Why Is the Photon One-Point Function Zero in QED?

taishizhiqiu
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In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore:

upload_2015-10-1_16-44-37.png


I really cannot justify this equation. Can anyone explain it to me?
 
Intuitively, the expression is essentially the diagram with the two fermion lines and vertex (by definition of j_{\mu}). Note the, "neglecting the external photon propagator." If you include the photon propagator as a factor, the expression gives rise to the same diagram as if the current were to be replaced by the photon field, i.e. the photon one-point correlation function.
 

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