NONMAGNETIC IMPURITIES IN SPIN GAP SYSTEMS

The effect of nonmagnetic impurities in the two-leg Heisenberg ladder system and other spin liquids with excitation gap is discussed. It is shown that the random depletion of spins introduces a random Berry pha se term into the nonlinear sigma model. The classical nature of the an tiferromagnetic correlation is enhanced by the topological decoherence , and the staggered susceptibility shows more singular behavior at low temperatures than the uniform antiferromagnetic Heisenberg chain.