Robust low energy Andreev bound states in semiconductor superconductor structures: Importance of partial separation of component Majorana bound states
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 recent experiments observing robust quantized zero bias conductance plateaus in semiconductor superconductor (SM SC) heterostructures. We clarify the mechanisms for the formation of such states in SM SC hybrid nanowires under the application of a parallel magnetic field. Specifically, 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 robustness of topologically trivial near zero energy ABSs, (2) this partial separation can be induced by monotonic smooth confinement potentials or by non monotonic valley or hill potentials, which can be induced by potential gates or by a position dependent work function difference, (3) 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, (4) these 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, and hence, in contrast to the claims in a recent theoretical work (Ref. [51] (5) 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.
Physical Review B