Manifestations of mesoscopic Jahn-Teller real-space pairing and clustering in YBa₂Cu₃O_7-δ

The manifestations of a mesoscopic Jahn-Teller real-space paring interaction in YBa₂Cu₃O_7-δ are described. The interaction of degenerate electronic stets with nonsimetric deformation, which acts on a length scale l∼2-3a∼ξ_s, where a is the lattice constant and ξ_s is the superconducting coherence length, gives rise to a mesoscopically inhomogeneous state of coexisting fermions and bosonic pairs that locally break the crystal symmetry. Lattice deformations observed in inelastic neutron scattering and broken-symmetry Raman scattering data are quantitatively examined and appear to confirm the existence of such an interaction. The possibility of pyro and piezoelectricity in YBa₂Cu₃O_7-δ is also predicted, in agreement with observations.