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.