Rapid suppression of the spin gap in Zn-doped ${\mathrm{CuGeO}}_3$ and ${\mathrm{SrCu}}_2$${\mathrm{O}}_3$

The influence of nonmagnetic impurities on the spectrum and dynamical spin structure factor of a model for ${\mathrm{CuGeO}}_3$ is studied. A simple extension to Zn-doped Sr ${\mathrm{Cu}}_2$ ${\mathrm{O}}_3$ is also discussed. Using exact diagonalization techniques and intuitive arguments we show that Zn doping introduces states in the spin-Peierls gap of ${\mathrm{CuGeO}}_3$. 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 ${\mathrm{CuGeO}}_3$ by the tendency to form spin singlets along the rungs in ${\mathrm{SrCu}}_2$${\mathrm{O}}_3$. © 1996 The American Physical Society.