Synthesis, characterization and physical properties of CDW material niobium selenide and superconducting niobium nitride nanostructures.

摘要

Nanostructures are an important system by which to study electronic properties in confined geometries. This dissertation describes synthesis and properties of both charge-density-wave (CDW) (NbSe3) and superconducting (NbN) nanostructures. NbSe3 nanostructures were synthesized using stoichiometric amounts of niobium and selenium powders in an evacuated quartz tube at 700 °C . These nanostructures were subsequently transformed into superconducting NbN nanostructures through annealing under ammonia gas atmosphere at temperatures up to 1000 °C.;Morphological characterizations were conducted with scanning electron microscopy. X-ray diffractions were used to identify the phases of the synthesized nanostructures. Four-probe resistive measurements on individual nanostructures were utilized to explore their CDW and superconducting properties in confined geometries. Magnetization measurements were also applied to reveal the superconductivity of NbN nanostructure bundles.;The physical properties of both individual NbSe3 nanoribbons and bulk NbSe3 crystals (for comparison) were studied. In bulk NbSe3, Shubnikov-de Hass (SdH) oscillations were seen at low temperatures when the applied field was along the c-axis, whereas negative magnetoresistance due to confinement of electron trajectories in a magnetic field was observed in nanoribbons. The observed anisotropy of magnetoresistance in NbSe3 for magnetic fields perpendicular and parallel to b-axis of lower temperature CDW phase was explained in terms of the ellipsoidal shape of the Fermi surface and hence the mass anisotropy.;Transport studies on individual superconducting NbN nanostructures revealed interesting oscillations in the magnetoresistance and critical current. The oscillations were periodic with more than one frequency. Comparisons with other reported work on magnetoresistance oscillations led to the conclusion that the oscillations were due to quantized flux passing through phase coherent loops of grains.