  It is common knowledge that a dark soliton can be excited in an ultra-cold
atomic gas by means of the phase imprinting method. We show that, for a
superfluid fermionic mixture, the standard phase imprinting procedure applied
to both components fails to create a state with symmetry properties identical
to those of the dark soliton solution of the Bogoliubov-de Gennes equations. To
produce a dark soliton in the BCS regime, a single component of the Fermi
mixture should be phase imprinted only.
