IS THE PSEUDOGAP IN THE HIGH T-C CUPRATES EVIDENCE FOR DYNAMIC VAN HOVE JAHN-TELLER EFFECTS

Angle-resolved photoemission has demonstrated that in optimally doped YBa2Cu3O7 (YBCO) and Bi2Sr2CaCu2O8, the van Rove singularity (vHs) is pinned close to the Fermi level. Such a pinning was predicted by the vHs scenario, due to strong electron-phonon coupling. This coupling arises for a novel Jahn-Teller (JT) effect, in which a structural distortion splits the degeneracy of the two vHs's. In the lanthanum cuprates, there appears to be a sequence of dynamic JT phases, possibly terminating in a low-temperature static JT phase. The highest-temperature JT phase corresponds to the onset of octahedral tilts in the La cuprates (pyramidal tilts in YBCO), and can be identified with the experimentally observed pseudogap phase. Here I will summarize recent experimental evidence for such a dynamic JT effect in the cuprates.