Half-integer Shapiro steps in strong ferromagnetic Josephson junctions
Ferromagnetic Josephson junctions have been considered to be potential building blocks for quantum-flux qubits. In this talk, I will show the experimental results of half-integer Shapiro steps in the strong ferromagnetic Josephson junction (Nb-NiFe-Nb) by investigating the current-phase relation under radio-frequency microwave excitation. The half-integer Shapiro steps are robust in a wide temperature range from T = 4 to 7 K. The coexistence of 0 and π states in the Josephson junctions with the spatial variation of the NiFe thickness might contribute to the half-integer Shapiro steps and this scenario is supported by the high-resolution transmission electron microscopy characterization. Our results might help the applications of FM Josephson junction-based flux qubit, and for future investigation of the quantum superposition states. *Y.Y., R.C., W.X., Y.M., Y.J., X.-C.X., and W.H. acknowledge the financial support from National Basic Research Programs of China (No. 2019YFA0308401), National Natural Science Foundation of China (No. 11974025), Beijing Natural Science Foundation (No. 1192009), and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB28000000). S.M. is supported by JST CREST Grants (No. JPMJCR19J4, No. JPMJCR1874, and No. JPMJCR20C1) and JSPS KAKENHI (No. 17H02927 and No. 20H01865) from MEXT, Japan. M.M. acknowledges the financial support from JSPS (No. JP20K03810 and No. JP21H04987).