Impurity-entanglement in dimerized spin chains

To quantify the entanglement caused by an impurity in an $S=\frac{1}{2}$ dimerized $J_1-J_2$ quantum spin chain, several different entanglement-measures have been utilized. We present the results of variational calculations of the impurity entanglement entropy as well as the negativity for a chain with an impurity attached at one end. We compare the results for both of these measures.