Raman-scattering study of the anomalous spin-Peierls state in heavily Mg-doped CuGeO3 under pressure -: art. no. 054412

By means of Raman scattering we have observed the appearance of the folded phonon modes and the two-magnetic-excitation bound state under high pressures in 3.5% Mg-doped CuGeO3 crystals. The spin-Peierls (SP) gap mode is also observed. These facts indicate clear evidence that the SP phase transition extinguished by Mg doping is revived by the application of high pressure. The revived SP state has a large spin gap and a very weak lattice dimerization when compared with pure sample, which is interpreted in terms of the strongly frustrated spin chain with a spontaneous spin gap. Moreover, a new phonon peak is observed above 2.8 GPa in the SP phase and even far above the SP transition temperature, indicating a pressure-induced structural phase transition. We propose a P-T phase diagram for this sample.