Boundary Quantum Entanglement of the XXZ Spin Chain with Boundary Impurities

The boundary quantum entanglement for the s=1/2 XXZ spin chain with boundary impurities is studied via the density matrix renormalization group (DMRG) method. It is shown that the entanglement entropy of the boundary bond (the impurity and the chain spin next to it) behaves differently in different phases. The relationship between the singular points of the boundary entropy and boundary quantum critical points is discussed.