Majorana-like Coulomb spectroscopy in the absence of zero-bias peaks

Hybrid semiconductor-superconductor devices hold great promise for realizing topological quantum computing with Majorana zero modes; 1-5; . However, multiple claims of Majorana detection, based on either tunnelling; 6-10; or Coulomb blockade (CB) spectroscopy; 11,12; , remain disputed. Here we devise an experimental protocol that allows us to perform both types of measurement on the same hybrid island by adjusting its charging energy via tunable junctions to the normal leads. This method reduces ambiguities of Majorana detections by checking the consistency between CB spectroscopy and zero-bias peaks in non-blockaded transport. Specifically, we observe junction-dependent, even-odd modulated, single-electron CB peaks in InAs/Al hybrid nanowires without concomitant low-bias peaks in tunnelling spectroscopy. We provide a theoretical interpretation of the experimental observations in terms of low-energy, longitudinally confined island states rather than overlapping Majorana modes. Our results highlight the importance of combined measurements on the same device for the identification of topological Majorana zero modes.