Logarithmic Corrections in Quantum Impurity Problems

The effect of a BULK marginal operator on BOUNDARY critical phenomena in two space-time dimensions is considered. The particular case of an open S=1/2 antiferromagnetic Heisenberg chain, corresponding to a Wess-Zumino-Witten non-linear sigma model, is solved. In this case, the needed renormalization group coefficient is associated with a novel operator product expansion in which THREE operators approach the same point. Resulting logarithmic corrections occurring in finite size calculations and nuclear magnetic resonance experiments are discussed.