Stefano Evangelisti - Università di Bologna #
Essential Singularity in the Renyi Entanglement Entropy of the 
1d XYZ spin-1/2 chain #
We study the  Renyi entropy of the 1-d XYZ spin-1/2 chain in 
the entirety of its phase diagram. The model has several quantum 
critical lines corresponding to rotated XXZ chains in their 
paramagnetic phase, and four tri-critical points where these 
phases join.<br/>
Two of these points are described by a  conformal FT and close 
to them the entropy scales as the logarithm of its mass gap (BKT 
PT).<br/>
The other two points are  not conformal and the entropy has a 
peculiar singular behavior in their neighbors, characteristic of 
an essential singularity (First order PT). Depending on the 
approach to these points, the entropy can take any positive 
value from 0 to &infin;. We propose  the entropy as an 
efficient tool to determine the nature of a PT