1. For one-particle tunneling between normal metal and a superconductor,
show qualitatively how the coherence factors
![$u_{\vec{k}}$](img1.png)
and
![$v_{\vec{k}}$](img2.png)
drop out from the expression for the tunneling current.
For one particle tunneling, one has to create single particle excitations
in a superconductor. It involves electron state
![$\vec{k}\uparrow$](img3.png)
as well as hole state
![$-\vec{k}\downarrow$](img4.png)
in such a way that the
coherence factors
![$u_{\vec{k}}$](img1.png)
and
![$v_{\vec{k}}$](img2.png)
drop out from
the expression for the tunneling current.
In the semiconductor model the superconducting gap is treated as the
band gap of the semiconductor. Howevere, one must remember that an
elementary excitation in a superconductor is a linear combination
of states above as well as below the superconducting gap.