content_image/347945.jpg content_image/347952.jpg content_image/347955.jpg content_image/347953.jpg content_image/347954.jpg content_image/347956.jpg content_image/347957.jpg
and finally: 

where ⃗ r = ⃗ r$_{2}$ − ⃗ r$_{1}$ . 

<img src='content_image/347945.jpg'>

<img src='content_image/347952.jpg'>

To evaluate the coherence length for both the magnetic and the electric order parameters we need to calculate 〈 δm$_{-k}$δm$_{k}$ 〉 and 〈 δp$_{-k}$δp$_{k}$ 〉 . Using (A7) and (A8) we obtain the following expressions: 

<img src='content_image/347955.jpg'>

<img src='content_image/347953.jpg'>

<img src='content_image/347954.jpg'>

The average values 〈 δm$_{-k}$δm$_{k}$ 〉 and 〈 δp$_{-k}$δp$_{k}$ 〉 can be calculated using the matrix A$_{k}$ defined in the Appendix A: 

<img src='content_image/347956.jpg'>

<img src='content_image/347957.jpg'>

