Impurities in S=1/2 Heisenberg antiferromagnetic chains: Consequences for neutron scattering and knight shift.

Non-magnetic impurities in an S=1/2 Heisenberg antiferromagnetic chain are studied using boundary conformal field theory techniques and finitetemperature quantum Monte Carlo simulations. We calculate the static structure function, Simp(k), measured in neutron scattering and the local susceptibility, χi measured in Knight shift experiments. Simp(k) becomes quite large near the antiferromagnetic wave-vector, and exhibits much stronger temperature dependence than the bulk structure function. χi has a large component which alternates and increases as a function of distance from the impurity. 75.10.Jm,75.20.Hr Typeset using REVTEX 1 Although the spin chain problem has been a popular topic for theoretical physicists since the early days of quantum mechanics, the correlation functions of the antiferromagnetic Heisenberg spin-1/2 chain could only be calculated with the help of modern quantum field theory [1]. Adding non-magnetic impurities to a spin-chain compound breaks the chains up into finite sections with essentially free boundary conditions. The correlation functions in the presence of such a boundary were calculated only rather recently [2]. These results provide a simple application of a general theory of conformally invariant boundary conditions which has been applied to a wide variety of quantum impurity problems in condensed matter and particle physics [3]. These functions exhibit a universal dependence on the boundary, at long distances and times. In this paper we wish to focus on a couple of applications of these results of experimental relevance: the impurity contribution to the static structure function, Simp(k, T ), and the local susceptibility, χi(T ). We derive analytic expressions for these quantities using field theory methods and compare them with finite-T Monte Carlo simulations using lengths of up to l = 128 with a varying number of time-steps up to 64 and several hundred thousand sweeps through each lattice. The Heisenberg Hamiltonian for the antiferromagnetic spin-1/2 chain H = J ∑