Transport properties and coherence in YBCO submicron structures.

The research activity has been mostly focused on the study of the transport properties of high critical temperature superconductor (HTS) nanostructures. The experiments have been carried on YBCO samples down to size width of 150 nm. This is one of the very first systematic studies of HTS devices at these length scales and represents an intermediate step towards the ideal nanoscale transport experiments, whose dimensions should be closer also to the HTS coherence length. Transport measurements at the nanoscale should unveil some secrets on the HTS physics. Sizes of the order of a few hundred nm guarantee a high level of homogeneity and reproducibility of physical phenomena, and give access to a series of experiments where the macroscopic quantum coherence manifest more pronounced properties than at larger scales. Samples have been patterned in shape of bridges and rings, where the ring radius and the width of channels have been varied from 1 um down to 150 nm. The reproducibility of superconducting transport properties as the critical current density (Jc) and the critical temperature (Tc) scale with the sample size. In optimal fabrication conditions the reduction of Tc and Jc is strictly below 10%, when we pass from 1 um down to 150 nm channel width, which is among the best results available in literature. Part of measurements have been addressed to the study of the magnetic and temperature dependence of the resistance. In ring shaped samples Little-Parks effect has been detected. Measurements on underdoped nano-rings can be directed to some relevant issues concerning flux periodicity anomalies induced by the d-wave order parameter .