Nonlinear microwave response of epitaxial YBaCuO films of varying oxygen content on MgO substrates

TL;DR

This study explores how the nonlinear microwave response of YBaCuO films varies with oxygen content, revealing the role of the superconducting order parameter and impurity states in MgO at different temperatures.

Contribution

It provides new insights into the relationship between oxygen doping levels and nonlinear microwave properties of YBaCuO films on MgO substrates.

Findings

Nonlinear surface resistance correlates with oxygen content.

Dielectric loss dominates below 20 K, affecting nonlinear response.

Resonant impurity absorption in MgO influences low-temperature behavior.

Abstract

We have investigated the nonlinear microwave properties of electron-beam coevaporated YBaCuO films on MgO, using stripline resonators at 2.3 GHz and temperatures 1.7-80 K. The oxygen content of the films ranged from strongly underdoped to overdoped. Above 20 K, the nonlinear response of the resonators was dominated by the superconductor. We could establish clear correlations between the nonlinear surface resistance, two-tone intermodulation (IMD), and oxygen content of the films, which indicate that the superconducting order parameter is important for the nonlinearities. An exponential rather than a power-law representation of the nonlinear current-voltage relation would be required to explain our results phenomenologically. Below 20 K, the dielectric loss tangent of MgO dominated the nonlinear response of the resonators. With increasing power, the dissipation losses decreased markedly, accompanied by enhanced IMD. The surface reactance passed through a shallow minimum at about 5 K, independent of power. We attribute these effects to resonant absorption by impurity states in MgO.