potential SRO with a proper parameter. The theoretical modeling is done with the Korringa-Kohn-Rostoker Greens function method. Therein, the extended multi-sublattice non-local coherent potential approximation is used to include SRO. As a model system, we use the binary solid solution Ag$_c$Pd$_{1-c}$ at three representative concentrations $c=0.25$, $0.5$ and $0.75$. The degree of SRO is varied from local ordering to local segregation through an intermediate completely uncorrelated state. We observe some pronounced features, which change over the whole energy range of the valence bands as a function of SRO in the alloy. These spectral variations should be traceable in modern photoemission experiments.
Experimental spectra from medium energy ion scattering were compared to Monte-Carlo simulations (employing the TRBS code) to obtain information on the scattering potential. The impact of uncertainties in the interatomic potential on quantification of sample properties such as thickness, composition or electronic stopping was investigated for different scattering geometries: backscattering and transmission. For backscattered He ions with tens of keV primary energy the scattering potential was found to overestimate the multiple scattering background in the energy spectra resulting in an uncertainty of < 3 % in quantitative analysis. Light ions transmitted through a sample for equivalent path length in the medium are only affected minorly by changes in the scattering potential. This effect becomes more distinct for heavier primary ions.
The effects of the tilt of the crystallographic orientation with respect to an incident electron probe on high-angle annular dark field (HAADF) imaging in aberration-corrected scanning transmission electron microscopy (STEM) have been investigated with experiments and simulations. A small specimen tilt can lead to unequal deviations of different atom species in the HAADF image and result in further relative displacement between anions and cations. Simulated HAADF images also confirm that the crystal tilt causes an artifact in atom polarization as well. The effect is derived from the scattering ability of different atoms.
Controlled excitation of materials can transiently induce changed or novel properties with many fundamental and technological implications. Especially, the concept of Floquet engineering, manipulation of the electronic structure via dressing with external lasers, has attracted some recent interest. Here we review the progress made in defining Floquet materials properties and give a special focus on their signatures in experimental observables as well as considering recent experiments realizing Floquet phases in solid state materials. We discuss how a wide range of experiments with non-equilibrium electronic structure can be viewed by employing Floquet theory as an analysis tool providing a different view of excitations in solids.
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