Growth and characterisation of uranium nanostructures

摘要

Uranium is the only element in the periodic table to exhibit a charge-density wave and superconductivity at ambient pressure. The competition between these effects in technologically important high-temperature superconducting systems has come under increasing scrutiny, and uranium offers a model system in which to study the CDW. However, the element is difficult to grow in single-crystals in the bulk. We describe the growth by magnetron sputtering and characterisation of single-crystal epitaxial thin- films of alpha-uranium in the (110) orientation on the Nb(110)/A-plane sapphire buffer layer/substrate system. We use X-ray scattering methods to determine the influence of the thickness of the component layers of the samples on the microstructure, and find that there is a non-trivial dependence of the microstructural state of the uranium layers on both the thickness of the uranium layers themselves and the thickness of the niobium buffer layers upon which they are grown. In particular, the widths of the uranium peaks decrease when the uranium layer thickness is increased, but increase when the buffer layer thickness is increased. An extensive review of the methods for characterisation of thin- film microstructures using X-ray diffraction is given, and several widely used models and interpretations are critically discussed, in particular those given in the many instances in which two-component line shapes are seen in transverse scans from thin- films. We also use X-ray diffraction from high-intensity synchrotron sources to characterise the charge-density wave state in these samples, and discuss the effect of uranium-layer thickness on its characteristics as a function of temperature. Important differences between the CDW seen in bulk uranium and in thin- films are seen and discussed in terms of the microstructure of the films. In particular, no incommensurate{commensurate transition is seen in the films, and a large intensity asymmetry is seen between the 2+2+1± and 2+2-1± CDW satellites. Furthermore, the correlation-length of the CDW is limited in the plane of the film, and dependent on the thickness of the uranium layers.