Critical behavior of lattice Schwinger model with topological term at   $\theta=\pi$ using Grassmann tensor renormalization group
  Lattice regularized Schwinger model with a so-called $\theta$ term is studied by using the Grassmann tensor renormalization group. We perform the Lee-Yang and Fisher zero analyses in order to investigate the phase structure at $\theta=\pi$. We find a first order phase transition at larger fermion mass. Both of the Lee-Yang zero and Fisher zero analyses indicate that the critical endpoint at which the first order phase transition terminates belongs to the Ising universality class. 
Yuya Shimizu and Yoshinobu Kuramashi
hep-lat cond-mat.str-el hep-th