RAPID SUPPRESSION OF THE SPIN GAP IN ZN-DOPED CUGEO3 AND SRCU2O3

The influence of nonmagnetic impurities on the spectrum and dynamical spin structure factor of a model for CuGeO3 is studied. A simple exten sion to Zn-doped SrCu2O3 is also discussed. Using exact diagonalizatio n techniques and intuitive arguments we show that Zn doping introduces states in the spin-Peierls gap of CuGeO3. This effect can be understo od easily in the large dimerization limit where doping by Zn creates ' 'loose'' S=1/2 spins, which interact with each other through very weak effective antiferromagnetic couplings. When the dimerization is small , a similar effect is observed but now with the free S=1/2 spins being the resulting S=1/2 ground state of severed chains with an odd number of sites. Experimental consequences of these results are discussed. I t is interesting to observe that the spin correlations along the chain s are enhanced by Zn doping according to the numerical data presented here. As recent numerical calculations have shown, similar arguments a pply to ladders with nonmagnetic impurities simply replacing the tende ncy to dimerization in CuGeO3 by the tendency to form spin singlets al ong the rungs in SrCu2O3.