Universal logarithmic correction to R\'enyi (Shannon) entropy in generic systems of critical quadratic fermions

TL;DR

This paper investigates the universal logarithmic correction to Rènyi (Shannon) entropy in critical quadratic fermion systems, revealing two classes with coefficients linked to the central charge and a new universal number depending on symmetry and the Rènyi index.

Contribution

It classifies the universal logarithmic correction coefficients for a broad class of critical quadratic fermion models, connecting them to conformal field theory parameters.

Findings

Coefficients are proportional to the central charge for systems with U(1) symmetry.

For systems without U(1) symmetry, coefficients depend on the Rènyi index, with a new universal number for α ≤ 1.

Coefficients are shown to be universal and related to the underlying conformal field theory.

Abstract

The R\'enyi (Shannon) entropy, i.e. $Re_{\alpha}(Sh)$, of the ground state of quantum systems in local bases normally show a volume-law behavior. For a subsystem of quantum chains at critical point there is an extra logarithmic subleading term with a coefficient which is universal. In this paper we study this coefficient for generic time-reversal translational invariant quadratic critical free fermions. These models can be parameterized by a complex function which has zeros on the unit circle. When the zeros on the unit circle do not have degeneracy and there is no zero outside of the unit circle we are able to classify the coefficient of the logarithm. In particular, we numerically calculate the R\'enyi (Shannon) entropy in configuration basis for wide variety of these models and show that there are two distinct classes. For systems with $U(1)$ symmetry the coefficient is proportional to the central charge, i.e. one half of the number of points that one can linearize the dispersion relation of the system; for all the values of $\alpha$ with transition point at $\alpha=4$. For systems without this symmetry, when $\alpha>1$ this coefficient is again proportional to the central charge. However, the coefficient for $\alpha\leq 1$ is a new universal number. Finally, by using the discrete version of Bisognano-Wichmann modular Hamiltonian of the Ising chain we show that these coefficients are universal and dependent on the underlying CFT.