Acoustic plasmons and cuprate superconductivity

The plasmon response is analyzed for a tight-binding model of two energy bands. The majority charge carriers originate from the Cu-O planes with copper orbitals of dₓ²-y² symmetry hybridized with the oxygen p states. The second band is more localized and yields low-energy acoustic plasmons that are well defined over a wide momentum range for appropriate positions of the Fermi energy EF. The use of bandwidths comparable to band-structure computations for YBa₂{Cu}₃O₇ yields a maximum energy FTHETA{}0.2 eV for the minority hole plasmon, which is a candidate for mediating superconducting pairing of the majority electrons. The transition temperature Tc is shown to correlate with the prefactor FTHETA as a function of EF in a weak-coupling analysis. If removal of oxygen raises EF, the corresponding decrease of FTHETA would be compatible with the observed drop in Tc for YBa₂{Cu}₃O_7-δ. The calculated structure factor reveals narrow acoustic plasmon peaks that should be accessible to electron-loss probes. Threshold structure in the electronic density of states for the narrower band may be related to observed Knight-shift variations.