Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaO[subscript x], 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO[subscript 3]) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO[subscript x] films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO[subscript 3], we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO[subscript x] and SrRuO[subscript 3] differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-1419807

Phylogeography of the common toad (Bufo bufo, Lissamphibia: Anura) in Switzerland

While Bufo bufo is widespread in Europe, the closely related B. spinosus is parapatrically distributed in Western Europe and Northern Africa. The exact course of the contact zone between both taxa, however, is still unknown. 24 samples from Switzerland were analysed using mitochondrial and nuclear markers combined with 243 previously published samples from Europe and Asia. No haplotypes of B. spinosus were detected in Switzerland, only two mitochondrial haplotypes of B. bufo: one north and south of the Alps, the other only north of the Alps. Both haplotypes are also widely distributed in Eastern and Central Europe. These results agree with a postglacial recolonisation of Central Europe from refugia on the Balkan Peninsula. The occurrence of one haplotype north and south of the Alps can be explained by colonisation either from the north by crossing the Alps or from the east along the southern edge of the Alps. The rapid postglacial recolonisation from Balkan refugia might have prevented the spread of southern haplotypes from Italy and of B. spinosus from France

Homogeneous freezing of water droplets for different volumes and cooling rates

To understand the crystallization of aqueous solutions in the atmosphere, biological specimens, or pharmaceutical formulations, the rate at which ice nucleates from pure liquid water must be quantified. There is still an orders-of-magnitude spread in the homogeneous nucleation rate of water measured using different instruments, with the most important source of uncertainty being that of the measured temperature. Microfluidic platforms can generate hundreds to thousands of monodisperse water-in-oil droplets, unachievable by most other techniques. However, most microfluidic devices previously used to quantify homogeneous ice nucleation rates have reported high temperature uncertainties, between +/- 0.3 and +/- 0.7 K. We use the recently developed Microfluidic Ice Nuclei Counter Zurich (MINCZ) to observe the freezing of spherical water droplets with two diameters (75 and 100 mu m) at two cooling rates (1 and 0.1 K min(-1)). By varying both droplet volume and cooling rate, we were able to probe a temperature range of 236.5-239.3 K with an accuracy of +/- 0.2 K, providing reliable data where previously determined nucleation rates suffered from large uncertainties and inconsistencies, especially at temperatures above 238 K. From these data and from Monte Carlo simulations, we demonstrate the importance of obtaining a sufficiently large dataset so that underlying nucleation rates are not overestimated at higher temperatures. Finally, we obtain new parameters for a previous parameterisation by fitting to our newly measured nucleation rates, enabling its use in applications where ice formation needs to be predicted.ISSN:1463-9084ISSN:1463-907

Spectromicroscopic insights for rational design of redox-based memristive devices

The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably