Generalized Gravitational Entropy from Various Matter Fields
  The generalized gravitational entropy proposed in recent by Lewkowycz and Maldacena [1] is extended to the systems of Boson fields, Fermion fields and Maxwell fields which have arbitrary frequency and mode numbers on the BTZ spacetime. We find the associated regular wave solution in each case and use it to calculate the exact gravitational entropy. The results show that there is a threshold frequency below which the Fermion fields could not contribute the generalized gravitational entropy. Also, the static and zero-mode solutions have no entropy, contrast to that in scalar fields. We also find that the entropy of the static scalar fields and non-static fermions is an increasing function of mode numbers and, after arriving the maximum entropy, it becomes a deceasing function approaching to a constant value. We calculate the gravitational entropy of Maxwell fields and use the duality between EM and scalar fields to explain its result. The gravitational entropy from DBI action is also studied. 
Wung-Hong Huang
hep-th gr-qc