O. Fujita

Electron Spin Resonance of Spin-Peierls Material CuGeO3

Magnetization and submillimeter wave ESR measurements have been performed on single crystals of CuGeO 3 using a pulsed high magnetic field. Magnetization jumps are observed at 12.8, 12.5 and 13.5 T for a -, b - and c -axes, respectively, at 4.2 K. The temperature dependences of the linewidth and the integrated intensity of the ESR signal are discussed in connection with the conventional spin-Peierls systems, and the excitation gap against the magnetic field is estimated when the field is applied along the a -axis. ESR has been observed in the magnetic phase above the critical field H c and the g -values have different values from those in the spin-Peierls phase. The origin of this difference is discussed.

Submillimeter Wave ESR Study of Spin Gap Excitations in CuGeO3.

Transitions between the ground singlet state and the excited triplet state have been observed in CuGeO 3 by means of submillimeter wave electron spin resonance. The energy gap at zero field is evaluated to be 570 GHz (2.36 meV) and this value is nearly identical to the gap at the zone center observed by inelastic neutron scattering. The absorption intensity shows strong field orientation dependence, but it shows no significant dependence on magnetic field intensity. These features have been explained by considering the existence of Dzyaloshinsky-Moriya (DM) antisymmetric exchange interaction. The doping effect on this singlet-triplet excitation has been also studied. A drastic broadening of the absorption line is observed by the doping of only 0.5 % of Si.

Magnetic phase diagram and antiferromagnetic resonance in

Magnetization and ESR measurements have been performed on Si-doped (y = 0.01 and 0.02) single crystals. The y = 0.01 sample shows a spin-Peierls transition at as well as undoped material and additionally show an antiferromagnetic state below . When an external field is applied to the c-axis below , two magnetization jumps due to the spin-flop and the transition from dimerized spin-Peierls phase to magnetic phase are observed at 0.98 T and 11.5 T, respectively, which means the coexistence of spin-Peierls and antiferromagnetic states. On the other hand, the y = 0.02 sample shows only an antiferromagnetic transition at .

Effect of pressure on the magnetic susceptibility in spin-Peierls cuprate, CuGeO3

Abstract AC susceptibility and magnetic balance measurements have been performed on a single crystalline CuGeO 3 under high pressure up to 8 GPa and 1.1 GPa, respectively. The spin-Peierls (SP) transition temperature ( T SP ) increases linearly with increasing pressure at a rate of 4.8 K/GPa. Curie-Weiss fitting was made for the susceptibility above T SP . Ferromagnetic transition was observed above 4.5 GPa at around 10 K.

The Observation of g-Shifts in Spin-Peierls Material CuGeO3 by Submillimeter Wave ESR

The g -shifts in spin-Peierls (SP) and magnetic (M) phases of the inorganic spin-Peierls material CuGeO 3 have been observed by submillimeter wave ESR. The temperature dependence of the g -shifts from U phase to SP or M phase showed a continuous change. Since such g -shifts were also observed in the organic spin-Peierls material, for example TTF-AuBDT, such g -shifts can be considered the universal phenomena due to the lattice distortion in the spin-Peierls materials. But, in the case of CuGeO 3 , the g -shifts are so large that we cannot explain them in the framework of the point charge model with considering the continuous lattice distortion.

Neutron Scattering Studies on Spin-Peierls Material CuGeO3

Crystal structure and phonon dynamics up to 150 meV of the spin-Peierls material CuGeO 3 have been investigated by neutron scattering experiments. We found a large lattice anomaly in the a-b plane, vertical to the one-dimensional antiferromagnetic spin chain, associated with additional diffraction peaks in the b * -axis at temperatures much higher than the spin-Peierls transition temperature. The phonon modes of the chain oxygens at 100 meV show consistent evolution with diffraction. Therefore, we conclude that the structural evolution of the chain oxygens in the a-b plane, which mediates the spin-spin interaction along the c -axis, drives the spin-Peierls transition.

Raman scattering of CuGeO3

Polarization and temperature dependence of Raman scattering spectra of CuGeO 3 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 5 2h ). 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 ).

TEMPERATURE DEPENDENCE OF THE SPIN-PEIERLS ENERGY GAP AND ANOMALOUS LINE SHAPES IN CUGEO3

Neutron-scattering measurements on a large single crystal of CuGeO{sub 3} have been used to determine the temperature dependence of the spin-Peierls energy gap. While the power-law behavior of the intensity of structural superlattice peaks is well fit by {ital I}({ital T}){proportional_to}({ital T}{sub {ital c}}{minus}{ital T}){sup 2{beta}} with an exponent of {beta}=0.33, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as {Delta}({ital T}){proportional_to}{ital I}{sup {ital a}} with {ital a}=1/3; the present results suggest an exponent of {ital a}{approx_equal}1/6 for CuGeO{sub 3}. An additional scattering cross section is observed in constant-{ital q} and constant-{ital E} scans creating a long tail extending to higher energies relating to a proposed scattering continuum. {copyright} {ital 1996 The American Physical Society.}

Raman Scattering Study of Phonon and Magnetic Excitations along the Magnetic Chain in CuGeO3

Detailed temperature dependence of ( c , c ) Raman scattering spectra has been measured for CuGeO 3 in the vicinity of spin-Peierls transition temperature ( T sp ). Below T sp , six peaks at 30, 105, 227, 370, 416, and 817 cm -1 have been observed. We newly observed a peak at 416 cm -1 . The energy of the 30 cm -1 peak decreases continuously with increasing temperature and becomes zero above T sp . The peak at 30 cm -1 is assigned as the soft mode in the spin-Peierls phase from the agreement with the temperature dependence of the super-lattice reflection intensity. The peak at 227 cm -1 is assigned as a two-magnon along the c -axis (Cu chain direction). The peaks at 370, 416, and 817 cm -1 are assigned as the phonons, which appear by the symmetry change. The new peaks of ( c , c ) are not sensitive to the magnetic field along the a -axis below 9 T at 4.7 K.

Dopant-bound spinons in Cu1-xZnxGeO3

Polarized inelastic light scattering experiments on Cu1 − xZnxGeO3 (0 ≤ x ≤ 0.045) single crystals show for x ≠ 0 a new distinct mode at nearly half the energy of the singlet response below the spin-Peierls transition. The temperature, magnetic field, polarization, and doping dependences of this mode are similar to those of the singlet bound state. The data are interpreted in terms of a spinon-assisted light scattering process. Position and form of the peak provide strong evidence for the presence of dopant-bound spinons in Cu1 − xZnxGeO3.