The anharmonic behavior of the oxygen modes in YBa2Cu3Ox compounds

Abstract

AbstractThe anharmonic behavior of the Raman‐active oxygen modes was examined in YBa2Cu316,18Ox compounds over the whole range of oxygen doping (6 < x < 7). The apex phonon of Ag symmetry shows a small anharmonicity in the range of 6.4 < x < 6.9, which can be attributed to the anharmonic behavior of this mode in one of the constituent phases (ortho‐II, ortho‐III, etc.), but not in the ortho‐I phase. The B1g mode does not show any anharmonicity over the whole range of oxygen doping, but characteristic spectral changes appear for x ≈ 6.3 and 6.7. At the lowest oxygen content studied (x = 6.03), there is a considerable amount of anharmonicity for the in‐phase phonon of the plane oxygen atoms, which remains large until x ≈ 6.3, at the phase transition to the tetragonal phase, where it drops to zero. It increases again for x ≈ 6.4 and then gradually decreases with doping to disappear at the optimal doping. If these changes are combined with the spectral modifications of the B1g and in‐phase modes, they indicate that local structural changes are induced by the transfer of carriers to the CuO2 planes at x ≈ 6.3, which are related to the reduction in anharmonicity, the pinning of the carriers and superconductivity. Copyright © 2001 John Wiley & Sons, Ltd.