Edge Magnetic Field in the xxz Spin-1/2 Chain

The critical behavior associated with a transverse magnetic field applied at the edge of a semi-infinite xxz S=1/2 chain is calculated using field theory techniques. Contrary to a recent claim, we find that the long-time behavior is given by a renormalization group fixed point corresponding to an infinite field which polarizes the spin at the edge. The zero temperature entropy and position-dependent magnetization are calculated.