Negative Differential Conductance in Cooper Pair Box Based on InSb Nanowire

A hybrid semiconductor-superconductor device can be driven into non-trivial topological regime and Majorana zero modes appears at the ends. A typical topological qubit could be constructed with such a hybrid island. Here we fabricated Cooper pair box (CPB) based on InSb island with aluminum. Negative differential conductance (NDC) was observed in the CPB. To figure out the source of NDC, we performed the measurement of the NDC as a function of the junction barrier, chemical potential of the superconducting island and magnetic field. As the junction barrier increases, the NDC moves to a higher bias voltage. Additionally, the NDC is insensitive to the chemical potential of the superconducting island. When the magnetic field increases to the critical magnetic field of aluminum, the NDC gradually disappears, indicating it originates from the superconductivity of island. Futhermore, the NDC does not appear in the bijunction device, which means the island plays a key role. These interesting phenomena provide a new approach to study the physics inside a hybrid island, and potentially instruct the research on topological qubit based on the hybrid island.