Symmetry breaking and nonlinear electrodynamics in the ceramic superconductor Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

For bulk samples of the ceramic superconductor Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ in a radio-frequency magnetic field and in a dc magnetic field $H_0$ we observe novel nonlinear behavior including very-high-order odd-harmonic generation if $H_0=0\mathrm{}$. If $H_0\ne 0$, there is additional even-harmonic generation. The even-harmonic power displays a very sharp dip at $H_0\sim 0$, of width Å 100 μG. This and related behavior can be semiquantitatively understood by modeling the system as a suitably averaged collection of flux-quantized supercurrent loops containing Josephson junctions.