Spin-Peierls State versus Néel State in Doped CuGeO3

The competition between nonmagnetic spin-Peierls (SP) and magnetic Néel ground states has been investigated in CuGe${\mathrm{O}}_3$ doped with Zn, Ni, Mn, and Si. The analysis of specific heat, $C_p$, data in CuGe${\mathrm{O}}_3$, measured as a function of magnetic field, supports a SP transition at $T_{\mathrm{SP}}=14\mathrm{K}$. The replacement of C${\mathrm{u}}^{2+}$ by Z${\mathrm{n}}^{2+}$, N${\mathrm{i}}^{2+}$, or M${\mathrm{n}}^{2+}$ reduces $T_{\mathrm{SP}}$ and stabilizes a Néel state, not a spin-glass phase as previously suggested. No significant difference in $C_p$ was observed for Zn- and Ni-doped samples. We suggest that the Néel state depends on $|S^{\prime }-S|$, where $S^{\prime }$ is the spin of the dopant and $S=\frac{1}{2}$.