Abstract
Topological insulator (TI) nanowires in proximity to conventional superconductors have been proposed as a tunable platform to realize topological superconductivity and Majorana zero modes. The tuning is done using an axial magnetic flux $\ensuremath{\phi}$ which allows transforming the system from trivial at $\ensuremath{\phi}=0$ to topologically nontrivial when half a magnetic flux quantum ${\ensuremath{\phi}}_{0}/2$ threads the cross-section of the wire. Here, we explore the expected topological transition in TI-wire-based Josephson junctions as a function of magnetic flux by probing the $4\ensuremath{\pi}$-periodic fraction of the supercurrent, which is considered an indicator of topological superconductivity. Our data suggest that this $4\ensuremath{\pi}$-periodic supercurrent is at lower magnetic field largely of trivial origin but that, at magnetic fields above $\ensuremath{\sim}{\ensuremath{\phi}}_{0}/4$, topological $4\ensuremath{\pi}$-periodic supercurrents take over.
Highlights
Majorana zero modes (MZMs), quasiparticle excitations having non-Abelian statistics, were predicted to form at topological superconductor boundaries [1,2,3,4,5] and might be a key component for fault-tolerant quantum computing [6,7]
Semiconductor nanowires with strong spin-orbit interaction in which conventional s-wave superconductors induce topological superconductivity have been the prevailing platform to search for MZMs [9,10,11,12,13]
The drawback is that the Fermi-level must be tuned into a material-dependent narrow gap opened by the Zeeman effect and that, despite great efforts, the MZM could not be proven beyond doubt [14]
Summary
Majorana zero modes (MZMs), quasiparticle excitations having non-Abelian statistics, were predicted to form at topological superconductor boundaries [1,2,3,4,5] and might be a key component for fault-tolerant quantum computing [6,7]. The subband structure of the wire features a gap at kx = 0, with kx the wave vector along the wire Missing Shapiro steps have been found in topologically trivial Josephson junctions [36]. What we contribute here is probing the topological transition in TI-wire-based Josephson junctions with its tunable subband structure by tracing the 4π periodic fraction of the supercurrent as a function of magnetic flux. While the prediction of 4π -periodic Josephson currents is based on microscopic Hamiltonians, the signature of I2π sin(φ) and I4π sin(φ/2) supercurrents on the Shapiro step spectrum depend on the normal resistance and capacitance of the junction. That we observe an increase of I4π /IC with φ with a maximum close to φ ∼ 0.5φ0 is one of the key results of this paper
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