The unique electronic structure found at interfaces between materials can allow unconventional quantum states to emerge. Here we report on the discovery of superconductivity in electron gases formed at interfaces between (111) oriented KTaO3 and insulating overlayers of either EuO or LaAlO3. The superconducting transition temperature, approaching 2.2 K, is about one order of magnitude higher than that of the LaAlO3/SrTiO3 system. Strikingly, similar electron gases at KTaO3 (001) interfaces remain normal down to 25 mK. The critical field and current-voltage measurements indicate that the superconductivity is two dimensional. In EuO/KTaO3 (111) samples, a spontaneous in-plane transport anisotropy is observed prior to the onset of superconductivity, suggesting the emergence of a distinct ‘stripe’ like phase, which is also revealed near the critical field.

No abstract

The exceptional mechanical strength of medium/high-entropy alloys has been attributed to hardening in random solid solutions. Here, we evidence non-random chemical mixing in a CrCoNi alloy, resulting from short-range ordering. A data-mining approach of electron nanodiffraction enabled the study, which is assisted by neutron scattering, atom probe tomography, and diffraction simulation using first-principles theory models. Two samples, one homogenized and one heat-treated, are observed. In both samples, results reveal two types of short-range-order inside nanoclusters that minimize the Cr–Cr nearest neighbors (L12) or segregate Cr on alternating close-packed planes (L11). The L11 is predominant in the homogenized sample, while the L12 formation is promoted by heat-treatment, with the latter being accompanied by a dramatic change in dislocation-slip behavior. These findings uncover short-range order and the resulted chemical heterogeneities behind the mechanical strength in CrCoNi, providing general opportunities for atomistic-structure study in concentrated alloys for the design of strong and ductile materials.

Materials with a layered Kagome lattice are expected to give rise to novel physics arising from band structures with topological properties, spin liquid behavior and the formation of skyrmions. Until now, most work on Kagome materials has been performed on bulk samples due to difficulties in thin film synthesis. Here, by using molecular beam epitaxy, layered Kagome-structured FeSn films are synthesized on (111) oriented LaAlO3 substrate. Both in-situ and ex-situ characterizations indicate these films are highly crystalline and c-axis oriented, with atomically smooth surfaces.However, the films grow as disconnected islands, with lateral dimensions on the micron scale. By patterning Pt electrodes using a focused electron beam, longitudinal and transverse resistance of single islands have been measured in magnetic fields. Our work opens a pathway for exploring mesoscale transport properties in thin films of Kagome materials and related devices.

No abstract

The development of bright, near-infrared-emissive quantum dots (QDs) is a necessary requirement for the realization of important new classes of technology. Specifically, there exist significant needs for brighter, heavy metal-free, near-infrared (NIR) QDs for applications with high radiative efficiency that span diverse applications, including down-conversion emitters for high-performance luminescent solar concentrators. We use a combination of theoretical and experimental approaches to synthesize bright, NIR luminescent InAs/InP/ZnSe QDs and elucidate fundamental material attributes that remain obstacles for development of near-unity NIR QD luminophores. First, using Monte Carlo ray tracing, we identify the atomic and electronic structural attributes of InAs core/shell, NIR emitters, whose luminescence properties can be tailored by synthetic design to match most beneficially those of high-performance, single-band-gap photovoltaic devices based on important semiconductor materials, such Si or GaAs. Second, we synthesize InAs/InP/ZnSe QDs based on the optical attributes found to maximize LSC performance and develop methods to improve the emissive qualities of NIR emitters with large, tunable Stokes ratios, narrow emission linewidths, and high luminescence quantum yields (here reaching 60 ± 2%). Third, we employ atomistic electronic structure calculations to explore charge carrier behavior at the nanoscale affected by interfacial atomic structures and find that significant exciton occupation of the InP shell occurs in most cases despite the InAs/InP type I bulk band alignment. Furthermore, the density of the valence band maximum state extends anisotropically through the (111) crystal planes to the terminal InP surfaces/interfaces, indicating that surface defects, such as unpassivated phosphorus dangling bonds, located on the (111) facets play an outsized role in disrupting the valence band maximum and quenching photoluminescence.

Transmission electron diffraction is a powerful and versatile structural probe for the characterization of a broad range of materials, from nanocrystalline thin films to single crystals. With recent developments in fast electron detectors and efficient computer algorithms, it now becomes possible to collect unprecedently large datasets of diffraction patterns (DPs) and process DPs to extract crystallographic information to form images or tomograms based on crystal structural properties, giving rise to data-driven electron microscopy. Critical to this kind of imaging is the type of crystallographic information being collected, which can be achieved with a judicious choice of electron diffraction techniques, and the efficiency and accuracy of DP processing, which requires the development of new algorithms. Here, we review recent progress made in data collection, new algorithms, and automated electron DP analysis. These progresses will be highlighted using application examples in materials research. Future opportunities based on smart sampling and machine learning are also discussed.

No abstract