# Another one on Lorentz Invariance

I recently read an author making the following argument in QFT:
if <m|A^0(t,0)|n>=B then <m|A^mu(t,0)|n>=(B/p^0)*p^mu by Lorentz invariance. Can anybody tell me under which circumstances this holds and how it comes about? I understand that <m|A^mu(t,0)|n> had to transform as a 4-vector but why should it be the momentum 4-vector?

dextercioby
Who's $A^{\mu}$ ? What does it stand for ?