Structural and spectroscopic evidence for stable chemical bonds and the correlation with high Tc superconductivity

It is discovered that in the high-Tc superconducting YBa2−xLaxCu3Oz system, by structural analysis and Raman spectroscopy, a stable ‘fixed triangle’ structure exists in the CuO2 planes. All chemical bonds and angles constituting the O(3)–Cu(2)–O(2) triangle are almost constant with doping. The frequencies and linewidths of the out-of-phase c-axis O(2)–O(3) buckling modes around 337 cm−1 and the in-plane Cu(2)–O(2) bond stretching modes around 534 cm−1 are both independent of the doping level, providing direct evidence for the stability of this fixed triangle. It was previously revealed that just these two phonons couple strongly with the antinodal and nodal electronic states, respectively, resulting in an anisotropic electron–phonon interaction in the cuprates. This work implies that the stability of the CuO2 subunit and phonons related to it might be quite important in inducing the d-wave high-Tc superconductivity and should be paid more attention.