Vortex Dynamics and Superconducting Phase Diagrams in Ta x Ge 1-x / Ge Multilayers with Coplanar Defects
The superconducting phase diagrams of amorphous multilayered Ta x Ge 1-x / Ge thin films have been studied over a large range of temperatures and magnetic fields by means of dc electrical transport measurements. These superconducting films belong to the class of extremely type-II superconductors, for which a multitude of superconducting phases has been predicted and experimentally verified. A thorough understanding of these phase diagrams is indispensable for future successful applications of high-temperature superconductors since some of the observed phases severely limit the zero-resistance current-carrying capacity of these materials. The Ta x Ge 1-x / Ge films in this study were prepared by vapour deposition under high vacuum conditions. The Ta-content varied between x = 0.31 and 0.37 and individual layer thicknesses ranged from about 3 to 15 nm. Tilting the sample substrates during the deposition resulted in coplanar defects with variable orientation and structure depending on the tilting angle. This way it was possible to study the interplay between magnetic flux lines and the material structure and defect morphology, respectively. Films with thin insulating Ge layers and thus strong interlayer coupling showed three dimensional behaviour over the complete range of fields and temperatures. The coplanar defect structure was able to extend the zero-resistance phase to significantly higher fields and temperatures for magnetic fields co-aligned with the defects. Strong support for the existence of a low-temperature glass phase was found in the case of aligned and misaligned magnetic fields. Increasing the insulating layer thickness lead to a cross-over to 2D behaviour depending on temperature and field as well as field orientation with respect to the defects. In the 2D phase regions the low-temperature zero-resistance glass phase may have disappeared entirely. Current-voltage characteristics measured in the low-temperature glass phases showed significant differences between the strongly and weakly coupled films. However the detailed temperature and field dependence of these current-voltage curves at low temperatures cannot be explained satisfactorily with existing theoretical models.