Entanglement in fermionic chains with interface defects



We study the ground-state entanglement of two halves of a critical transverse Ising chain, separated by an interface defect. From the relation to a two-dimensional Ising model with a defect line we obtain an exact expression for the continuously varying effective central charge which governs the asymptotic behaviour of the entanglement entropy. The result is relevant also for other fermionic chains.