Robust low-energy Andreev bound states in semiconductor-superconductor structures: Importance of partial separation of component Majorana bound states

Abstract

Robust topologically trivial low-energy Andreev bound states (ABSs) induced by position-dependent effective potentials have recently come under renewed focus, in light of a remarkable set of experiments observing robust quantized zero-bias conductance plateaus in semiconductor-superconductor heterostructures. We show that (1) the partial spatial separation of the wave functions of the component Majorana bound states (MBSs) is crucial for the creation and stability of topologically trivial near-zero-energy Andreev bound states, (2) the signs of the spin polarizations of the component MBSs can be either the same or opposite, depending on the profile of the inducing potential, and (3) the spin polarizations do not play a fundamental role in generating vastly different coupling strengths to local probes and/or ensuring the robustness of the near-zero-energy ABS. Consequently, in contrast to recent theoretical claims (Vuik et al., arXiv:1806.02801), we find that a robust, quantized zero-bias conductance plateau of height 2e^2/h measured in the topologically trivial regime necessarily requires partially separated ABSs (ps-ABSs), independent of the relative signs of the spin-polarizations. In addition, we show that (4) well-defined energy splitting oscillations involve MBSs characterized by exponential tails pointing toward each other, and (5) ps-ABSs generated by the tunnel barrier itself produce zero-bias conductance peaks with a characteristic width that increases strongly with the applied magnetic field. Finally, we propose (6) a quantitative scheme for analyzing the stability of Majorana modes based on probability distributions of splitting susceptibilities and show that a ps-ABS mode can be remarkably robust when judged based on its signature in a charge tunneling experiment, but, in essence, is topologically unprotected.