Magnetic characterization of synthetic titanomagnetites: Quantifying the recording fidelity of ideal synthetic analogs

A series of four synthetic basalts comprising titanomagnetite (Fe3-xTixO4) grains of varied size and titanium content have been produced by a glass-ceramic method. Complementary characterization techniques of X-ray diffractometry, secondary electron microscopy, and transmission electron microscopy (TEM) demonstrate the reaction product composition consisted of mainly Fe3-xTixO4, pyroxene hedenbergite, fayalite, and SiO2. The samples exhibit bimodal distributions of larger (<2 µm) and smaller Fe3-xTixO4 particles (<50 nm in diameter), the latter found inside pyroxene crystals, as well as the sporadic occurance of dendritic Fe3-xTixO4 structures. Magnetic measurements show their bulk characteristics fall into two groups: Ti-rich titanomagnetite samples with varying Ti content; and near-stoichiometric magnetite. The TEM technique of off-axis electron holography allowed for visualization of the magnetic behavior of the synthetic Fe3-xTixO4 grains. Energy dispersive X-ray analysis and off-axis electron holography confirmed the small Fe3-xTixO4 grains (<50 nm) confined within glassy pyroxene regions to be Fe-rich and single domain, carrying strong magnetic signals, compared to the relatively magnetically weak larger Fe3-xTixO4 grains (x ∼ 0.6). The large grains in the pure magnetite sample are shown to be pseudo-single domain in nature. The quenching process involved in synthesis is considered similar to that of pillow basalts found at mid-ocean ridges and hence the reaction products are thought ideal in terms of characterization and understanding, for the purpose of studying natural systems

Effect of high oxygen deficiency in nano-confined bismuth sesquioxide

Bismuth sesquioxidein its cubic form, i.e.delta-Bi2O3,is the fastest oxygen ionic conductor knownwith important applications in energy technologies.However, the material is unstable asit undergoes to high-density polymorphic transitionsand degradation. In this work, we show that delta-Bi2O3can be stabilized both at high and low temperatures (T &lt; 775°C) under low oxygen partial pressure (pO2&lt; 10-5atm), where the material is nanostructured in multi-layered thin film coherent heterostructures with yttriumstabilized zirconia(YSZ).DFT calculation confirms sucha form of metastability, also showingthat highoxygen defect concentration favorsthe cubic phase.Moreover, high oxygen deficiencyin the nanoionics leadsto an unexpected“two-regime” conductivitywith high values(sigma&gt; 1 S cm-1at 600 °C)at high pO2and lowerionic conductivity (sigma~0.1 S cm-1at 600 °C) at low pO2.Ionic conductivity at low pO2occurs with high activation energy (Ea &gt; 1.5 eV), suggestingthusa drastic decrease in mobility for high concentration of defects

Nanowire quantum dots tuned to atomic resonances

Quantum dots tuned to atomic resonances represent an emerging field of hybrid quantum systems where the advantages of quantum dots and natural atoms can be combined. Embedding quantum dots in nanowires boosts these systems with a set of powerful possibilities, such as precise positioning of the emitters, excellent photon extraction efficiency and direct electrical contacting of quantum dots. Notably, nanowire structures can be grown on silicon substrates, allowing for a straightforward integration with silicon-based photonic devices. In this work we show controlled growth of nanowire-quantum-dot structures on silicon, frequency tuned to atomic transitions. We grow GaAs quantum dots in AlGaAs nanowires with a nearly pure crystal structure and excellent optical properties. We precisely control the dimensions of quantum dots and their position inside nanowires, and demonstrate that the emission wavelength can be engineered over the range of at least $30\,nm$ around $765\,nm$. By applying an external magnetic field we are able to fine tune the emission frequency of our nanowire quantum dots to the $D_{2}$ transition of $^{87}$Rb. We use the Rb transitions to precisely measure the actual spectral linewidth of the photons emitted from a nanowire quantum dot to be $9.4 \pm 0.7 \mu eV$, under non-resonant excitation. Our work brings highly-desirable functionalities to quantum technologies, enabling, for instance, a realization of a quantum network, based on an arbitrary number of nanowire single-photon sources, all operating at the same frequency of an atomic transition.Comment: main text (20 pages, 3 figures) plus supplementary information, Nano Letters (2018