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 extension to Zn-doped Sr Cu₂ O₃ is also discussed. Using exact diagonalization techniques and intuitive arguments we show that Zn doping introduces states in the spin-Peierls gap of CuGeO₃. This effect can be understood 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. It is interesting to observe that the spin correlations along the chains are enhanced by Zn doping according to the numerical data presented here. As recent numerical calculations have shown, similar arguments apply to ladders with nonmagnetic impurities simply replacing the tendency to dimerization in CuGeO₃ by the tendency to form spin singlets along the rungs in SrCu₂O₃.