Why does my Feynman diagram get distorted?

documentclass[letterpaper,aps,floatfix,twocolumn]{revtex4}
%documentclass{article}
%
usepackage{graphicx}
usepackage{grffile}
usepackage{epsfig}
usepackage{amsmath}
usepackage{amssymb}
usepackage{hyperref}
usepackage{alltt}
usepackage{setspace}
usepackage[english]{babel}
usepackage{euscript}
usepackage{tikz}
usepackage{yfonts}
usepackage{tikz-feynman}
usepackage[utf8]{inputenc}
usepackage{feynmf}
usepackage{natbib}
usepackage{theoremref}
usepackage{amsthm}


begin{document}

feynmandiagram [horizontal= a to be] 
{ i1 -- [scalar] a[dot] -- [scalar] i2, 
a -- [photon, half left] b, 
b --[photon, half left] a, 
f1 -- [scalar] b -- [scalar] f2, };

end{document}

I tried the above code under TikZ.

However, the diagram I get looks like the rightmost diagram…..

I cannot understand why the diagram looks so weird…..Maybe because I am writing on the paper on two columns….

I tried everything but the diagram wouldn’t change and it is extremely frustrating…

Could anyone please help me?