We report novel non-linear phenomena in the ab-plane microwave impedance of
YBaCu2O7−δ single crystals. The Rs vs. Hrf data are well
described by the non-linear RSJ model : ϕ˙+sinϕ=irfcosωt. The entire crystal behaves like a single Josephson junction. The
extraordinary coherence of the data suggests an intrinsic mechanism.Comment: 2 pages,1 figure, Submitted to Proc. of M^2SHTSC-V (Beijing), also
available at http://sagar.physics.neu.edu/preprint