Origin of current phase relation in Josepshon junction.

[YHFa]

At the superconductor-superconductor point contact regime,
two Andreev bound states carries supercurrent through the S-weak links-S interface.
According to literature, current can be simply expressd
Eq1) I_S=(1/Phi_0)*dE_A/d Phi
here I_S : supercurrent, Phi_0 = flux quantum, E_A : Andreev bound state energy, Phi : Phase difference between two superconductor.
Do you have any idea about the origin of this relation?

 

[DrDu]

Maybe you could provide a reference?