Block Entropy of the XYZ Model: Exact Results and Numerical Study

In the first part of this contribution we review the recent analytical results obtained by Ercolessi, Evangelisti and Ravanini regarding the block entropy of the XYZ spin-1/2 model. Using the corner transfer matrix formalism the exact value of the von Neumann entropy for a semi-infinite block whitin an infinite chain is obtained for all possible choices of the exchange couplings, including the fully anisotropic case. Remarkably, it is also possible to compute the block entropy for the quantum sine-Gordon model by taking a suitable scaling limit of the XYZ spin chain. The second part is devoted to a numerical investigation of these results, using the density-matrix renormalisation group algorithm. We discuss the even/odd finite-size effects and argue that the expected asymptotic values are correctly reproduced provided that the ground-state (quasi-)degeneracy is properly taken into account. In fact we show how the amount of entanglement is related to an interplay of the chain length and the symmetry breaking fields.