Zero-crossing Shapiro steps in focused-ion-beam-tailored high-Tc superconducting microstructures
Abstract
Microwave response of S-shaped Bi2Sr2CaCu2O8+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 fp5 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 non-zero 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 fp. 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.