Researchers could not confirm that a feature that supposedly signals the presence of Majorana bound states - the unusual quasiparticles that may become the cornerstone of topological quantum computing - was in fact due to elusive Majorana particles, in full-shell semiconductor/superconductor nanowires. Rather, this feature, known as zero bias conductance peak, can arise from another quantum phenomenon in these hybrid nanowire structures, the authors say. In recent years, intense research has been conducted on nanowire-based semiconductor-superconductor hybrid systems because predictions suggest that a topological superconductor state with Majorana zero modes (MZMs) can be engineered from them. Even though several experiments in such platforms have reported on signatures compatible with MZMs, the interpretation that they demonstrate the presence of Majorana particles has been challenged; studies have shown that a quantum state known as the Andreev bound state - which can appear inside the superconducting gap of such structures - can mimic MZMs. A 2020 study in Science introduced an alternative platform for realizing topological superconductivity and thus for finding Majorana bound states. It proposed Majorana modes could be found in a semiconducting nanowire fully wrapped by a superconducting shell. Here, Marco Valentini and colleagues report experiments with such nanowire systems. Instead of confirming MZMs as they had hoped, however, the authors found that the zero-bias peaks they observed were a product of Andreev levels in their system. According to Valentini et al., the findings did help to constrain crucial experimental parameters for evaluating topological superconductivity in this system.
