Zero-crossing Shapiro steps in high- T_c superconducting microstructures tailored by a focused ion beam

Microwave response of S-shaped Bi₂Sr₂CaCu₂O_8+x (BI-2212) micron-scale samples, in which the supercurrent was forced to flow perpendicular to the crystal layers, was investigated. A treatment with a focused ion beam allowed us to reduce the plasma frequency down to f_p∼5 GHz at T=0.3 K in naturally stacked Josephson junctions in a crystal. We observed Shapiro steps at frequencies as low as ∼5 GHz. Well-developed zero-crossing Shapiro steps were observed at frequencies as low as ∼10 GHz. They appeared as constant-voltage plateaus with a nonzero voltage occurring at zero bias current. We confirmed that zero-crossing Shapiro steps in the Bi-2212 stacked junctions can be observed when the irradiated frequency is sufficiently larger than f_p. The observed high-order fractional steps in the microwave responses indicate that the interlayer-coupled Bi-2212 Josephson junctions have nonsinusoidal current-phase relation. Based on the temperature dependence of the steps, we also showed that the finite slope of the steps is due to the enhancement of the phase-diffusion effect.