Kondo effect in XXZ spin chains

The Kondo effect in a one-dimensional spin-1/2 XXZ model in the gapless XY regime (-1 < Delta less than or equal to 1) is studied both analytically and numerically. In our model an impurity spin (S = 1/2) is coupled to a single spin in the XXZ spin chain. Perturbative renormalization-group (RG) analysis is performed for various limiting cases to deduce low-ene;gy fixed points. It is shown that in the ground state the impurity spin is screened by forming a singlet with a spin in the host XXZ chain. In the antiferromagnetic side (0 < Delta less than or equal to 1) the host chain is cut into two semi-infinite chains by the singlet. In the ferromagnetic side (-1 < Delta < 0); on the other hand, the host XXZ chain remains as a single chain through "healing" of a weakened bond;in the low-energy (long-distance) limit. The density-matrix renormalization-group method is used to study the size scaling of finite-size energy gaps and the power-law decay of correlation functions in the ground state. The numerical results are in good agreement with the predictions of the RG analysis. Low-temperature behaviors of specific heat and susceptibility are also discussed.