Critical phenomena at the spin-Peierls transition in doped CuGeO3

We have examined the critical properties of several lightly doped samples of CuGeO3 through x-ray diffraction measurements of the order parameter. We obtain an exponent β consistent with that obtained for pure CuGeO3, where β=0.345±0.03, for 0.1% Zn-doped, 0.2% Si-doped, and 0.4% Si-doped samples. Measurements on two 0.1% Cd-doped samples produce results clearly inconsistent with those for the pure compound and well described by mean field behavior. This change in critical phenomena from three-dimensional behavior to mean field behavior is interpreted as a consequence of local strain fields induced by the presence of larger dopant ions. Lattice constant measurements on the same doped samples indicate the presence of spontaneous strains which scale with the square of the order parameter, as was the case for pure CuGeO3.