YBCO nanowires to study nanoscale ordering in High-Tc Superconductors

Doctoral thesis, 2016

We present the realization of YBa2Cu3O7-δ (YBCO) nanowires for both basic physics studies, which could help to elucidate the microscopic mechanisms of High critical Temperature Superconductors (HTS), and for novel applications. The first part of the thesis describes an improved nanopatterning procedure, based on e-beam lithography in combination with an amorphous carbon mask and a very gentle Ar+ ion etching. By using this procedure, nanowires preserving pristine superconducting properties, characterized by critical current densities very close to the theoretical depairing limit, have been achieved. These structures represent model systems for the study of HTS: their superconducting properties are very close to the as grown films. We have grown YBCO thin films covering a broad oxygen doping range going from the slightly overdoped regime down to the strongly underdoped region of the phase diagram. We have been able to trace the entire underdoped part of the phase diagram, and to show that the most peculiar features encountered in single crystals are well reproduced in our thin films. The core of the thesis describes an experiment done to study the effect of the nanoscale ordering on the superconducting properties of our nanowires. The presence of a local charge density wave (CDW) order has been recently demonstrated to be ubiquitous among all the cuprate families; if associated to a local modulations of Cooper pair density, CDWs might affect the absolute value of the critical current density of nanowires patterned at different in plane angles. We have used YBCO nanowires, fabricated on untwinned films, as a function of the oxygen doping and with dimensions of the same order as the CDW correlation length. By measuring the current voltage characteristic (IVC) of nanowires with the same width, patterned at different angles with respect to a fixed in plane direction of the substrate, we have revealed a cosinusoidal modulation of the critical current density for the narrowest width w (of the order of 65 nm). This dependence, that smears out for wider nanowire dimensions, can represent one of the first evidence of the existence of a pair density wave in YBCO. In the second part of the thesis we have focused on applications. YBCO nanoSQUIDs, employing very short nanowires in the so-called Dayem bridge configuration, have been fabricated and characterized. These devices, working in the whole temperature range from 300 mK up to the critical temperature TC (close to 85 K), have revealed an ultra low-white noise, below 1 µΦ0Hz-1/2 above 10 kHz, corresponding to a predicted spin sensitivity of 50 µBHz-1/2. The homogeneity of the nanowires has also given a boost to the realization of devices, aimed at the detection of single photons. We have shown that in ultrathin (7-8 unit cell thick) YBCO nanowires a hot spot is formed within the wire: as a consequence, the nanowires are driven from the superconducting directly to the normal state, analogously to NbN nanowires commonly used as single photon detectors.