Even to Odd Fermion Parity Transition in Majorana Islands based on Al-InSb Hybrid System

Different topological qubit schemes, based on one-dimensional nanowires with proximity-induced superconductivity, have been proposed. Most of them consist of multiple superconducting islands in the Coulomb blockade regime, each hosting pairs of Majorana zero modes (MZMs). We build such Majorana islands in InSb nanowires with epitaxial Al shells. The measurement of Coulomb diamonds reveals 2e periodicity with respect to the gate voltage, originating from the even fermion parity being protected by a hard superconducting gap. When a magnetic field is applied parallel to the nanowire, the 2e periodicity gradually changes to 1e-periodic oscillations with alternating regions of even and odd fermion parity. This behavior is related to the appearance of low-energy states, signalling either the appearance of MZMs in the topological regime or the presence of non-topological Andreev bound states. By sweeping across large ranges of plunger gate and tunneling gate voltages, we found that some features are very sensitive to the gate settings, and thus easily construed in terms of localized Andreev bound states. In other cases, the 1e oscillations are more robust and accompanied by an excitation gap in finite bias measurements, which could be interpreted as due to the presence of MZMs.