  The asymptotic dynamical correlation functions in one-dimensional spin chains
are described by power-laws. The corresponding exponents characterize different
bulk and boundary critical behavior. We present novel results for the
logarithmic contribution to the boundary correlations of an isotropic
Heisenberg chain. The exponent of the logarithm, $\lambda=1$, is derived using
a renormalization group technique. We confirm our analytical results by
comparing with numerical quantum Monte Carlo data.
