On relation between unitary gauge and gauge given by $\xi$-limiting   process
  It is shown that in general the gauge given by the limit of $\xi \to 0$ in $R_\xi$ gauge does not necessarily agree with the unitary gauge by examining the symmetry breaking of two steps from ${\rm SU(3) \otimes U(1)}_N$ to ${\rm SU(2) \otimes U(1)}_Y$ and then to ${\rm U(1)}_{em}$. 
T. Kiyan, T. Maekawa, M. Masuda and H. Taira