  For the pristine phosphorene nanoribbons (PNRs) with edge states, there exist
two categories of edge bands near the Fermi energy (EF), i.e., the
shuttle-shaped twofold-degenerate and the near-flat simple degenerate edge
bands. However, the usual experimental measurement may not distinguish the
difference between the two categories of edge bands. Here we study the varying
rule for the edge bands of PNRs under an external electrostatic field. By using
the KWANT code based on the tight-binding approach, we find that the
twofold-degenerate edge bands can be divided into two separated shuttles until
the degeneracy is completely removed and a gap near EFis opened under a
sufficiently strong in-plane electric field. Importantly, each shuttle from the
ribbon upper or lower edge outmost atoms is identified according to the local
density of states. However, under a small off-plane field the shuttle-shaped
bands are easily induced into two near-flat bands contributed from the edge
atoms of the top and bottom sublayers, respectively. The evidence provides the
edge and sublayer degrees of freedom (DOF) for the PNRs with shuttle-shaped
edge bands, of which is obviously different from another category PNRs
intrinsically with near-flat edge bands. This is because that the former
category of ribbons solely have four zigzag-like atomic configurations at the
edges in each unit cell, which also results in that the property is robust
against the point defect in the ribbon center area. As an application,
furthermore, based on this issue we propose a homogenous junction of a
shuttle-edge-band PNR attached by two electric gates. Interestingly, the
transport property of the junction with field manipulation well reflects the
characteristics of the two DOFs. These findings may provide a further
understanding on PNRs and initiate new developments in PNR-based electronics.
