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

The nonlinear microwave response of superconducting YBa${}_2$ Cu ${}_3$ O${}_{\text{7−δ}}$ thin films and crystals was studied quantitatively by measuring the third-order harmonic amplitude $E_{\text{3ω}}$ $\mathrm{(\omega }$ : 35 GHz, $j_{\mathrm{RF}}{\text{⩽3×10}}^4$ A/cm${}^2)$ . We find $E_{\text{3ω}}$ 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 $E_{\text{3ω}}$ -signal close to $T_c.$ Both effects together lead to a minimum in $E_{\text{3ω}}$ 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.