Unusual Corrections to the Scaling of the Entanglement Entropy of the Excited states in Conformal Field Theory

Abstract: In this paper we study the scaling of the correction of the Renyi entropy of the excited states in systems described, in the continuum limit, by a conformal field theory (CFT). These corrections scale as $L^{-\frac{2\Delta}{n}}$, where $L$ is the system size and $\Delta$ is the scaling dimension of a relevant bulk operator located around the singularities of the Riemann surface $\mathcal{R}_n$. Their name is due to their explicit dependence on the Riemann surface $\mathcal{R}_n$. Their presence has been detected in several works on the entanglement entropy in finite size systems, both in the ground and the excited states. Here, we present a general study of these corrections based on the perturbation expansion on $\mathcal{R}_n$. Some of the terms in this expansion are divergent and they will be cured with addition cut-offs. These cut-offs will determine how these corrections scale with the system size $L$. Exact numerical computations of the Renyi entropy of the excited states of the XX model are provided and they confirm our theoretical prediction on the scaling of corrections. They allow also a comparison with the other works present in the literature finding that the corrections, for the excited states, have the exact same form of the ones of the ground state case multiplied by a model dependend function of $n$ and $l/L$.