Nonlinear Microwave Switching Response of Bscco Single Crystals

Measurements of the surface impedance, Zs, in Bi 2 Sr 2 Ca 1 Cu 2 O 8+ single crystal with microwave currents owing along the ^ c axis show clear evidence of a step-like nonlinearity. The surface resistance switches between apparently quantized levels for microwave eld strength changes of less than 1 mG. This non-linear response can arise from the presence of intrinsic Josephson junctions along the ^ c axis of these samples driven by the microwave current. The work described in this paper was motivated by a chance observation during the study of the microwave response of high quality Bi 2 Sr 2 Ca 1 Cu 2 O 8+ (BSCCO) single crystals. While measuring the surface impedance, Z s (T), of these crystals, an unusual step-like switching feature was observed in the cavity resonances. (See Fig. 1) These eeects are only observed in the presence of microwave currents induced along the ^ c axis, and seem qualitatively consistent with the underdamped response of intrinsic Josephson junctions (JJs) in these samples. The measurements are performed in a Nb cavity 1 operating in the TE 011 mode at 10 GHz which is held at xed temperature, T 4.2 K. The sample sits on a sapphire rod at the center of the cavity so that its temperature can be varied independent of that of the cavity. The microwave eld at the center of the cavity induces circulating microwave currents in the sample. By orienting the sample ^ c axis k H rf these currents ow entirely in the ab plane, while for ^ c ? H rf current ows along the ^ c axis as well as in the ab plane. All of the measurements reported here are for the latter geometry, and the ^ c axis response dominates due to the large anisotropy. The sample surface impedance, Z s = R s +iX s , is typically determined from changes in the quality factor and resonance frequency of the perturbed cavity