Characterizing Superfluid Stiffness in Cuprate Superconductors - Using Coplanar Waveguide Resonators

Abstract

High Temperature Superconductors (HTS), despite being discovered almost four decades ago, remain an unsolved puzzle that demands innovative approaches and advanced experimental techniques. The fundamental mechanism behind Cooperpair formation is still unknown, posing a major challenge to efforts aimed at increasing the critical temperature. Recently, the observation of superconductivity in Magic-Angle Twisted Bilayer Graphene (MATBG), driven by moiré-physics and flat-band phenomena, has inspired new perspectives on HTS. This connection arises from striking similarities between cuprates and MATBG, both hosting multiple correlated electronic phases, including superconductivity, although MATBG exhibits a much lower critical temperature. The increased density of states in flat bands naturally enhance the pairing critical temperature but simultaneously suppress superfluid stiffness due to the diverging effective mass, hindering superconductivity. However, the quantum metric in multiband systems, such as MATBG, restores finite stiffness, enabling a superconducting state. Analogous moiré-like effects can be induced in HTS thin films via substratedriven superpotentials. For instance, quasi-periodic one-dimensional potentials from nanometer-scale facets on (110) MgO substrates significantly influence ultrathin YBCO films, promoting electronic nematicity, a precursor to flat-band physics. In this thesis, we investigate superfluid stiffness, which is proportional to 1/λ2 (with λ being the London penetration depth) in YBCO films of varying thickness grown on different substrates. Our measurements reveal a pronounced increase in penetration depth for films on (110) MgO compared to LSAT substrates lacking surface nanostructures, indicating flatter electronic bands. These findings support the possibility of engineering band structures in YBCO thin films, paving the way toward tuning critical temperatures to higher values.