Studying local orders in YBCO by nanoscale confinement

Abstract

In 1986 scientists were amazed by the discovery of materials that conducted electricity without any resistance at temperatures much higher than previously thought, an accomplishment awarded the Nobel Prize in Physics already the following year. This led to great excitement and the hope one would be able to design materials that would be ‘superconducting’, as the phenomenon is called, even at room temperature, the holy grail of superconductivity research. The possibility to dispense the use of expensive and cumbersome cooling would indeed revolutionize many technologies like electrified transports, and also lay the foundation for the next generation of green innovations. However, despite intense research, many open questions remain and the mechanism behind high temperature superconductivity still represent a puzzle, far from being fully understood. Recently, the most common idea is that the comprehension of the superconducting mechanism cannot prescind from the understanding of the normal state of these materials. This is also unconventional, and populated by a constellation of local orders, characterized by nanoscale lengths: here, charge, spin, lattice and orbitals have a role, but their entwining and mutual relations are still not fully understood. The core of cuprate high-Tc superconductors (HTS) is represented by the CuO2 planes, where superconductivity sets in and where all the symmetry breaking orders reside. In order to succeed in understanding these fascinating materials, an innovative strategy is to confine the CuO2 planes at the nanoscale in HTS, and to use strain and confinement as a knob to tune the orders both in the superconducting and in the normal state. This can be done only if HTS are shrunk in thin film form, preserving the bulk quality. In this way, confining the orders on the same scale of their characteristic lengths, one may expect the locality of charge, spin and current orders to be enhanced, possibly simplifying the physics at play. The confinement can be obtained in two ways, either by nanopatterning c-axis oriented HTS thin film, where the CuO2 planes take the shape of the nanostructures, or depositing a-axis oriented HTS nm-thin films, where the CuO2 planes form nanoribbons, constrained on either side by vacuum and the underlying substrate. In this thesis work we have followed both of these strategies, using c-axis and a-axis oriented YBa2Cu3O7−δ thin films.