Spin-Peierls and antiferromagnetic phases in Cu1−xZnxGeO3: A neutron-scattering study

Comprehensive neutron-scattering studies were carried out on a series of high-quality single crystals of Cu${}_{1-x}$Zn${}_x$GeO${}_3$. The Zn concentration $x$ was determined for each sample using electron probe microanalysis. The measured Zn concentrations were found to be 40–80 % lower than the target values. Nevertheless, the measured concentrations cover a wide range which enables a systematic study of the effects due to Zn doping. We have confirmed the coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low temperatures and the measured phase diagram is presented. Most surprisingly, long-range AF ordering occurs even in the lowest available Zn concentration $x=0.42\%$, which places important constraints on theoretical models of the AF-SP coexistence. Magnetic excitations are also examined in detail. The AF excitations are sharp at low energies and show no considerable broadening as $x$ increases, indicating that the AF ordering remains long ranged for $x$ up to 4.7%. On the other hand, the SP phase exhibits increasing disorder as $x$ increases, as shown from the broadening of the SP excitations as well as the dimer reflection peaks.