Search

The authors study, as a promising candidate for photovoltaic applications, the Mott insulator LaVO${}_{3}$, epitaxially grown as a thin film on SrTiO${}_{3}$. They elucidate the electronic properties by electrical transport and photoemission measurements. The origin of the conducting interface is identified to be electronic reconstruction due to the polar discontinuity between film and substrate. The authors find a potential gradient in the film and a downward band bending in the substrate, with the electrons residing in interfacial Ti states.

Envisaged applications of Skyrmions in magnetic memory and logic devices crucially depend on the stability and mobility of these topologically nontrivial magnetic textures in thin films. We present for the first time quantitative maps of the magnetic induction that provide evidence for a 3D modulation of the Skyrmionic spin texture. The projected in-plane magnetic induction maps as determined from in-line and off-axis electron holography carry the clear signature of Bloch Skyrmions. However, the magnitude of this induction is much smaller than the values expected for homogeneous Bloch Skyrmions that extend throughout the thickness of the film. This finding can only be understood if the underlying spin textures are modulated along the out-of-plane $z$ direction. The projection of (the in-plane magnetic induction of) helices is further found to exhibit thickness-dependent lateral shifts, which show that this $z$ modulation is accompanied by an (in-plane) modulation along the $x$ and $y$ directions.

We report on the development of electron holographic tomography towards a versatile potential measurement technique, overcoming several limitations, such as a limited tilt range, previously hampering a reproducible and accurate electrostatic potential reconstruction in three dimensions. Most notably, tomographic reconstruction is performed on optimally sampled polar grids taking into account symmetry and other spatial constraints of the nanostructure. Furthermore, holographic tilt series acquisition and alignment have been automated and adapted to three dimensions. We demonstrate 6 nm spatial and 0.2 V signal resolution by reconstructing various, previously hidden, potential details of a GaAs/AlGaAs core-shell nanowire. The improved tomographic reconstruction opens pathways towards the detection of minute potentials in nanostructures and an increase in speed and accuracy in related techniques such as x-ray tomography.

Differential phase contrast (DPC) in a scanning transmission electron microscope is a widely employed technique for probing electromagnetic fields on the nanoscale. We show that the DPC signal corresponds to the averaged lateral probability current of the scattered electron probe. Based on this result we discuss the interpretation of DPC in terms of the projected electric and magnetic fields and the influence of experimental parameters thereon. We further show that DPC can be interpreted as a quantum weak measurement and that the reciprocal broad beam illumination technique is given by an astigmatic transport of intensity reconstruction.

The distribution and movement of charge is fundamental to many physical phenomena, particularly for applications involving nanoparticles, nanostructures, and electronic devices. However, there are very few ways of quantifying charge at the necessary length scale. Here, we show that aberration-corrected electron holography is capable of counting the charge on individual nanoparticles to a precision of one elementary unit of charge. We present a method that measures charges within predefined contours by directly applying Gauss's law at the nanoscale. We perform a statistical analysis to reveal the relationship between the size of the contours and the precision of the charge measurement and present strategies to optimize the spatial and signal resolution for the presented method.

Domain walls (DWs) substantially influence a large number of applications involving ferroelectric materials due to their limited mobility when shifted during polarization switching. The discovery of greatly enhanced conduction at ${\mathrm{BiFeO}}_{3}$ DWs has highlighted yet another role of DWs as a local material state with unique properties. However, the lack of precise information on the local atomic structure is still hampering microscopical understanding of DW properties. Here, we examine the atomic structure of ${\mathrm{BiFeO}}_{3}$ 109\ifmmode^\circ\else\textdegree\fi{} DWs with pm precision by a combination of high-angle annular dark-field scanning transmission electron microscopy and a dedicated structural analysis. By measuring simultaneously local polarization and strain, we provide direct experimental proof for the straight DW structure predicted by ab initio calculations as well as the recently proposed theory of diffuse DWs, thus resolving a long-standing discrepancy between experimentally measured and theoretically predicted DW mobilities.

BaTiO${}_{3}$ (BTO) nanocubes of about 50 nm in size were investigated by means of off-axis electron holography combined with in situ heating above and below the Curie temperature. It is shown that reconstructed electron phase maps of the BTO nanocubes are associated with the ferroelectric polarization pattern through a mechanism that could either be direct imaging of the internal potential or lattice variations that are common in ferroelectric domain patterns.

In ${\text{YMn}}_{2}{\text{O}}_{5}$, the Mn atoms occupy two nonequivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i.e., the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure, Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure changes upon the substitution of Mn by the non-Jahn-Teller cation ${\text{Fe}}^{3+}$. Therefore, we synthesized a series of ${\text{YMn}}_{2\ensuremath{-}x}{\text{Fe}}_{x}{\text{O}}_{5}$ powder samples with $x=0$, 0.5, and 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis as well as density-functional theory (DFT) to investigate the two nonequivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order, all plausible options of substitution of Fe for Mn are discussed. On the basis of these evaluations, the EXAFS and XANES behavior is analyzed and appropriate crystallographic weights are assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple-scattering theory, we conclude only the $4h$ Wyckoff site to be occupied by Fe [occupancy refined is $(100\ifmmode\pm\else\textpm\fi{}3)\mathrm{%}$ in case of $x=1$]. Furthermore, taking the XANES spectra into account, we are able to verify the EXAFS results and additionally explain the differences in the $\text{Mn}\text{ }K$ XANES spectra in dependence on $x$ to be caused by changes in the dipole transitions to $4p$ final states. From quantitative pre-edge analysis an oxidation number of $+4$ for the Mn atom for $x=1$ is determined whereas the Fe valence is shown to be unchanged. Since the substitution process only involves one Wyckoff site, the experimentally observed limit to a maximum amount of $x=1$ is explained. Additionally, a possible disorder, discussed in the literature, is not proven for our samples. With DFT calculations, the experimental findings are verified on the basis of the total energy of the different possible electronic configurations. Crystal-field effects are identified to be responsible for the site-selective substitution of Fe for Mn.