Metamorphosis of Andreev bound states into Majorana bound states in pristine nanowires

We show theoretically that a clean superconducting spin-orbit-coupled nanowire with finite chemical potential has two distinct non-topological regimes as a function of Zeeman splitting: one is characterized by finite-energy in-gap Andreev bound states (ABS), while the other has only extended bulk states. The Andreev bound state regime is characterized by strong features in the tunneling spectra creating a “gap closure” signature. However, no “gap reopening” signature should be apparent above the topological quantum phase transition (TQPT), in agreement with the most recent experimental observations. The gap closure feature is not a signature of the trivial gap of extended bulk states closing at the transition, but rather reflects the coming together of the ABS in systems with high chemical potential. Our theoretical finding establishes the generic intrinsic ABS on the trivial side of the topological quantum phase transition as the main contributors to the tunneling conductance spectra, providing a generic interpretation of existing experiments in clean Majorana nanowires. Our work also explains why experimental tunnel conductance spectra generically have gap closing features below the TQPT, but no gap opening features above it.