Magnetic-Field and Pressure Effects on the Magnetic Excitations in the Disordered Spin-Peierls Systems

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1. Raman scattering in CuGeO_3(1) In Zn-, Si- and Ni-doped CuGeO_3, we studied Raman scattering from the folded phonon, which was folded onto Γ point from the Brillouin zone boundary by the lattice dimerization, and found that the doping suppressed the spin-Peierls (SP) transition and reduced the lattice distortion. We also observed it even in the antiferromagnetic phase of the lightly doped samples, indicating the existence of the lattice dimerization in this phase. In the heavily doped samples its spectrum broadened with a higher-energy tail, indicating the existence of the uniform antiferromagnetic phase possessing a short-range lattice dimerization.(2) Breakdown of the selection rule in the doped CuGeO_3 enabled us to observe a SP gap excitation. It decreased in energy and broadened with increasing dopant concentration. The two-magneticexcitation bound state changed into a resonant state and finally disappeared with increasing dopant concentration. These features were demonstrated … More theoretically by using Green's function, taking into account the shortening of the lifetime of the magnetic excitations.(3) We studied the magnetic excitations in the incommensurate phase under high magnetic fields. In the incommensurate phase the SP gap excitation changed to a soliton-like excitation whose frequency increased with increasing magnetic field. On the other hand, the intensity of the two-magnetic-excitation bound/resonant state rapidly decreased.(4) High-pressure equipment with a diamond anvil cell has been installed in order to study the high-pressure effect on the SP transition by means of Raman spectroscopy. We found successive structural phase transitions to the pyroxene-type structures under strictly hydrostatic pressures at room temperature. The pyroxene-type structures have already been reported to emerge in pure crystal only under quasihydrostatic pressures.(5) It is well known that the SP phase disappears in heavily doped CuGeO_3. We demonstrated by observing the folded phonon that the SP phase revised in heavily Mg-doped crystal when the pressure was applied to it at low temperatures.2. Raman scattering in α'-NaV_2O_5(1) We observed three folded phonons and the first- and second-order Raman spectra from magnetic excitations in the low-temperature phase. We inferred the lattice symmetry of the low-temperature phase from the results of the polarization characteristics. The Na^+-ion deficiency in α'-Na_<1-δ>V_2O_5 cut the spin chain, and it suppressed the phase transition and reduced the lattice distortion. We compared these results with CuGeO_3.(2) We observed the Fano effect between the V-O bending phonon and the d - d electronic Raman band of the V^<4+> ion. This effect weakened with increasing concentration of the Na^+-ion deficiency. These spectra were demonstrated theoretically, taking into account a phonon-phonon relaxation process. Less