The compact U(1) lattice gauge theory is described by the action
$$S_0=-\frac{1}{g^2}\sum_\square \cos\left(\sum_{l\in\partial \square}A_l\right),$$
where the gauge connection $A_l\in$U(1) is defined on the link $l$. I was told that this theory in 2+1D spacetime is dual to a U(1) XY model on the dual lattice, described by the following action
$$S_1=-\chi\sum_l \cos\left(\sum_{i\in\partial l}\theta_i\right)-K\sum_i\cos(\theta_i),$$
where the XY variable $\theta_i\in$U(1) is defined on the dual site $i$. It was said that the K term in the action is to take into account the instanton effect in the compact U(1) lattice gauge theory (which I don't understand). However when I tried to derive the the dual theory, I arrived at the following integer XY model (or height model?)
$$S_2=-\chi\sum_l \cos\left(\sum_{i\in\partial l}m_i\right),$$
with the integer variable $m_i\in\mathbb{Z}$ defined on the dual site $i$. Because the Pontryagin dual group of U(1) is simply $\mathbb{Z}$ but not U(1), so I believe that the U(1) gauge theory $S_0$ should dual to an integer XY model $S_2$, and this duality is exact. But every book or paper that I have encountered did not mention anything about $S_2$, instead they all point to the U(1) XY model $S_1$. Therefore I was forced to conjecture that the integer XY model is equivalent to the U(1) XY model with additional K term. Can anyone tell me if my conjecture is correct or not? How to go from $S_2$ to $S_1$ (or maybe directly from $S_0$ to $S_1$)? How is the K term being added? What is the physical meaning of the K term?

This post imported from StackExchange Physics at 2014-04-05 17:26 (UCT), posted by SE-user Everett You