Raman scattering from the spin-gap mode in the Ni-doped spin-Peierls compoundCuGeO3

Abstract

By means of Raman scattering we have studied the spin-Peierls ͑SP͒ transition in Ni-doped CuGeO 3 crystals. The folded-phonon peak extremely broadens and the two-magnetic-excitation mode disappears when the Ni concentration is above about 2.0%, indicating that the SP phase collapses. The spin-gap mode is activated in lightly Ni-doped samples. This peak neither splits nor shifts under magnetic fields in the SP phase, but its frequency increases in the incommensurate phase. The activation of the spin-gap mode is interpreted in terms of a mixing between the spin-singlet ground state and the S z = 0 triplet excited one by staggered fields around the doped impurities. Meanwhile, the two-magnetic-excitation mode gradually diminishes in intensity with increasing magnetic field in the incommensurate phase.

Spin-gap mode in the charge-ordered phase ofNaV2O5studied by Raman scattering under high pressures

“…This fact strongly suggests that the 64 cm −1 mode is assigned as it because the spin-gap mode at the magnetic ⌫ point can be observed by Raman scattering when the spin-singlet ground state is mixed with the spin-triplet excited state. 27,28 Moreover, we emphasize that the corresponding peak above 1.15 GPa is also assigned as the spin-gap mode.…”

“…28 This idea is also applicable to the present case. Then we may consider only the wave functions on these four rungs in the primitive unit cell.…”

“…Moreover, the first-order Raman scattering from magnetic excitation by the spin-orbit interaction mechanism is forbidden in the spin-gap systems. 28 The spin system above T c can be modeled by twodimensional Hamiltonian in the ab plane…”

“…This mechanism does not work in the spin-gap state because the total spin is quenched completely in the spin-singlet ground state. 28 In the second-order magnetic Raman process of magnetic materials, the exchange integral works between the pair of magnetic ions. This mechanism plays an essential role in the spin-singlet ground state because the total spin S total and its z component S total z are conserved in this Raman process.…”

“…This mechanism plays an essential role in the spin-singlet ground state because the total spin S total and its z component S total z are conserved in this Raman process. 28 The Raman Hamiltonian in the photon-induced exchange-interaction mechanism is fundamentally given as 28,[35][36][37][38] …”