Eigenstate entanglement scaling for critical interacting spin chains

With increasing subsystem size and energy, bipartite entanglement entropies of energy eigenstates cross over from the groundstate scaling to a volume law. In previous work, we pointed out that, when strong or weak eigenstate thermalization (ETH) applies, all or, respectively, almost follow single crossover function. The functions are determined by entropy thermal states assume universal forms in quantum-critical regimes. This was demonstrated field-theoretical arguments analysis large systems non-interacting fermions bosons. Here, substantiate such properties for integrable non-integrable interacting spin-1/2 chains at criticality using exact diagonalization. particular, analyze XXZ transverse-field Ising models with without next-nearest-neighbor interactions. Indeed, can be described function following conformal field theory. Furthermore, validity ETH these models. Even relatively small system sizes that simulated, distributions sharply peaked around corresponding ensembles.