Noise and electrical properties of YBCO nanostructures

Doktorsavhandling, 2020

This thesis work deals with the investigation of noise properties in cuprate High critical Temperature Superconductor (HTS) YBCO nanoscale devices. Here the aim is to get a better understanding of nanoscale fluctuations in the normal state of HTS from which superconductivity evolves. The observation of fluctuations in the electronic properties might offer valuable clues toward the microscopic mechanism leading to superconductivity in HTS, which still represent one of the main unsolved problems in solid-state physics. In this respect, the YBCO nanodevices are implemented as tools to obtain new experimental signatures, which can deliver new insights about the complex properties of HTS materials. Since cuprate HTS undergo various nano-scale ordering transitions upon cooling and variation of hole doping, being able to study transport properties on the nanoscale is of utmost importance. In this respect, resistance noise properties of YBCO nanowires are studied as a function of temperature and hole doping. Indications of nematic fluctuations, that is local time dependent fluctuations of the in-plane conductivity anisotropy, have been observed in a wide temperature range above the superconducting transition. The observed fluctuations might be related to so-called charge stripe fluctuations, which represent a possible microscopic mechanism for superconductivity in these materials.

However, the interest in HTS nanostructures is not purely academic. The technological application of YBCO weak links in SQUID, is a major focus of research in the field. In this thesis, we present a novel fabrication process of HTS weak links: the nanoscale Grooved Dayem Bridge (GDB). Here, the layout of the bridge and the weak link inside the bridge are realized during one single lithography process on a YBCO film grown on a single crystal substrate. This results in high-quality weak links with IcRn products as high as 550 µV and differential resistances much larger than those observed in bare Dayem bridges at T=77 K. Moreover, the GDB greatly simplifies the fabrication procedure compared to grain boundary based JJs. We have used YBCO GDBs as novel nanoscale building blocks in HTS SQUID magnetometers coupled to an in plane pickup loop, which have been characterized via transport and noise measurements at T= 77 K. These devices exhibit large voltage modulations (ΔV =27-50 µV), low values of white magnetic flux noise, 6 µΦ0/\sqrt{Hz}, and corresponding magnetic field noise, 63 fT/\sqrt{Hz}, at T=77 K. Therefore, GDB based SQUIDs combine the nanofabrication advantages and the device reproducibility, which are typical of Dayem bridges, with the performances, i.e. low magnetic flux and field noise, of state-of-the-art SQUIDs based on grain boundary JJs. The achieved magnetic field noise paves the way for the realization of a single layer YBCO magnetometer with magnetic field noise below 20 fT/\sqrt(Hz).