Spin-Peierls transition in CuGeO₃and impurity-induced ordered phases in low-dimensional spin-gap systems

The research on spin-Peierls transition and impurity-induced antiferromagnetic state(s) in CuGeO 3 is reviewed. In particular the recent progress of the studies of the compositional phase diagram will be discussed. The discovery of two antiferromagnetic phases in Mg-doped CuGeO 3 (Masuda T et al 1998 Phys. Rev. Lett. 80 4566) opened a new aspect of this kind of research. The transition between dimerized antiferromagnetic and uniform antiferromagnetic phases is of the first order as a function of the impurity concentration. This phenomenon was studied in detail by the susceptibility, neutron diffraction, synchrotron x-ray diffraction and other methods. The existence of this transition was firmly confirmed and it was revealed that the impurity-induced phase diagram of CuGeO 3 is very complex. Another important problem is whether the impurity-induced ordered phase in the spin-gap system is unique to the S = ½ spin-gap system or not. As to this problem a new Haldane-gap compound PbNi 2 V 2 O 8 was found (Uchiyama Y et al 1999 Phys. Rev. Lett. 83 632). This compound has relatively strong inter-chain interaction. It was also found that the substitution of Mg 2+ ions ( S = 0) for Ni 2+ ions ( S = 1) induces antiferromagnetic phase in PbNi 2 V 2 O 8 . From these experimental results it was shown that the occurrence of the impurity-induced ordered phase is a common feature of quasi-one-dimensional spin systems with relatively strong inter-chain interaction.