  In recent work, we discussed the difference between electrons and holes in
energy band in solids from a many-particle point of view, originating in the
electron-electron interaction[1], and from a single particle point of view,
originating in the electron-ion interaction[2]. We proposed that
superconductivity in solids only occurs when the Fermi level is close to the
top of a band (hole carriers), that it originates in `undressing' of carriers
from $both$ the electron-electron and the electron-ion interaction, and that as
a consequence holes in the normal state behave like electrons in the
superconducting state[3]. However, the connection between both undressing
effects was left unclear, as was left unclear how the transformation from hole
behavior to electron behavior occurs. Here we clarify these questions by
showing that the same electron-electron interaction physics that promotes
pairing of hole carriers and undressing of carriers from the electron-electron
interaction leads to undressing of carriers from the electron-ion interaction
and transforms the behavior of carriers from hole-like to electron-like.
Furthermore this phenomenon is connected with the expulsion of negative charge
that we predict to occur in superconductors. These unexpected connections
support the validity of our theoretical framework, the theory of hole
superconductivity, to explain superconductivity in solids.
