Observation of anomalous Shapiro steps in ballistic Josephson junctions of InAs nanowires

Advances in semiconductor technology provide the high electric controllability and long mean free path in various kinds of nanostructure. Especially proximity-induced superconductivity in semiconductor nanowire (NW)-superconductor junctions provides intriguing platforms to study exotic superconducting phenomena occurring in the ballistic transport regime.
For example, the quantized conductance due to Majorana Fermions (MFs) [1], and high-efficient Cooper pair splitting [2] have been observed in a superconducting junction of a single NW, and double NW, respectively.
Here we report on observation of anomalous Shapiro steps measured in Josephson junctions (JJs) of InAs NWs. The Shapiro steps appear on not only integer multiples of hf/2e but also the half-integer multiples. We attribute these anomalous steps to the skewed current-phase relation (CPR) expected in the ballistic JJs. Our results contribute to establish the relationship between the CPR and Shapiro step anomalies, which is also important for study of MFs.
[1] H. Zhang, et al., Nature 556, 74–79 (2018)
[2] K. Ueda, et al., Sci. Adv. 5, eaaw2194 (2019)