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

TL;DR

This study demonstrates the observation of zero-crossing Shapiro steps in focused-ion-beam-tailored high-$T_c$ superconducting microstructures, revealing insights into their microwave response and phase dynamics at low frequencies.

Contribution

It provides the first experimental evidence of zero-crossing Shapiro steps in Bi-2212 microstructures with tailored plasma frequencies, highlighting the nonsinusoidal current-phase relation.

Findings

Zero-crossing Shapiro steps observed at frequencies as low as 10 GHz.

High-order fractional steps indicate nonsinusoidal current-phase relation.

Phase diffusion effects influence the slope of the Shapiro steps.

Abstract

Microwave response of S-shaped Bi$_2$Sr$_2$CaCu2O$_{8+x}$ (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$$\sim$5 GHz at $T$=0.3 K in naturally stacked Josephson junctions in a crystal. We observed Shapiro steps at frequencies as low as $\sim$5 GHz. Well-developed zero-crossing Shapiro steps were observed at frequencies as low as $\sim$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 $f_p$. 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.