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

Marco Valentini, Maksim Borovkov, Elsa Prada, Sara Marti-Sanchez, Marc Botifoll, Andrea Hofmann, Jordi Arbiol, Ramon Aguado, Pablo San-Jose, Georgios Katsaros

Published 2022-03-15Version 1

Hybrid semiconductor-superconductor devices hold great promise for realizing topological quantum computing with Majorana zero modes. However, multiple claims of Majorana detection, based on either tunneling or Coulomb spectroscopy, remain disputed. Here we devise an experimental protocol that allows to perform both types of measurements 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 Coulomb spectroscopy and zero bias peaks in non-blockaded transport. Specifically we observe junction-dependent, even-odd modulated, single-electron Coulomb peaks without concomitant low-bias peaks in tunneling spectroscopy. We interpret the experimental observations as the result of low-energy longitudinally-confined island states rather than overlapping Majorana modes. Our method therefore highlights the necessity of combined measurements on the same device for a definite identification of topological Majorana zero modes.