The thermal expansion, specific heat and magnetization of CuGe 1- x Si x O 3 have been studied. Although the Néel temperature T N of the D + A F phase, where the antiferromagnetic ( A F ) phase coexists with the dimerized ( D ) one, is proportional to x up to 0.02 in H =0 as in Cu 1- x Zn x GeO 3 , T N of the M + A F phase, where the A F phase coexists with the magnetic ( M ) one above 10 T, suddenly increases with x between x =0.005 and 0.008. The A F properties in the D + A F phase are quite unusual in contrast with the normal ones in the A F state for x =0.02. This is because the A F properties in the D + A F phase are largely influenced by the existence of dimerization as a background, which exists below T SP , and the quantum effect plays an important role in this phenomenon.

The thermal expansion, specific heat and magnetization of CuGe1-xSixO3 have been studied. Although the Neel temperature T-N Of the D + AF phase, where the antiferromagnetic (AF) phase coexists with the dimerized (D) one, is proportional to x up to 0.02 in H = 0 as in Cu1-xZnxGeO3, T-N Of the M + AF phase, where the AF phase coexists with the magnetic (M) one above 10 T, suddenly increases with x between x = 0.005 and 0.008. The AF properties in the D + AF phase are quite unusual in contrast with the normal ones in the AF state for x = 0.02. This is because the AF properties in the D + AF phase are largely influenced by the existence of dimerization as a background, which exists below T-SP, and the quantum effect plays an important role in this phenomenon.

Inelastic neutron scattering have been performed on the spin-Peierls compound CuGeO3 in order to investigate the quantum effect on the magnetic excitation. In one-dimensional Heisenberg antiferromagnet system with S=1/2, the quantum effect is so strong that there is no long range order, and the existence of so-called ‘Spin Wave Continuum’ above the des Cloizeaux-Pearson mode can be expected. In this work, the whole spectrum of the magnetic excitation in the Brillouin zone has been firstly elucidated and it has made clear the quantum effect on the ground-state of the spin-Peierls system.

Raman scattering and infrared absorption spectra of CuGeO3 and Y2BaNiO5 have been measured. CuGeO3 is a typical inorganic spin-Peierls system with S=1/2 and Y2BaNiO5 is a recently discovered Haldane system with S=1. Two magnon excitation along the c*-axis is found for CuGeO3. Additional two-magnon excitations due to one dimensional fluctuation are observed for both system. For CuGeO3 the mode at about 30cm−1 is the soft mode due to the lattice distortion, not due to the spin gap. The normal mode calculations for CuGeO3 show that the crystal structure is stabilized by the Ge-O frame.

Submillimeter wave ESR and far infrared transmission measurements of first inorganic spin-Peierls system CuGeO3 have been performed using the single crystals. Our new results are discussed in connection with the magnetic properties of the spin-Peierls systems and the recent neutron measurements of CuGeO3.

The dimerization in the spin-Peierls state of CuGeO3 has been studied by neutron diffraction under high pressures up to 1.65 GPa. Effects of pressure on the chain direction along the c axis are fairly small; however, the dimerization of Cu along this direction tends to decrease. In comparison with this small change, the effects on the oxygen displacements in the ab plane are rather large. The shift of oxygen atoms decreases also along the a axis; on the other hand, the shift along the b axis increases. These characteristic changes in the dimerization together with those in the bond lengths and bond angles are briefly discussed in connection with the large enhancement of the transition temperature T-sp and also with the decrease in the exchange energy J which were recently observed under high pressure.

Polarization and temperature dependence of Raman scattering spectra of CuGeO3 has been measured from room temperature to 5 K. The observed phonons at room temperature are well assigned by the group theoretical estimation for Pbmm(D-2h(5)). The qualitative change for all possible Raman scattering spectra has not been found from room temperature to 14 K. The energy of the five inter-chain phonons increases with decreasing temperature. Below spin-Peierls transition temperature T-sp( 14 K), five peaks at 27, 107, 230, 370, and 820 cm(-1) have been observed for the (c, c) geometry while the peaks at 370 and 820 cm(-1) appear for (b, c).