Third-order nonlinear microwave response of YBa2Cu3O7−δ thin films and single crystals

The nonlinear microwave response of superconducting YBa2Cu3O7−δ thin films and crystals was studied quantitatively by measuring the third-order harmonic amplitude E3ω (ω: 35 GHz, jRF⩽3×104 A/cm2). We find E3ω to be dominated by two extrinsic effects: (1) the interaction with flux lines in weak links, leading to an increased nonlinear response at low temperatures, and (2) rf related flux line creation at film edges, yielding a high E3ω-signal close to Tc. Both effects together lead to a minimum in E3ω around 40–60 K, which makes this temperature interval interesting for rf applications. Intrinsic nonlinearities are smaller by orders of magnitude and could not be detected.