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Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high-enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing both high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a novel plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage from EELS data and transport measurements on the bulk single crystals of BLSO. These results represent a unique way of studying heterogeneities in solids, understanding structure-property relationships at the nanoscale, and opening the way to leveraging nanoscale doping texture in the design of nanophotonic devices.

The group-IV diamond-structure elements are known to host a variety of planar defects, including {001} planar defects in C and {001}, {111} and {113} planar defects in Si and Ge. Among the {001} planar defects, the Humble defect, known for some time to occur in Ge, has recently also been observed in Si/Ge alloys, but the details of its electronic structure remain poorly understood. Here we perform first-principles density functional calculations to study Humble defects in both Ge and Ge$_{0.8}$Si$_{0.2}$. We also measure the Si L$_{2,3}$-edge electron energy loss spectra both at the defect and in a bulk-like region far from the defect, and compare with theoretical calculations on corresponding Si sites in our first-principles calculations. We find that inclusion of core-hole effects in the theory is essential for reproducing the observed L$_{2,3}$ edge spectra, and that once they are included, the results provide a set of fingerprints for different types of local atomic bonding environments in Ge$_{0.8}$Si$_{0.2}$. Our first-principles calculations reveal that the Humble defects have a tendency to enlarge the electronic band gap, which may have potential uses in band engineering. The use of hybrid functionals for an improved description of the band gap in these systems is also discussed., Comment: 11 pages, 10 figures; Comments are welcome

Liquid-phase chemical exfoliation is ideal to achieve industry scale production of two-dimensional (2D) materials for a wide range of application such as printable electronics, catalysis and energy storage. However, many impactful 2D materials with potentials in quantum technologies can only be studied in lab settings due to their air-sensitivity, and loss of physical performance after chemical processing. Here, we report a simple chemical exfoliation method to create a stable, aqueous, surfactant-free, superconducting ink containing phase-pure 1T'-WS$_2$ monolayers that are isotructural to the air-sensitive topological insulator 1T'-WTe$_2$. We demonstrate that thin films can be cast on both hard and flexible substrates. The printed film is metallic at room temperature and superconducting below 7.3 K, shows strong anisotropic unconventional superconducting behavior with an in-plane and out-of-plane upper critical magnetic field of 30.1 T and 5.3 T, has a critical current of 44 mA, and is stable at ambient conditions for at least 30 days. Our results show that chemical processing can provide an engineering solution, which makes non-trivial 2D materials that used to be only studied in laboratories commercially accessible., Comment: 19 pages, 4 figures

We report a new \{001\} planar defect found in SiGe nanopillars. The defect structure, determined by atomic resolution electron microscopy, matches the Humble defect model proposed for diamond. We also investigated several possible variants of the Humble structure using first principles calculations and found that the one lowest in energy was also in best agreement with the STEM images. The pillar composition has been analyzed with electron energy loss spectroscopy, which hints at how the defect is formed. Our results show that the structure and formation process of defects in nanostructured group IV semiconductors can be different from their bulk counterparts.

BaSnO$_3$ exhibits the highest carrier mobility among perovskite oxides, making it ideal for oxide electronics. Collective charge carrier oscillations, plasmons, are expected to arise in this material, thus providing a tool to control the nanoscale optical field for optoelectronics applications. Here, we demonstrate the existence of relatively long-lived plasmons supported by high-mobility charge carriers in La-doped BaSnO$_3$ (BLSO). By exploiting the high spatial and energy resolution of electron energy-loss spectroscopy with a focused beam in a scanning transmission electron microscope, we systematically investigate the dispersion, confinement ratio, and damping of infrared localized surface plasmons (LSP) in BLSO nanoparticles. We find that the LSPs in BLSO are highly spatially confined compared to those sustained by noble metals and have relatively low loss and high quality factor compared to other doped oxides. Further analysis clarifies the relation between plasmon damping and carrier mobility in BLSO. Our results support the use of nanostructured degenerate semiconductors for plasmonic applications in the infrared region and establish a relevant alternative to more traditional plasmonic materials.

Orientation dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community., Comment: 9 pages and 9 figures

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a "demon," could exist in three-dimensional metals containing more than one species of charge carrier. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral, and do not couple to light, so have never been detected in an equilibrium, three-dimensional metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals, optical properties of metal nanoparticles, "soundarons" in Weyl semimetals, and high temperature superconductivity in, for example, metal hydrides. Here, we present evidence for a demon in Sr$_2$RuO$_4$ from momentum-resolved electron energy-loss spectroscopy (M-EELS). Formed of electrons in the $\beta$ and $\gamma$ bands, the demon is gapless with a room temperature velocity $v=1.065 \pm 0.12 \times 10^5$ m/s and critical momentum $q_c=0.08$ reciprocal lattice units. Its spectral weight violates low-energy partial sum rules, affirming its neutral character. Our study confirms a 66-year old prediction and suggests that demons may be a pervasive feature of multiband metals., Comment: 14 pages, 4 figures ; Revised to include a complete analysis of the gapless collective mode